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PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression. They fall into two
major categories: list operators and named unary operators. These differ in their
precedence relationship with a following comma. (See the precedence table in perlop.)
List operators take more than one argument, while unary operators can never take more than
one argument. Thus, a comma terminates the argument of a unary operator, but merely
separates the arguments of a list operator. A unary operator generally provides scalar
context to its argument, while a list operator may provide either scalar or list contexts
for its arguments. If it does both, scalar arguments come first and list argument follow,
and there can only ever be one such list argument. For instance, "splice" has three
scalar arguments followed by a list, whereas "gethostbyname" has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide
list context for elements of the list) are shown with LIST as an argument. Such a list
may consist of any combination of scalar arguments or list values; the list values will be
included in the list as if each individual element were interpolated at that point in the
list, forming a longer single-dimensional list value. Commas should separate literal
elements of the LIST.
Any function in the list below may be used either with or without parentheses around its
arguments. (The syntax descriptions omit the parentheses.) If you use parentheses, the
simple but occasionally surprising rule is this: It looks like a function, therefore it is
a function, and precedence doesn't matter. Otherwise it's a list operator or unary
operator, and precedence does matter. Whitespace between the function and left
parenthesis doesn't count, so sometimes you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the "use warnings" pragma, it can warn you about this. For example,
the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither unary nor list
operators. These include such functions as "time" and "endpwent". For example,
"time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context, nonabortive failure is
generally indicated in scalar context by returning the undefined value, and in list
context by returning the empty list.
Remember the following important rule: There is no rule that relates the behavior of an
expression in list context to its behavior in scalar context, or vice versa. It might do
two totally different things. Each operator and function decides which sort of value
would be most appropriate to return in scalar context. Some operators return the length
of the list that would have been returned in list context. Some operators return the
first value in the list. Some operators return the last value in the list. Some
operators return a count of successful operations. In general, they do what you want,
unless you want consistency.
A named array in scalar context is quite different from what would at first glance appear
to be a list in scalar context. You can't get a list like "(1,2,3)" into being in scalar
context, because the compiler knows the context at compile time. It would generate the
scalar comma operator there, not the list concatenation version of the comma. That means
it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the
same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and
"undef" otherwise, as is usually mentioned in the descriptions below. This is different
from the C interfaces, which return "-1" on failure. Exceptions to this rule include
"wait", "waitpid", and "syscall". System calls also set the special $! variable on
failure. Other functions do not, except accidentally.
Extension modules can also hook into the Perl parser to define new kinds of keyword-headed
expression. These may look like functions, but may also look completely different. The
syntax following the keyword is defined entirely by the extension. If you are an
implementor, see "PL_keyword_plugin" in perlapi for the mechanism. If you are using such
a module, see the module's documentation for details of the syntax that it defines.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like some keywords
and named operators) arranged by category. Some functions appear in more than one place.
Any warnings, including those produced by keywords, are described in perldiag and
warnings.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "fc", "hex", "index", "lc", "lcfirst", "length",
"oct", "ord", "pack", "q//", "qq//", "reverse", "rindex", "sprintf", "substr",
"tr///", "uc", "ucfirst", "y///"
"fc" is available only if the "fc" feature is enabled or if it is prefixed with
"CORE::". The "fc" feature is enabled automatically with a "use v5.16" (or higher)
declaration in the current scope.
Regular expressions and pattern matching
"m//", "pos", "qr//", "quotemeta", "s///", "split", "study"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt",
"srand"
Functions for real @ARRAYs
"each", "keys", "pop", "push", "shift", "splice", "unshift", "values"
Functions for list data
"grep", "join", "map", "qw//", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof", "fileno",
"flock", "format", "getc", "print", "printf", "read", "readdir", "readline",
"rewinddir", "say", "seek", "seekdir", "select", "syscall", "sysread", "sysseek",
"syswrite", "tell", "telldir", "truncate", "warn", "write"
"say" is available only if the "say" feature is enabled or if it is prefixed with
"CORE::". The "say" feature is enabled automatically with a "use v5.10" (or higher)
declaration in the current scope.
Functions for fixed-length data or records
"pack", "read", "syscall", "sysread", "sysseek", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat",
"mkdir", "open", "opendir", "readlink", "rename", "rmdir", "select", "stat",
"symlink", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your Perl program
"break", "caller", "continue", "die", "do", "dump", "eval", "evalbytes", "exit",
"__FILE__", "goto", "last", "__LINE__", "next", "__PACKAGE__", "redo", "return",
"sub", "__SUB__", "wantarray"
"break" is available only if you enable the experimental "switch" feature or use the
"CORE::" prefix. The "switch" feature also enables the "default", "given" and "when"
statements, which are documented in "Switch Statements" in perlsyn. The "switch"
feature is enabled automatically with a "use v5.10" (or higher) declaration in the
current scope. In Perl v5.14 and earlier, "continue" required the "switch" feature,
like the other keywords.
"evalbytes" is only available with the "evalbytes" feature (see feature) or if
prefixed with "CORE::". "__SUB__" is only available with the "current_sub" feature or
if prefixed with "CORE::". Both the "evalbytes" and "current_sub" features are
enabled automatically with a "use v5.16" (or higher) declaration in the current scope.
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "state", "use"
"state" is available only if the "state" feature is enabled or if it is prefixed with
"CORE::". The "state" feature is enabled automatically with a "use v5.10" (or higher)
declaration in the current scope.
Miscellaneous functions
"defined", "formline", "lock", "prototype", "reset", "scalar", "undef"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx//",
"readpipe", "setpgrp", "setpriority", "sleep", "system", "times", "wait", "waitpid"
Keywords related to Perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientation
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen",
"recv", "send", "setsockopt", "shutdown", "socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop", "shmctl",
"shmget", "shmread", "shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent", "getgrgid", "getgrnam",
"getlogin", "getpwent", "getpwnam", "getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent",
"getnetbyaddr", "getnetbyname", "getnetent", "getprotobyname", "getprotobynumber",
"getprotoent", "getservbyname", "getservbyport", "getservent", "sethostent",
"setnetent", "setprotoent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Non-function keywords
"and", "AUTOLOAD", "BEGIN", "CHECK", "cmp", "CORE", "__DATA__", "default", "DESTROY",
"else", "elseif", "elsif", "END", "__END__", "eq", "for", "foreach", "ge", "given",
"gt", "if", "INIT", "le", "lt", "ne", "not", "or", "UNITCHECK", "unless", "until",
"when", "while", "x", "xor"
Portability
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix
environments, the functionality of some Unix system calls may not be available or details
of the available functionality may differ slightly. The Perl functions affected by this
are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump",
"endgrent", "endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec",
"fcntl", "flock", "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent",
"getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp",
"getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpwnam", "getpwuid",
"getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link", "lstat",
"msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select",
"semctl", "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
"setpriority", "setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget",
"shmread", "shmwrite", "socket", "socketpair", "stat", "symlink", "syscall", "sysopen",
"system", "times", "truncate", "umask", "unlink", "utime", "wait", "waitpid"
For more information about the portability of these functions, see perlport and other
available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X DIRHANDLE
-X A file test, where X is one of the letters listed below. This unary operator takes
one argument, either a filename, a filehandle, or a dirhandle, and tests the
associated file to see if something is true about it. If the argument is omitted,
tests $_, except for "-t", which tests STDIN. Unless otherwise documented, it returns
1 for true and '' for false. If the file doesn't exist or can't be examined, it
returns "undef" and sets $! (errno). With the exception of the "-l" test they all
follow symbolic links because they use "stat()" and not "lstat()" (so dangling
symlinks can't be examined and will therefore report failure).
Despite the funny names, precedence is the same as any other named unary operator.
The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link (false if symlinks aren't
supported by the file system).
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII or UTF-8 text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other
platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still
works as expected, however: only single letters following a minus are interpreted as
file tests.
These operators are exempt from the "looks like a function rule" described above.
That is, an opening parenthesis after the operator does not affect how much of the
following code constitutes the argument. Put the opening parentheses before the
operator to separate it from code that follows (this applies only to operators with
higher precedence than unary operators, of course):
-s($file) + 1024 # probably wrong; same as -s($file + 1024)
(-s $file) + 1024 # correct
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and
"-X" is by default based solely on the mode of the file and the uids and gids of the
user. There may be other reasons you can't actually read, write, or execute the file:
for example network filesystem access controls, ACLs (access control lists), read-only
filesystems, and unrecognized executable formats. Note that the use of these six
specific operators to verify if some operation is possible is usually a mistake,
because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and
"-W" tests always return 1, and "-x" and "-X" return 1 if any execute bit is set in
the mode. Scripts run by the superuser may thus need to do a "stat" to determine the
actual mode of the file, or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more
accurate results than the bare "stat" mode bits. When under "use filetest 'access'",
the above-mentioned filetests test whether the permission can(not) be granted using
the access(2) family of system calls. Also note that the "-x" and "-X" tests may
under this pragma return true even if there are no execute permission bits set (nor
any extra execute permission ACLs). This strangeness is due to the underlying system
calls' definitions. Note also that, due to the implementation of "use filetest
'access'", the "_" special filehandle won't cache the results of the file tests when
this pragma is in effect. Read the documentation for the "filetest" pragma for more
information.
The "-T" and "-B" tests work as follows. The first block or so of the file is
examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's
a "-T" file. Otherwise, that same portion of the file is examined for odd characters
such as strange control codes or characters with the high bit set. If more than a
third of the characters are strange, it's a "-B" file; otherwise it's a "-T" file.
Also, any file containing a zero byte in the examined portion is considered a binary
file. (If executed within the scope of a uselocale which includes "LC_CTYPE", odd
characters are anything that isn't a printable nor space in the current locale.) If
"-T" or "-B" is used on a filehandle, the current IO buffer is examined rather than
the first block. Both "-T" and "-B" return true on an empty file, or a file at EOF
when testing a filehandle. Because you have to read a file to do the "-T" test, on
most occasions you want to use a "-f" against the file first, as in "next unless -f
$file && -T $file".
If any of the file tests (or either the "stat" or "lstat" operator) is given the
special filehandle consisting of a solitary underline, then the stat structure of the
previous file test (or "stat" operator) is used, saving a system call. (This doesn't
work with "-t", and you need to remember that "lstat" and "-l" leave values in the
stat structure for the symbolic link, not the real file.) (Also, if the stat buffer
was filled by an "lstat" call, "-T" and "-B" will reset it with the results of "stat
_"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test
operators, in a way that "-f -w -x $file" is equivalent to "-x $file && -w _ && -f _".
(This is only fancy syntax: if you use the return value of "-f $file" as an argument
to another filetest operator, no special magic will happen.)
Portability issues: "-X" in perlport.
To avoid confusing would-be users of your code with mysterious syntax errors, put
something like this at the top of your script:
use 5.010; # so filetest ops can stack
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as accept(2) does. Returns the packed
address if it succeeded, false otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the
newly opened file descriptor, as determined by the value of $^F. See "$^F" in
perlvar.
alarm SECONDS
alarm
Arranges to have a SIGALRM delivered to this process after the specified number of
wallclock seconds has elapsed. If SECONDS is not specified, the value stored in $_ is
used. (On some machines, unfortunately, the elapsed time may be up to one second less
or more than you specified because of how seconds are counted, and process scheduling
may delay the delivery of the signal even further.)
Only one timer may be counting at once. Each call disables the previous timer, and an
argument of 0 may be supplied to cancel the previous timer without starting a new one.
The returned value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN,
and starting from Perl 5.8 part of the standard distribution) provides "ualarm". You
may also use Perl's four-argument version of "select" leaving the first three
arguments undefined, or you might be able to use the "syscall" interface to access
setitimer(2) if your system supports it. See perlfaq8 for details.
It is usually a mistake to intermix "alarm" and "sleep" calls, because "sleep" may be
internally implemented on your system with "alarm".
If you want to use "alarm" to time out a system call you need to use an "eval"/"die"
pair. You can't rely on the alarm causing the system call to fail with $! set to
"EINTR" because Perl sets up signal handlers to restart system calls on some systems.
Using "eval"/"die" always works, modulo the caveats given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
my $nread = sysread $socket, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
Portability issues: "alarm" in perlport.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the
familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3)
manpage for more information.
Portability issues: "atan2" in perlport.
bind SOCKET,NAME
Binds a network address to a socket, just as bind(2) does. Returns true if it
succeeded, false otherwise. NAME should be a packed address of the appropriate type
for the socket. See the examples in "Sockets: Client/Server Communication" in
perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems
where the run-time libraries distinguish between binary and text files. If FILEHANDLE
is an expression, the value is taken as the name of the filehandle. Returns true on
success, otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS- and Windows-based systems) "binmode" is necessary
when you're not working with a text file. For the sake of portability it is a good
idea always to use it when appropriate, and never to use it when it isn't appropriate.
Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.
In other words: regardless of platform, use "binmode" on binary data, like images, for
example.
If LAYER is present it is a single string, but may contain multiple directives. The
directives alter the behaviour of the filehandle. When LAYER is present, using
binmode on a text file makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing
binary data. This includes turning off possible CRLF translation and marking it as
bytes (as opposed to Unicode characters). Note that, despite what may be implied in
"Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is not simply the
inverse of ":crlf". Other layers that would affect the binary nature of the stream
are also disabled. See PerlIO, and the discussion about the PERLIO environment
variable in perlrun.
The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are
called I/O layers. The open pragma can be used to establish default I/O layers.
The LAYER parameter of the "binmode" function is described as "DISCIPLINE" in
"Programming Perl, 3rd Edition". However, since the publishing of this book, by many
known as "Camel III", the consensus of the naming of this functionality has moved from
"discipline" to "layer". All documentation of this version of Perl therefore refers
to "layers" rather than to "disciplines". Now back to the regularly scheduled
documentation...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)". ":utf8" just marks
the data as UTF-8 without further checking, while ":encoding(UTF-8)" checks the data
for actually being valid UTF-8. More details can be found in PerlIO::encoding.
In general, "binmode" should be called after "open" but before any I/O is done on the
filehandle. Calling "binmode" normally flushes any pending buffered output data (and
perhaps pending input data) on the handle. An exception to this is the ":encoding"
layer that changes the default character encoding of the handle. The ":encoding"
layer sometimes needs to be called in mid-stream, and it doesn't flush the stream.
":encoding" also implicitly pushes on top of itself the ":utf8" layer because
internally Perl operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all
conspire to let the programmer treat a single character ("\n") as the line terminator,
irrespective of external representation. On many operating systems, the native text
file representation matches the internal representation, but on some platforms the
external representation of "\n" is made up of more than one character.
All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single
character to end each line in the external representation of text (even though that
single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE
FEED on Unix and most VMS files). In other systems like OS/2, DOS, and the various
flavors of MS-Windows, your program sees a "\n" as a simple "\cJ", but what's stored
in text files are the two characters "\cM\cJ". That means that if you don't use
"binmode" on these systems, "\cM\cJ" sequences on disk will be converted to "\n" on
input, and any "\n" in your program will be converted back to "\cM\cJ" on output.
This is what you want for text files, but it can be disastrous for binary files.
Another consequence of using "binmode" (on some systems) is that special end-of-file
markers will be seen as part of the data stream. For systems from the Microsoft
family this means that, if your binary data contain "\cZ", the I/O subsystem will
regard it as the end of the file, unless you use "binmode".
"binmode" is important not only for "readline" and "print" operations, but also when
using "read", "seek", "sysread", "syswrite" and "tell" (see perlport for more
details). See the $/ and "$\" variables in perlvar for how to manually set your input
and output line-termination sequences.
Portability issues: "binmode" in perlport.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the
CLASSNAME package. If CLASSNAME is an empty string, it is interpreted as referring to
the "main" package. If CLASSNAME is omitted, the current package is used. Because a
"bless" is often the last thing in a constructor, it returns the reference for
convenience. Always use the two-argument version if a derived class might inherit the
method doing the blessing. See perlobj for more about the blessing (and blessings) of
objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with
all lowercase names are considered reserved for Perl pragmas. Builtin types have all
uppercase names. To prevent confusion, you may wish to avoid such package names as
well. It is advised to avoid the class name 0, because much code erroneously uses the
result of "ref" as a truth value.
See "Perl Modules" in perlmod.
break
Break out of a "given" block.
"break" is available only if the "switch" feature is enabled or if it is prefixed with
"CORE::". The "switch" feature is enabled automatically with a "use v5.10" (or higher)
declaration in the current scope.
caller EXPR
caller
Returns the context of the current pure perl subroutine call. In scalar context,
returns the caller's package name if there is a caller (that is, if we're in a
subroutine or "eval" or "require") and the undefined value otherwise. caller never
returns XS subs and they are skipped. The next pure perl sub will appear instead of
the XS sub in caller's return values. In list context, caller returns
# 0 1 2
my ($package, $filename, $line) = caller;
Like "__FILE__" and "__LINE__", the filename and line number returned here may be
altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.
With EXPR, it returns some extra information that the debugger uses to print a stack
trace. The value of EXPR indicates how many call frames to go back before the current
one.
# 0 1 2 3 4
my ($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);
Here, $subroutine is the function that the caller called (rather than the function
containing the caller). Note that $subroutine may be "(eval)" if the frame is not a
subroutine call, but an "eval". In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the frame is created by a "require" or
"use" statement, $evaltext contains the text of the "eval EXPR" statement. In
particular, for an "eval BLOCK" statement, $subroutine is "(eval)", but $evaltext is
undefined. (Note also that each "use" statement creates a "require" frame inside an
"eval EXPR" frame.) $subroutine may also be "(unknown)" if this particular subroutine
happens to have been deleted from the symbol table. $hasargs is true if a new
instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints
that the caller was compiled with. $hints corresponds to $^H, and $bitmask
corresponds to "${^WARNING_BITS}". The $hints and $bitmask values are subject to
change between versions of Perl, and are not meant for external use.
$hinthash is a reference to a hash containing the value of "%^H" when the caller was
compiled, or "undef" if "%^H" was empty. Do not modify the values of this hash, as
they are the actual values stored in the optree.
Note that the only types of call frames that are visible are subroutine calls and
"eval". Other forms of context, such as "while" or "foreach" loops or "try" blocks are
not considered interesting to "caller", as they do not alter the behaviour of the
"return" expression.
Furthermore, when called from within the DB package in list context, and with an
argument, caller returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before "caller" had
a chance to get the information. That means that caller(N) might not return
information about the call frame you expect it to, for "N > 1". In particular,
@DB::args might have information from the previous time "caller" was called.
Be aware that setting @DB::args is best effort, intended for debugging or generating
backtraces, and should not be relied upon. In particular, as @_ contains aliases to
the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain
modifications the subroutine makes to @_ or its contents, not the original values at
call time. @DB::args, like @_, does not hold explicit references to its elements, so
under certain cases its elements may have become freed and reallocated for other
variables or temporary values. Finally, a side effect of the current implementation
is that the effects of "shift @_" can normally be undone (but not "pop @_" or other
splicing, and not if a reference to @_ has been taken, and subject to the caveat about
reallocated elements), so @DB::args is actually a hybrid of the current state and
initial state of @_. Buyer beware.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to
the directory specified by $ENV{HOME}, if set; if not, changes to the directory
specified by $ENV{LOGDIR}. (Under VMS, the variable $ENV{'SYS$LOGIN'} is also
checked, and used if it is set.) If neither is set, "chdir" does nothing and fails.
It returns true on success, false otherwise. See the example under "die".
On systems that support fchdir(2), you may pass a filehandle or directory handle as
the argument. On systems that don't support fchdir(2), passing handles raises an
exception.
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the
numeric mode, which should probably be an octal number, and which definitely should
not be a string of octal digits: 0644 is okay, but "0644" is not. Returns the number
of files successfully changed. See also "oct" if all you have is a string.
my $cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
my $mode = "0644"; chmod oct($mode), "foo"; # this is better
my $mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass filehandles among the files. On
systems that don't support fchmod(2), passing filehandles raises an exception.
Filehandles must be passed as globs or glob references to be recognized; barewords are
considered filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the "Fcntl" module:
use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.
Portability issues: "chmod" in perlport.
chomp VARIABLE
chomp( LIST )
chomp
This safer version of "chop" removes any trailing string that corresponds to the
current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the "English" module).
It returns the total number of characters removed from all its arguments. It's often
used to remove the newline from the end of an input record when you're worried that
the final record may be missing its newline. When in paragraph mode ("$/ = ''"), it
removes all trailing newlines from the string. When in slurp mode ("$/ = undef") or
fixed-length record mode ($/ is a reference to an integer or the like; see perlvar),
"chomp" won't remove anything. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
my @array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the
"each" iterator in the process.
You can actually chomp anything that's an lvalue, including an assignment:
chomp(my $cwd = `pwd`);
chomp(my $answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters
removed is returned.
Note that parentheses are necessary when you're chomping anything that is not a simple
variable. This is because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) =
`pwd`;", rather than as "chomp( $cwd = `pwd` )" which you might expect. Similarly,
"chomp $a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop
Chops off the last character of a string and returns the character chopped. It is
much more efficient than "s/.$//s" because it neither scans nor copies the string. If
VARIABLE is omitted, chops $_. If VARIABLE is a hash, it chops the hash's values, but
not its keys, resetting the "each" iterator in the process.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is
returned.
Note that "chop" returns the last character. To return all but the last character,
use "substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list
must be the numeric uid and gid, in that order. A value of -1 in either position is
interpreted by most systems to leave that value unchanged. Returns the number of
files successfully changed.
my $cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass filehandles among the files. On
systems that don't support fchown(2), passing filehandles raises an exception.
Filehandles must be passed as globs or glob references to be recognized; barewords are
considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp(my $user = <STDIN>);
print "Files: ";
chomp(my $pattern = <STDIN>);
my ($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
my @ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're
the superuser, although you should be able to change the group to any of your
secondary groups. On insecure systems, these restrictions may be relaxed, but this is
not a portable assumption. On POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
Portability issues: "chown" in perlport.
chr NUMBER
chr Returns the character represented by that NUMBER in the character set. For example,
"chr(65)" is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face.
Negative values give the Unicode replacement character (chr(0xfffd)), except under the
bytes pragma, where the low eight bits of the value (truncated to an integer) are
used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by default internally not encoded
as UTF-8 for backward compatibility reasons.
See perlunicode for more about Unicode.
chroot FILENAME
chroot
This function works like the system call by the same name: it makes the named
directory the new root directory for all further pathnames that begin with a "/" by
your process and all its children. (It doesn't change your current working directory,
which is unaffected.) For security reasons, this call is restricted to the superuser.
If FILENAME is omitted, does a "chroot" to $_.
NOTE: It is mandatory for security to "chdir("/")" ("chdir" to the root directory)
immediately after a "chroot", otherwise the current working directory may be outside
of the new root.
Portability issues: "chroot" in perlport.
close FILEHANDLE
close
Closes the file or pipe associated with the filehandle, flushes the IO buffers, and
closes the system file descriptor. Returns true if those operations succeed and if no
error was reported by any PerlIO layer. Closes the currently selected filehandle if
the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open"
on it, because "open" closes it for you. (See "open".) However, an explicit "close"
on an input file resets the line counter ($.), while the implicit close done by "open"
does not.
If the filehandle came from a piped open, "close" returns false if one of the other
syscalls involved fails or if its program exits with non-zero status. If the only
problem was that the program exited non-zero, $! will be set to 0. Closing a pipe
also waits for the process executing on the pipe to exit--in case you wish to look at
the output of the pipe afterwards--and implicitly puts the exit status value of that
command into $? and "${^CHILD_ERROR_NATIVE}".
If there are multiple threads running, "close" on a filehandle from a piped open
returns true without waiting for the child process to terminate, if the filehandle is
still open in another thread.
Closing the read end of a pipe before the process writing to it at the other end is
done writing results in the writer receiving a SIGPIPE. If the other end can't handle
that, be sure to read all the data before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle,
usually the real filehandle name or an autovivified handle.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just like connect(2). Returns true if it
succeeded, false otherwise. NAME should be a packed address of the appropriate type
for the socket. See the examples in "Sockets: Client/Server Communication" in
perlipc.
continue BLOCK
continue
When followed by a BLOCK, "continue" is actually a flow control statement rather than
a function. If there is a "continue" BLOCK attached to a BLOCK (typically in a
"while" or "foreach"), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a "for" loop in C. Thus it can be
used to increment a loop variable, even when the loop has been continued via the
"next" statement (which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block; "last" and "redo"
behave as if they had been executed within the main block. So will "next", but since
it will execute a "continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is equivalent to using an empty one, logically enough,
so "next" goes directly back to check the condition at the top of the loop.
When there is no BLOCK, "continue" is a function that falls through the current "when"
or "default" block instead of iterating a dynamically enclosing "foreach" or exiting a
lexically enclosing "given". In Perl 5.14 and earlier, this form of "continue" was
only available when the "switch" feature was enabled. See feature and "Switch
Statements" in perlsyn for more information.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes the
cosine of $_.
For the inverse cosine operation, you may use the "Math::Trig::acos" function, or use
this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming
that you actually have a version there that has not been extirpated as a potential
munition).
"crypt" is a one-way hash function. The PLAINTEXT and SALT are turned into a short
string, called a digest, which is returned. The same PLAINTEXT and SALT will always
return the same string, but there is no (known) way to get the original PLAINTEXT from
the hash. Small changes in the PLAINTEXT or SALT will result in large changes in the
digest.
There is no decrypt function. This function isn't all that useful for cryptography
(for that, look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is
a bit of a misnomer. Instead it is primarily used to check if two pieces of text are
the same without having to transmit or store the text itself. An example is checking
if a correct password is given. The digest of the password is stored, not the
password itself. The user types in a password that is "crypt"'d with the same salt as
the stored digest. If the two digests match, the password is correct.
When verifying an existing digest string you should use the digest as the salt (like
"crypt($plain, $digest) eq $digest"). The SALT used to create the digest is visible
as part of the digest. This ensures "crypt" will hash the new string with the same
salt as the digest. This allows your code to work with the standard "crypt" and with
more exotic implementations. In other words, assume nothing about the returned string
itself nor about how many bytes of SALT may matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt,
followed by 11 bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of
PLAINTEXT mattered. But alternative hashing schemes (like MD5), higher level security
schemes (like C2), and implementations on non-Unix platforms may produce different
strings.
When choosing a new salt create a random two character string whose characters come
from the set "[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand
64, rand 64]"). This set of characters is just a recommendation; the characters
allowed in the salt depend solely on your system's crypt library, and Perl can't
restrict what salts "crypt" accepts.
Here's an example that makes sure that whoever runs this program knows their password:
my $pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp(my $word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The "crypt" function is unsuitable for hashing large quantities of data, not least of
all because you can't get the information back. Look at the Digest module for more
robust algorithms.
If using "crypt" on a Unicode string (which potentially has characters with codepoints
above 255), Perl tries to make sense of the situation by trying to downgrade (a copy
of) the string back to an eight-bit byte string before calling "crypt" (on that copy).
If that works, good. If not, "crypt" dies with "Wide character in crypt".
Portability issues: "crypt" in perlport.
dbmclose HASH
[This function has been largely superseded by the "untie" function.]
Breaks the binding between a DBM file and a hash.
Portability issues: "dbmclose" in perlport.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH
is the name of the hash. (Unlike normal "open", the first argument is not a
filehandle, even though it looks like one). DBNAME is the name of the database
(without the .dir or .pag extension if any). If the database does not exist, it is
created with protection specified by MASK (as modified by the "umask"). To prevent
creation of the database if it doesn't exist, you may specify a MODE of 0, and the
function will return a false value if it can't find an existing database. If your
system supports only the older DBM functions, you may make only one "dbmopen" call in
your program. In older versions of Perl, if your system had neither DBM nor ndbm,
calling "dbmopen" produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not
set them. If you want to test whether you can write, either use file tests or try
setting a dummy hash entry inside an "eval" to trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on
large DBM files. You may prefer to use the "each" function to iterate over large DBM
files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the
various dbm approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call
"dbmopen":
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
Portability issues: "dbmopen" in perlport.
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other than the undefined
value "undef". If EXPR is not present, $_ is checked.
Many operations return "undef" to indicate failure, end of file, system error,
uninitialized variable, and other exceptional conditions. This function allows you to
distinguish "undef" from other values. (A simple Boolean test will not distinguish
among "undef", zero, the empty string, and "0", which are all equally false.) Note
that since "undef" is a valid scalar, its presence doesn't necessarily indicate an
exceptional condition: "pop" returns "undef" when its argument is an empty array, or
when the element to return happens to be "undef".
You may also use "defined(&func)" to check whether subroutine "func" has ever been
defined. The return value is unaffected by any forward declarations of "func". A
subroutine that is not defined may still be callable: its package may have an
"AUTOLOAD" method that makes it spring into existence the first time that it is
called; see perlsub.
Use of "defined" on aggregates (hashes and arrays) is no longer supported. It used to
report whether memory for that aggregate had ever been allocated. You should instead
use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is defined, not whether
the key exists in the hash. Use "exists" for the latter purpose.
Examples:
print if defined $switch{D};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined" and are then surprised to discover that
the number 0 and "" (the zero-length string) are, in fact, defined values. For
example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds and $1 is defined, although it matched "nothing". It
didn't really fail to match anything. Rather, it matched something that happened to
be zero characters long. This is all very above-board and honest. When a function
returns an undefined value, it's an admission that it couldn't give you an honest
answer. So you should use "defined" only when questioning the integrity of what
you're trying to do. At other times, a simple comparison to 0 or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies an element or slice of a hash, "delete" deletes the
specified elements from that hash so that "exists" on that element no longer returns
true. Setting a hash element to the undefined value does not remove its key, but
deleting it does; see "exists".
In list context, usually returns the value or values deleted, or the last such element
in scalar context. The return list's length corresponds to that of the argument list:
deleting non-existent elements returns the undefined value in their corresponding
positions. When a key/value hash slice is passed to "delete", the return value is a
list of key/value pairs (two elements for each item deleted from the hash).
"delete" may also be used on arrays and array slices, but its behavior is less
straightforward. Although "exists" will return false for deleted entries, deleting
array elements never changes indices of existing values; use "shift" or "splice" for
that. However, if any deleted elements fall at the end of an array, the array's size
shrinks to the position of the highest element that still tests true for "exists", or
to 0 if none do. In other words, an array won't have trailing nonexistent elements
after a delete.
WARNING: Calling "delete" on array values is strongly discouraged. The notion of
deleting or checking the existence of Perl array elements is not conceptually
coherent, and can lead to surprising behavior.
Deleting from %ENV modifies the environment. Deleting from a hash tied to a DBM file
deletes the entry from the DBM file. Deleting from a "tied" hash or array may not
necessarily return anything; it depends on the implementation of the "tied" package's
DELETE method, which may do whatever it pleases.
The "delete local EXPR" construct localizes the deletion to the current block at run
time. Until the block exits, elements locally deleted temporarily no longer exist.
See "Localized deletion of elements of composite types" in perlsub.
my %hash = (foo => 11, bar => 22, baz => 33);
my $scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach my $key (keys %HASH) {
delete $HASH{$key};
}
foreach my $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which
is the customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its final operation is an element or
slice of an aggregate:
delete $ref->[$x][$y]{$key};
delete $ref->[$x][$y]->@{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete $ref->[$x][$y]->@[$index1, $index2, @moreindices];
die LIST
"die" raises an exception. Inside an "eval" the exception is stuffed into $@ and the
"eval" is terminated with the undefined value. If the exception is outside of all
enclosing "eval"s, then the uncaught exception is printed to "STDERR" and perl exits
with an exit code indicating failure. If you need to exit the process with a specific
exit code, see "exit".
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
Most of the time, "die" is called with a string to use as the exception. You may
either give a single non-reference operand to serve as the exception, or a list of two
or more items, which will be stringified and concatenated to make the exception.
If the string exception does not end in a newline, the current script line number and
input line number (if any) and a newline are appended to it. Note that the "input
line number" (also known as "chunk") is subject to whatever notion of "line" happens
to be currently in effect, and is also available as the special variable $.. See "$/"
in perlvar and "$." in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better
sense when the string "at foo line 123" is appended. Suppose you are running script
"canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
If LIST was empty or made an empty string, and $@ already contains an exception value
(typically from a previous "eval"), then that value is reused after appending
"\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST was empty or made an empty string, and $@ contains an object reference that
has a "PROPAGATE" method, that method will be called with additional file and line
number parameters. The return value replaces the value in $@; i.e., as if "$@ = eval
{ $@->PROPAGATE(__FILE__, __LINE__) };" were called.
If LIST was empty or made an empty string, and $@ is also empty, then the string
"Died" is used.
You can also call "die" with a reference argument, and if this is trapped within an
"eval", $@ contains that reference. This permits more elaborate exception handling
using objects that maintain arbitrary state about the exception. Such a scheme is
sometimes preferable to matching particular string values of $@ with regular
expressions.
Because Perl stringifies uncaught exception messages before display, you'll probably
want to overload stringification operations on exception objects. See overload for
details about that. The stringified message should be non-empty, and should end in a
newline, in order to fit in with the treatment of string exceptions. Also, because an
exception object reference cannot be stringified without destroying it, Perl doesn't
attempt to append location or other information to a reference exception. If you want
location information with a complex exception object, you'll have to arrange to put
the location information into the object yourself.
Because $@ is a global variable, be careful that analyzing an exception caught by
"eval" doesn't replace the reference in the global variable. It's easiest to make a
local copy of the reference before any manipulations. Here's an example:
use Scalar::Util "blessed";
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err)
&& $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
If an uncaught exception results in interpreter exit, the exit code is determined from
the values of $! and $? with this pseudocode:
exit $! if $!; # errno
exit $? >> 8 if $? >> 8; # child exit status
exit 255; # last resort
As with "exit", $? is set prior to unwinding the call stack; any "DESTROY" or "END"
handlers can then alter this value, and thus Perl's exit code.
The intent is to squeeze as much possible information about the likely cause into the
limited space of the system exit code. However, as $! is the value of C's "errno",
which can be set by any system call, this means that the value of the exit code used
by "die" can be non-predictable, so should not be relied upon, other than to be non-
zero.
You can arrange for a callback to be run just before the "die" does its deed, by
setting the $SIG{__DIE__} hook. The associated handler is called with the exception
as an argument, and can change the exception, if it sees fit, by calling "die" again.
See "%SIG" in perlvar for details on setting %SIG entries, and "eval" for some
examples. Although this feature was to be run only right before your program was to
exit, this is not currently so: the $SIG{__DIE__} hook is currently called even inside
"eval"ed blocks/strings! If one wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes
strange action at a distance, this counterintuitive behavior may be fixed in a future
release.
See also "exit", "warn", and the Carp module.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of
commands indicated by BLOCK. When modified by the "while" or "until" loop modifier,
executes the BLOCK once before testing the loop condition. (On other statements the
loop modifiers test the conditional first.)
"do BLOCK" does not count as a loop, so the loop control statements "next", "last", or
"redo" cannot be used to leave or restart the block. See perlsyn for alternative
strategies.
do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a Perl
script:
# load the exact specified file (./ and ../ special-cased)
do '/foo/stat.pl';
do './stat.pl';
do '../foo/stat.pl';
# search for the named file within @INC
do 'stat.pl';
do 'foo/stat.pl';
"do './stat.pl'" is largely like
eval `cat stat.pl`;
except that it's more concise, runs no external processes, and keeps track of the
current filename for error messages. It also differs in that code evaluated with "do
FILE" cannot see lexicals in the enclosing scope; "eval STRING" does. It's the same,
however, in that it does reparse the file every time you call it, so you probably
don't want to do this inside a loop.
Using "do" with a relative path (except for ./ and ../), like
do 'foo/stat.pl';
will search the @INC directories, and update %INC if the file is found. See "@INC" in
perlvar and "%INC" in perlvar for these variables. In particular, note that whilst
historically @INC contained '.' (the current directory) making these two cases
equivalent, that is no longer necessarily the case, as '.' is not included in @INC by
default in perl versions 5.26.0 onwards. Instead, perl will now warn:
do "stat.pl" failed, '.' is no longer in @INC;
did you mean do "./stat.pl"?
If "do" can read the file but cannot compile it, it returns "undef" and sets an error
message in $@. If "do" cannot read the file, it returns undef and sets $! to the
error. Always check $@ first, as compilation could fail in a way that also sets $!.
If the file is successfully compiled, "do" returns the value of the last expression
evaluated.
Inclusion of library modules is better done with the "use" and "require" operators,
which also do automatic error checking and raise an exception if there's a problem.
You might like to use "do" to read in a program configuration file. Manual error
checking can be done this way:
# Read in config files: system first, then user.
# Beware of using relative pathnames here.
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump EXPR
dump
This function causes an immediate core dump. See also the -u command-line switch in
perlrun, which does the same thing. Primarily this is so that you can use the undump
program (not supplied) to turn your core dump into an executable binary after having
initialized all your variables at the beginning of the program. When the new binary
is executed it will begin by executing a "goto LABEL" (with all the restrictions that
"goto" suffers). Think of it as a goto with an intervening core dump and
reincarnation. If "LABEL" is omitted, restarts the program from the top. The "dump
EXPR" form, available starting in Perl 5.18.0, allows a name to be computed at run
time, being otherwise identical to "dump LABEL".
WARNING: Any files opened at the time of the dump will not be open any more when the
program is reincarnated, with possible resulting confusion by Perl.
This function is now largely obsolete, mostly because it's very hard to convert a core
file into an executable. As of Perl 5.30, it must be invoked as "CORE::dump()".
Unlike most named operators, this has the same precedence as assignment. It is also
exempt from the looks-like-a-function rule, so "dump ("foo")."bar"" will cause "bar"
to be part of the argument to "dump".
Portability issues: "dump" in perlport.
each HASH
each ARRAY
When called on a hash in list context, returns a 2-element list consisting of the key
and value for the next element of a hash. In Perl 5.12 and later only, it will also
return the index and value for the next element of an array so that you can iterate
over it; older Perls consider this a syntax error. When called in scalar context,
returns only the key (not the value) in a hash, or the index in an array.
Hash entries are returned in an apparently random order. The actual random order is
specific to a given hash; the exact same series of operations on two hashes may result
in a different order for each hash. Any insertion into the hash may change the order,
as will any deletion, with the exception that the most recent key returned by "each"
or "keys" may be deleted without changing the order. So long as a given hash is
unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on
why hash order is randomized. Aside from the guarantees provided here the exact
details of Perl's hash algorithm and the hash traversal order are subject to change in
any release of Perl.
After "each" has returned all entries from the hash or array, the next call to "each"
returns the empty list in list context and "undef" in scalar context; the next call
following that one restarts iteration. Each hash or array has its own internal
iterator, accessed by "each", "keys", and "values". The iterator is implicitly reset
when "each" has reached the end as just described; it can be explicitly reset by
calling "keys" or "values" on the hash or array, or by referencing the hash (but not
array) in list context. If you add or delete a hash's elements while iterating over
it, the effect on the iterator is unspecified; for example, entries may be skipped or
duplicated--so don't do that. Exception: It is always safe to delete the item most
recently returned by "each", so the following code works properly:
while (my ($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
Tied hashes may have a different ordering behaviour to perl's hash implementation.
The iterator used by "each" is attached to the hash or array, and is shared between
all iteration operations applied to the same hash or array. Thus all uses of "each"
on a single hash or array advance the same iterator location. All uses of "each" are
also subject to having the iterator reset by any use of "keys" or "values" on the same
hash or array, or by the hash (but not array) being referenced in list context. This
makes "each"-based loops quite fragile: it is easy to arrive at such a loop with the
iterator already part way through the object, or to accidentally clobber the iterator
state during execution of the loop body. It's easy enough to explicitly reset the
iterator before starting a loop, but there is no way to insulate the iterator state
used by a loop from the iterator state used by anything else that might execute during
the loop body. To avoid these problems, use a "foreach" loop rather than
"while"-"each".
This prints out your environment like the printenv(1) program, but in a different
order:
while (my ($key,$value) = each %ENV) {
print "$key=$value\n";
}
Starting with Perl 5.14, an experimental feature allowed "each" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
As of Perl 5.18 you can use a bare "each" in a "while" loop, which will set $_ on
every iteration. If either an "each" expression or an explicit assignment of an
"each" expression to a scalar is used as a "while"/"for" condition, then the condition
actually tests for definedness of the expression's value, not for its regular truth
value.
while (each %ENV) {
print "$_=$ENV{$_}\n";
}
To avoid confusing would-be users of your code who are running earlier versions of
Perl with mysterious syntax errors, put this sort of thing at the top of your file to
signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays
use 5.018; # so each assigns to $_ in a lone while test
See also "keys", "values", and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file or if FILEHANDLE is
not open. FILEHANDLE may be an expression whose value gives the real filehandle.
(Note that this function actually reads a character and then "ungetc"s it, so isn't
useful in an interactive context.) Do not read from a terminal file (or call
"eof(FILEHANDLE)" on it) after end-of-file is reached. File types such as terminals
may lose the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using "eof()" with empty
parentheses is different. It refers to the pseudo file formed from the files listed
on the command line and accessed via the "<>" operator. Since "<>" isn't explicitly
opened, as a normal filehandle is, an "eof()" before "<>" has been used will cause
@ARGV to be examined to determine if input is available. Similarly, an "eof()" after
"<>" has returned end-of-file will assume you are processing another @ARGV list, and
if you haven't set @ARGV, will read input from "STDIN"; see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each
file, whereas "eof()" will detect the end of the very last file only. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl, because the input
operators typically return "undef" when they run out of data or encounter an error.
eval EXPR
eval BLOCK
eval
"eval" in all its forms is used to execute a little Perl program, trapping any errors
encountered so they don't crash the calling program.
Plain "eval" with no argument is just "eval EXPR", where the expression is understood
to be contained in $_. Thus there are only two real "eval" forms; the one with an
EXPR is often called "string eval". In a string eval, the value of the expression
(which is itself determined within scalar context) is first parsed, and if there were
no errors, executed as a block within the lexical context of the current Perl program.
This form is typically used to delay parsing and subsequent execution of the text of
EXPR until run time. Note that the value is parsed every time the "eval" executes.
The other form is called "block eval". It is less general than string eval, but the
code within the BLOCK is parsed only once (at the same time the code surrounding the
"eval" itself was parsed) and executed within the context of the current Perl program.
This form is typically used to trap exceptions more efficiently than the first, while
also providing the benefit of checking the code within BLOCK at compile time. BLOCK
is parsed and compiled just once. Since errors are trapped, it often is used to check
if a given feature is available.
In both forms, the value returned is the value of the last expression evaluated inside
the mini-program; a return statement may also be used, just as with subroutines. The
expression providing the return value is evaluated in void, scalar, or list context,
depending on the context of the "eval" itself. See "wantarray" for more on how the
evaluation context can be determined.
If there is a syntax error or runtime error, or a "die" statement is executed, "eval"
returns "undef" in scalar context, or an empty list in list context, and $@ is set to
the error message. (Prior to 5.16, a bug caused "undef" to be returned in list
context for syntax errors, but not for runtime errors.) If there was no error, $@ is
set to the empty string. A control flow operator like "last" or "goto" can bypass the
setting of $@. Beware that using "eval" neither silences Perl from printing warnings
to STDERR, nor does it stuff the text of warning messages into $@. To do either of
those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the
BLOCK or EXPR using "nowarnings'all'". See "warn", perlvar, and warnings.
Note that, because "eval" traps otherwise-fatal errors, it is useful for determining
whether a particular feature (such as "socket" or "symlink") is implemented. It is
also Perl's exception-trapping mechanism, where the "die" operator is used to raise
exceptions.
Before Perl 5.14, the assignment to $@ occurred before restoration of localized
variables, which means that for your code to run on older versions, a temporary is
required if you want to mask some, but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}
There are some different considerations for each form:
String eval
Since the return value of EXPR is executed as a block within the lexical context
of the current Perl program, any outer lexical variables are visible to it, and
any package variable settings or subroutine and format definitions remain
afterwards.
Under the "unicode_eval" feature
If this feature is enabled (which is the default under a "use 5.16" or higher
declaration), EXPR is considered to be in the same encoding as the surrounding
program. Thus if "useutf8" is in effect, the string will be treated as being
UTF-8 encoded. Otherwise, the string is considered to be a sequence of
independent bytes. Bytes that correspond to ASCII-range code points will have
their normal meanings for operators in the string. The treatment of the other
bytes depends on if the "'unicode_strings"" feature is in effect.
In a plain "eval" without an EXPR argument, being in "useutf8" or not is
irrelevant; the UTF-8ness of $_ itself determines the behavior.
Any "useutf8" or "noutf8" declarations within the string have no effect, and
source filters are forbidden. ("unicode_strings", however, can appear within
the string.) See also the "evalbytes" operator, which works properly with
source filters.
Variables defined outside the "eval" and used inside it retain their original
UTF-8ness. Everything inside the string follows the normal rules for a Perl
program with the given state of "useutf8".
Outside the "unicode_eval" feature
In this case, the behavior is problematic and is not so easily described.
Here are two bugs that cannot easily be fixed without breaking existing
programs:
o It can lose track of whether something should be encoded as UTF-8 or not.
o Source filters activated within "eval" leak out into whichever file scope
is currently being compiled. To give an example with the CPAN module
Semi::Semicolons:
BEGIN { eval "use Semi::Semicolons; # not filtered" }
# filtered here!
"evalbytes" fixes that to work the way one would expect:
use feature "evalbytes";
BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
# not filtered
Problems can arise if the string expands a scalar containing a floating point
number. That scalar can expand to letters, such as "NaN" or "Infinity"; or,
within the scope of a "use locale", the decimal point character may be something
other than a dot (such as a comma). None of these are likely to parse as you are
likely expecting.
You should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the
variable $x. (Although case 2 has misleading double quotes making the reader
wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave
in the same way: they run the code '$x', which does nothing but return the value
of $x. (Case 4 is preferred for purely visual reasons, but it also has the
advantage of compiling at compile-time instead of at run-time.) Case 5 is a place
where normally you would like to use double quotes, except that in this particular
situation, you can just use symbolic references instead, as in case 6.
An "eval ''" executed within a subroutine defined in the "DB" package doesn't see
the usual surrounding lexical scope, but rather the scope of the first non-DB
piece of code that called it. You don't normally need to worry about this unless
you are writing a Perl debugger.
The final semicolon, if any, may be omitted from the value of EXPR.
Block eval
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap
run-time errors without incurring the penalty of recompiling each time. The
error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
If you want to trap errors when loading an XS module, some problems with the
binary interface (such as Perl version skew) may be fatal even with "eval" unless
$ENV{PERL_DL_NONLAZY} is set. See perlrun.
Using the "eval {}" form as an exception trap in libraries does have some issues.
Due to the current arguably broken state of "__DIE__" hooks, you may wish not to
trigger any "__DIE__" hooks that user code may have installed. You can use the
"local $SIG{__DIE__}" construct for this purpose, as this example shows:
# a private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again,
which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be
fixed in a future release.
"eval BLOCK" does not count as a loop, so the loop control statements "next",
"last", or "redo" cannot be used to leave or restart the block.
The final semicolon, if any, may be omitted from within the BLOCK.
evalbytes EXPR
evalbytes
This function is similar to a string eval, except it always parses its argument (or $_
if EXPR is omitted) as a string of independent bytes.
If called when "useutf8" is in effect, the string will be assumed to be encoded in
UTF-8, and "evalbytes" will make a temporary copy to work from, downgraded to
non-UTF-8. If this is not possible (because one or more characters in it require
UTF-8), the "evalbytes" will fail with the error stored in $@.
Bytes that correspond to ASCII-range code points will have their normal meanings for
operators in the string. The treatment of the other bytes depends on if the
"'unicode_strings"" feature is in effect.
Of course, variables that are UTF-8 and are referred to in the string retain that:
my $a = "\x{100}";
evalbytes 'print ord $a, "\n"';
prints
256
and $@ is empty.
Source filters activated within the evaluated code apply to the code itself.
"evalbytes" is available starting in Perl v5.16. To access it, you must say
"CORE::evalbytes", but you can omit the "CORE::" if the "evalbytes" feature is
enabled. This is enabled automatically with a "use v5.16" (or higher) declaration in
the current scope.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never returns; use "system" instead
of "exec" if you want it to return. It fails and returns false only if the command
does not exist and it is executed directly instead of via your system's command shell
(see below).
Since it's a common mistake to use "exec" instead of "system", Perl warns you if
"exec" is called in void context and if there is a following statement that isn't
"die", "warn", or "exit" (if warnings are enabled--but you always do that, right?).
If you really want to follow an "exec" with some other statement, you can use one of
these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, this calls execvp(3) with the arguments in
LIST. If there is only one element in LIST, the argument is checked for shell
metacharacters, and if there are any, the entire argument is passed to the system's
command shell for parsing (this is "/bin/sh -c" on Unix platforms, but varies on other
platforms). If there are no shell metacharacters in the argument, it is split into
words and passed directly to "execvp", which is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program
you are executing about its own name, you can specify the program you actually want to
run as an "indirect object" (without a comma) in front of the LIST, as in "exec
PROGRAM LIST". (This always forces interpretation of the LIST as a multivalued list,
even if there is only a single scalar in the list.) Example:
my $shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results are subject to its
quirks and capabilities. See "`STRING`" in perlop for details.
Using an indirect object with "exec" or "system" is also more secure. This usage
(which also works fine with "system") forces interpretation of the arguments as a
multivalued list, even if the list had just one argument. That way you're safe from
the shell expanding wildcards or splitting up words with whitespace in them.
my @args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing
it "surprise" an argument. The second version didn't; it tried to run a program named
"echo surprise", didn't find it, and set $? to a non-zero value indicating failure.
On Windows, only the "exec PROGRAM LIST" indirect object syntax will reliably avoid
using the shell; "exec LIST", even with more than one element, will fall back to the
shell if the first spawn fails.
Perl attempts to flush all files opened for output before the exec, but this may not
be supported on some platforms (see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle" on any open
handles to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods
on your objects.
Portability issues: "exec" in perlport.
exists EXPR
Given an expression that specifies an element of a hash, returns true if the specified
element in the hash has ever been initialized, even if the corresponding value is
undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
exists may also be called on array elements, but its behavior is much less obvious and
is strongly tied to the use of "delete" on arrays.
WARNING: Calling "exists" on array values is strongly discouraged. The notion of
deleting or checking the existence of Perl array elements is not conceptually
coherent, and can lead to surprising behavior.
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined and defined only if it
exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the
specified subroutine has ever been declared, even if it is undefined. Mentioning a
subroutine name for exists or defined does not count as declaring it. Note that a
subroutine that does not exist may still be callable: its package may have an
"AUTOLOAD" method that makes it spring into existence the first time that it is
called; see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a
hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the most deeply nested array or hash element will not spring into existence
just because its existence was tested, any intervening ones will. Thus "$ref->{"A"}"
and "$ref->{"A"}->{"B"}" will spring into existence due to the existence test for the
$key element above. This happens anywhere the arrow operator is used, including even
here:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
Use of a subroutine call, rather than a subroutine name, as an argument to "exists" is
an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit
Evaluates EXPR and exits immediately with that value. Example:
my $ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally
recognized values for EXPR are 0 for success and 1 for error; other values are subject
to interpretation depending on the environment in which the Perl program is running.
For example, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will
cause the mailer to return the item undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want
to trap whatever error happened. Use "die" instead, which can be trapped by an
"eval".
The "exit" function does not always exit immediately. It calls any defined "END"
routines first, but these "END" routines may not themselves abort the exit. Likewise
any object destructors that need to be called are called before the real exit. "END"
routines and destructors can change the exit status by modifying $?. If this is a
problem, you can call "POSIX::_exit($status)" to avoid "END" and destructor
processing. See perlmod for details.
Portability issues: "exit" in perlport.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted,
gives "exp($_)".
fc EXPR
fc Returns the casefolded version of EXPR. This is the internal function implementing
the "\F" escape in double-quoted strings.
Casefolding is the process of mapping strings to a form where case differences are
erased; comparing two strings in their casefolded form is effectively a way of asking
if two strings are equal, regardless of case.
Roughly, if you ever found yourself writing this
lc($this) eq lc($that) # Wrong!
# or
uc($this) eq uc($that) # Also wrong!
# or
$this =~ /^\Q$that\E\z/i # Right!
Now you can write
fc($this) eq fc($that)
And get the correct results.
Perl only implements the full form of casefolding, but you can access the simple folds
using "casefold()" in Unicode::UCD and "prop_invmap()" in Unicode::UCD. For further
information on casefolding, refer to the Unicode Standard, specifically sections 3.13
"Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available
at <https://www.unicode.org/versions/latest/>, as well as the Case Charts available at
<https://www.unicode.org/charts/case/>.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as within
"usefeature'unicode_strings", as "lc" does, with the single exception of "fc" of LATIN
CAPITAL LETTER SHARP S (U+1E9E) within the scope of "uselocale". The foldcase of this
character would normally be "ss", but as explained in the "lc" section, case changes
that cross the 255/256 boundary are problematic under locales, and are hence
prohibited. Therefore, this function under locale returns instead the string
"\x{17F}\x{17F}", which is the LATIN SMALL LETTER LONG S. Since that character itself
folds to "s", the string of two of them together should be equivalent to a single
U+1E9E when foldcased.
While the Unicode Standard defines two additional forms of casefolding, one for Turkic
languages and one that never maps one character into multiple characters, these are
not provided by the Perl core. However, the CPAN module "Unicode::Casing" may be used
to provide an implementation.
"fc" is available only if the "fc" feature is enabled or if it is prefixed with
"CORE::". The "fc" feature is enabled automatically with a "use v5.16" (or higher)
declaration in the current scope.
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value returned
work just like "ioctl" below. For example:
use Fcntl;
my $flags = fcntl($filehandle, F_GETFL, 0)
or die "Can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". Like "ioctl", it
maps a 0 return from the system call into "0 but true" in Perl. This string is true
in boolean context and 0 in numeric context. It is also exempt from the normal
"Argument "..." isn't numeric" warnings on improper numeric conversions.
Note that "fcntl" raises an exception if used on a machine that doesn't implement
fcntl(2). See the Fcntl module or your fcntl(2) manpage to learn what functions are
available on your system.
Here's an example of setting a filehandle named $REMOTE to be non-blocking at the
system level. You'll have to negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
my $flags = fcntl($REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
Portability issues: "fcntl" in perlport.
__FILE__
A special token that returns the name of the file in which it occurs. It can be
altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn.
fileno FILEHANDLE
fileno DIRHANDLE
Returns the file descriptor for a filehandle or directory handle, or undefined if the
filehandle is not open. If there is no real file descriptor at the OS level, as can
happen with filehandles connected to memory objects via "open" with a reference for
the third argument, -1 is returned.
This is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-
handling operations. If FILEHANDLE is an expression, the value is taken as an
indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying
descriptor:
if (fileno($this) != -1 && fileno($this) == fileno($that)) {
print "\$this and \$that are dups\n";
} elsif (fileno($this) != -1 && fileno($that) != -1) {
print "\$this and \$that have different " .
"underlying file descriptors\n";
} else {
print "At least one of \$this and \$that does " .
"not have a real file descriptor\n";
}
The behavior of "fileno" on a directory handle depends on the operating system. On a
system with dirfd(3) or similar, "fileno" on a directory handle returns the underlying
file descriptor associated with the handle; on systems with no such support, it
returns the undefined value, and sets $! (errno).
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false
on failure. Produces a fatal error if used on a machine that doesn't implement
flock(2), fcntl(2) locking, or lockf(3). "flock" is Perl's portable file-locking
interface, although it locks entire files only, not records.
Two potentially non-obvious but traditional "flock" semantics are that it waits
indefinitely until the lock is granted, and that its locks are merely advisory. Such
discretionary locks are more flexible, but offer fewer guarantees. This means that
programs that do not also use "flock" may modify files locked with "flock". See
perlport, your port's specific documentation, and your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing portable
programs. (But if you're not, you should as always feel perfectly free to write for
your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB.
These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic
names if you import them from the Fcntl module, either individually, or as a group
using the ":flock" tag. LOCK_SH requests a shared lock, LOCK_EX requests an exclusive
lock, and LOCK_UN releases a previously requested lock. If LOCK_NB is bitwise-or'ed
with LOCK_SH or LOCK_EX, then "flock" returns immediately rather than blocking waiting
for the lock; check the return status to see if you got it.
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before
locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it
requires that FILEHANDLE be open with write intent. These are the semantics that
lockf(3) implements. Most if not all systems implement lockf(3) in terms of fcntl(2)
locking, though, so the differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with
read intent to use LOCK_SH and requires that it be open with write intent to use
LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would
need to use the more system-specific "fcntl" for that. If you like you can force Perl
to ignore your system's flock(2) function, and so provide its own fcntl(2)-based
emulation, by passing the switch "-Ud_flock" to the Configure program when you
configure and build a new Perl.
Here's a mailbox appender for BSD systems.
# import LOCK_* and SEEK_END constants
use Fcntl qw(:flock SEEK_END);
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
# and, in case we're running on a very old UNIX
# variant without the modern O_APPEND semantics...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);
On systems that support a real flock(2), locks are inherited across "fork" calls,
whereas those that must resort to the more capricious fcntl(2) function lose their
locks, making it seriously harder to write servers.
See also DB_File for other "flock" examples.
Portability issues: "flock" in perlport.
fork
Does a fork(2) system call to create a new process running the same program at the
same point. It returns the child pid to the parent process, 0 to the child process,
or "undef" if the fork is unsuccessful. File descriptors (and sometimes locks on
those descriptors) are shared, while everything else is copied. On most systems
supporting fork(2), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the dominant
paradigm for multitasking over the last few decades.
Perl attempts to flush all files opened for output before forking the child process,
but this may not be supported on some platforms (see perlport). To be safe, you may
need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle"
on any open handles to avoid duplicate output.
If you "fork" without ever waiting on your children, you will accumulate zombies. On
some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc
for more examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT
that are actually connected by a pipe or socket, even if you exit, then the remote
server (such as, say, a CGI script or a backgrounded job launched from a remote shell)
won't think you're done. You should reopen those to /dev/null if it's any issue.
On some platforms such as Windows, where the fork(2) system call is not available,
Perl can be built to emulate "fork" in the Perl interpreter. The emulation is
designed, at the level of the Perl program, to be as compatible as possible with the
"Unix" fork(2). However it has limitations that have to be considered in code
intended to be portable. See perlfork for more details.
Portability issues: "fork" in perlport.
format
Declare a picture format for use by the "write" function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may call it, too. It
formats (see perlform) a list of values according to the contents of PICTURE, placing
the output into the format output accumulator, $^A (or $ACCUMULATOR in English).
Eventually, when a "write" is done, the contents of $^A are written to some
filehandle. You could also read $^A and then set $^A back to "". Note that a format
typically does one "formline" per line of form, but the "formline" function itself
doesn't care how many newlines are embedded in the PICTURE. This means that the "~"
and "~~" tokens treat the entire PICTURE as a single line. You may therefore need to
use multiple formlines to implement a single record format, just like the "format"
compiler.
Be careful if you put double quotes around the picture, because an "@" character may
be taken to mean the beginning of an array name. "formline" always returns true. See
perlform for other examples.
If you are trying to use this instead of "write" to capture the output, you may find
it easier to open a filehandle to a scalar ("open my $fh, ">", \$output") and write to
that instead.
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE, or the
undefined value at end of file or if there was an error (in the latter case $! is
set). If FILEHANDLE is omitted, reads from STDIN. This is not particularly
efficient. However, it cannot be used by itself to fetch single characters without
waiting for the user to hit enter. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
my $key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an exercise to the
reader.
The "POSIX::getattr" function can do this more portably on systems purporting POSIX
compliance. See also the "Term::ReadKey" module on CPAN.
getlogin
This implements the C library function of the same name, which on most systems returns
the current login from /etc/utmp, if any. If it returns the empty string, use
"getpwuid".
my $login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
Portability issues: "getlogin" in perlport.
getpeername SOCKET
Returns the packed sockaddr address of the other end of the SOCKET connection.
use Socket;
my $hersockaddr = getpeername($sock);
my ($port, $iaddr) = sockaddr_in($hersockaddr);
my $herhostname = gethostbyaddr($iaddr, AF_INET);
my $herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a PID of 0 to get the
current process group for the current process. Will raise an exception if used on a
machine that doesn't implement getpgrp(2). If PID is omitted, returns the process
group of the current process. Note that the POSIX version of "getpgrp" does not
accept a PID argument, so only "PID==0" is truly portable.
Portability issues: "getpgrp" in perlport.
getppid
Returns the process id of the parent process.
Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX
thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that
used LinuxThreads, this emulation has since been removed. See the documentation for
$$ for details.
Portability issues: "getppid" in perlport.
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a user. (See
getpriority(2).) Will raise a fatal exception if used on a machine that doesn't
implement getpriority(2).
"WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER" imported from
"RESOURCE CONSTANTS" in POSIX.
Portability issues: "getpriority" in perlport.
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines are the same as their counterparts in the system C library. In list
context, the return values from the various get routines are as follows:
# 0 1 2 3 4
my ( $name, $passwd, $gid, $members ) = getgr*
my ( $name, $aliases, $addrtype, $net ) = getnet*
my ( $name, $aliases, $port, $proto ) = getserv*
my ( $name, $aliases, $proto ) = getproto*
my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
my ( $name, $passwd, $uid, $gid, $quota,
$comment, $gcos, $dir, $shell, $expire ) = getpw*
# 5 6 7 8 9
(If the entry doesn't exist, the return value is a single meaningless true value.)
The exact meaning of the $gcos field varies but it usually contains the real name of
the user (as opposed to the login name) and other information pertaining to the user.
Beware, however, that in many system users are able to change this information and
therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The
$passwd and $shell, user's encrypted password and login shell, are also tainted, for
the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in
which case you get the other thing, whatever it is. (If the entry doesn't exist you
get the undefined value.) For example:
my $uid = getpwnam($name);
my $name = getpwuid($num);
my $name = getpwent();
my $gid = getgrnam($name);
my $name = getgrgid($num);
my $name = getgrent();
# etc.
In getpw*() the fields $quota, $comment, and $expire are special in that they are
unsupported on many systems. If the $quota is unsupported, it is an empty scalar. If
it is supported, it usually encodes the disk quota. If the $comment field is
unsupported, it is an empty scalar. If it is supported it usually encodes some
administrative comment about the user. In some systems the $quota field may be
$change or $age, fields that have to do with password aging. In some systems the
$comment field may be $class. The $expire field, if present, encodes the expiration
period of the account or the password. For the availability and the exact meaning of
these fields in your system, please consult getpwnam(3) and your system's pwd.h file.
You can also find out from within Perl what your $quota and $comment fields mean and
whether you have the $expire field by using the "Config" module and the values
"d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and "d_pwexpire". Shadow
password files are supported only if your vendor has implemented them in the intuitive
fashion that calling the regular C library routines gets the shadow versions if you're
running under privilege or if there exists the shadow(3) functions as found in System
V (this includes Solaris and Linux). Those systems that implement a proprietary
shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space-separated list of the login names
of the members of the group.
For the gethost*() functions, if the "h_errno" variable is supported in C, it will be
returned to you via $? if the function call fails. The @addrs value returned by a
successful call is a list of raw addresses returned by the corresponding library call.
In the Internet domain, each address is four bytes long; you can unpack it by saying
something like:
my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
my $iaddr = inet_aton("127.1"); # or whatever address
my $name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
my $straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket;
my $packed_ip = gethostbyname("www.perl.org");
my $ip_address;
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}
Make sure "gethostbyname" is called in SCALAR context and that its return value is
checked for definedness.
The "getprotobynumber" function, even though it only takes one argument, has the
precedence of a list operator, so beware:
getprotobynumber $number eq 'icmp' # WRONG
getprotobynumber($number eq 'icmp') # actually means this
getprotobynumber($number) eq 'icmp' # better this way
If you get tired of remembering which element of the return list contains which return
value, by-name interfaces are provided in standard modules: "File::stat",
"Net::hostent", "Net::netent", "Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These override the normal built-ins, supplying
versions that return objects with the appropriate names for each field. For example:
use File::stat;
use User::pwent;
my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks as though they're the same method calls (uid), they aren't,
because a "File::stat" object is different from a "User::pwent" object.
Many of these functions are not safe in a multi-threaded environment where more than
one thread can be using them. In particular, functions like "getpwent()" iterate per-
process and not per-thread, so if two threads are simultaneously iterating, neither
will get all the records.
Some systems have thread-safe versions of some of the functions, such as
"getpwnam_r()" instead of "getpwnam()". There, Perl automatically and invisibly
substitutes the thread-safe version, without notice. This means that code that safely
runs on some systems can fail on others that lack the thread-safe versions.
Portability issues: "getpwnam" in perlport to "endservent" in perlport.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you
don't know the address because you have several different IPs that the connection
might have come in on.
use Socket;
my $mysockaddr = getsockname($sock);
my ($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a given LEVEL. Options may
exist at multiple protocol levels depending on the socket type, but at least the
uppermost socket level SOL_SOCKET (defined in the "Socket" module) will exist. To
query options at another level the protocol number of the appropriate protocol
controlling the option should be supplied. For example, to indicate that an option is
to be interpreted by the TCP protocol, LEVEL should be set to the protocol number of
TCP, which you can get using "getprotobyname".
The function returns a packed string representing the requested socket option, or
"undef" on error, with the reason for the error placed in $!. Just what is in the
packed string depends on LEVEL and OPTNAME; consult getsockopt(2) for details. A
common case is that the option is an integer, in which case the result is a packed
integer, which you can decode using "unpack" with the "i" (or "I") format.
Here's an example to test whether Nagle's algorithm is enabled on a socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "getsockopt TCP_NODELAY: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ",
$nodelay ? "off\n" : "on\n";
Portability issues: "getsockopt" in perlport.
glob EXPR
glob
In list context, returns a (possibly empty) list of filename expansions on the value
of EXPR such as the standard Unix shell /bin/csh would do. In scalar context, glob
iterates through such filename expansions, returning undef when the list is exhausted.
This is the internal function implementing the "<*.c>" operator, but you can use it
directly. If EXPR is omitted, $_ is used. The "<*.c>" operator is discussed in more
detail in "I/O Operators" in perlop.
Note that "glob" splits its arguments on whitespace and treats each segment as
separate pattern. As such, "glob("*.c *.h")" matches all files with a .c or .h
extension. The expression "glob(".* *")" matches all files in the current working
directory. If you want to glob filenames that might contain whitespace, you'll have
to use extra quotes around the spacey filename to protect it. For example, to glob
filenames that have an "e" followed by a space followed by an "f", use one of:
my @spacies = <"*e f*">;
my @spacies = glob '"*e f*"';
my @spacies = glob q("*e f*");
If you had to get a variable through, you could do this:
my @spacies = glob "'*${var}e f*'";
my @spacies = glob qq("*${var}e f*");
If non-empty braces are the only wildcard characters used in the "glob", no filenames
are matched, but potentially many strings are returned. For example, this produces
nine strings, one for each pairing of fruits and colors:
my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
This operator is implemented using the standard "File::Glob" extension. See
File::Glob for details, including "bsd_glob", which does not treat whitespace as a
pattern separator.
If a "glob" expression is used as the condition of a "while" or "for" loop, then it
will be implicitly assigned to $_. If either a "glob" expression or an explicit
assignment of a "glob" expression to a scalar is used as a "while"/"for" condition,
then the condition actually tests for definedness of the expression's value, not for
its regular truth value.
Portability issues: "glob" in perlport.
gmtime EXPR
gmtime
Works just like "localtime", but the returned values are localized for the standard
Greenwich time zone.
Note: When called in list context, $isdst, the last value returned by gmtime, is
always 0. There is no Daylight Saving Time in GMT.
Portability issues: "gmtime" in perlport.
goto LABEL
goto EXPR
goto &NAME
The "goto LABEL" form finds the statement labeled with LABEL and resumes execution
there. It can't be used to get out of a block or subroutine given to "sort". It can
be used to go almost anywhere else within the dynamic scope, including out of
subroutines, but it's usually better to use some other construct such as "last" or
"die". The author of Perl has never felt the need to use this form of "goto" (in
Perl, that is; C is another matter). (The difference is that C does not offer named
loops combined with loop control. Perl does, and this replaces most structured uses
of "goto" in other languages.)
The "goto EXPR" form expects to evaluate "EXPR" to a code reference or a label name.
If it evaluates to a code reference, it will be handled like "goto &NAME", below.
This is especially useful for implementing tail recursion via "goto __SUB__".
If the expression evaluates to a label name, its scope will be resolved dynamically.
This allows for computed "goto"s per FORTRAN, but isn't necessarily recommended if
you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
As shown in this example, "goto EXPR" is exempt from the "looks like a function" rule.
A pair of parentheses following it does not (necessarily) delimit its argument.
"goto("NE")."XT"" is equivalent to "goto NEXT". Also, unlike most named operators,
this has the same precedence as assignment.
Use of "goto LABEL" or "goto EXPR" to jump into a construct is deprecated and will
issue a warning. Even then, it may not be used to go into any construct that requires
initialization, such as a subroutine, a "foreach" loop, or a "given" block. In
general, it may not be used to jump into the parameter of a binary or list operator,
but it may be used to jump into the first parameter of a binary operator. (The "="
assignment operator's "first" operand is its right-hand operand.) It also can't be
used to go into a construct that is optimized away.
The "goto &NAME" form is quite different from the other forms of "goto". In fact, it
isn't a goto in the normal sense at all, and doesn't have the stigma associated with
other gotos. Instead, it exits the current subroutine (losing any changes set by
"local") and immediately calls in its place the named subroutine using the current
value of @_. This is used by "AUTOLOAD" subroutines that wish to load another
subroutine and then pretend that the other subroutine had been called in the first
place (except that any modifications to @_ in the current subroutine are propagated to
the other subroutine.) After the "goto", not even "caller" will be able to tell that
this routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a
code reference or a block that evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In
particular, it is not limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
element) and returns the list value consisting of those elements for which the
expression evaluated to true. In scalar context, returns the number of times the
expression was true.
my @foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
my @foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to modify the elements
of the LIST. While this is useful and supported, it can cause bizarre results if the
elements of LIST are not variables. Similarly, grep returns aliases into the original
list, much as a for loop's index variable aliases the list elements. That is,
modifying an element of a list returned by grep (for example, in a "foreach", "map" or
another "grep") actually modifies the element in the original list. This is usually
something to be avoided when writing clear code.
See also "map" for a list composed of the results of the BLOCK or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding numeric value. If EXPR
is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
$valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
A hex string consists of hex digits and an optional "0x" or "x" prefix. Each hex
digit may be preceded by a single underscore, which will be ignored. Any other
character triggers a warning and causes the rest of the string to be ignored (even
leading whitespace, unlike "oct"). Only integers can be represented, and integer
overflow triggers a warning.
To convert strings that might start with any of 0, "0x", or "0b", see "oct". To
present something as hex, look into "printf", "sprintf", and "unpack".
import LIST
There is no builtin "import" function. It is just an ordinary method (subroutine)
defined (or inherited) by modules that wish to export names to another module. The
"use" function calls the "import" method for the package used. See also "use",
perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within another, but without the wildcard-
like behavior of a full regular-expression pattern match. It returns the position of
the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted,
starts searching from the beginning of the string. POSITION before the beginning of
the string or after its end is treated as if it were the beginning or the end,
respectively. POSITION and the return value are based at zero. If the substring is
not found, "index" returns -1.
Find characters or strings:
index("Perl is great", "P"); # Returns 0
index("Perl is great", "g"); # Returns 8
index("Perl is great", "great"); # Also returns 8
Attempting to find something not there:
index("Perl is great", "Z"); # Returns -1 (not found)
Using an offset to find the second occurrence:
index("Perl is great", "e", 5); # Returns 10
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use
this function for rounding: one because it truncates towards 0, and two because
machine representations of floating-point numbers can sometimes produce
counterintuitive results. For example, "int(-6.725/0.025)" produces -268 rather than
the correct -269; that's because it's really more like -268.99999999999994315658
instead. Usually, the "sprintf", "printf", or the "POSIX::floor" and "POSIX::ceil"
functions will serve you better than will "int".
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require "sys/ioctl.ph"; # probably in
# $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't
have the correct definitions you'll have to roll your own, based on your C header
files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the
Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or
written depending on the FUNCTION; a C pointer to the string value of SCALAR will be
passed as the third argument of the actual "ioctl" call. (If SCALAR has no string
value but does have a numeric value, that value will be passed rather than a pointer
to the string value. To guarantee this to be true, add a 0 to the scalar before using
it.) The "pack" and "unpack" functions may be needed to manipulate the values of
structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily
determine the actual value returned by the operating system:
my $retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from "Argument "..." isn't numeric" warnings
on improper numeric conversions.
Portability issues: "ioctl" in perlport.
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the
value of EXPR, and returns that new string. Example:
my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its first argument.
Compare "split".
keys HASH
keys ARRAY
Called in list context, returns a list consisting of all the keys of the named hash,
or in Perl 5.12 or later only, the indices of an array. Perl releases prior to 5.12
will produce a syntax error if you try to use an array argument. In scalar context,
returns the number of keys or indices.
Hash entries are returned in an apparently random order. The actual random order is
specific to a given hash; the exact same series of operations on two hashes may result
in a different order for each hash. Any insertion into the hash may change the order,
as will any deletion, with the exception that the most recent key returned by "each"
or "keys" may be deleted without changing the order. So long as a given hash is
unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on
why hash order is randomized. Aside from the guarantees provided here the exact
details of Perl's hash algorithm and the hash traversal order are subject to change in
any release of Perl. Tied hashes may behave differently to Perl's hashes with respect
to changes in order on insertion and deletion of items.
As a side effect, calling "keys" resets the internal iterator of the HASH or ARRAY
(see "each") before yielding the keys. In particular, calling "keys" in void context
resets the iterator with no other overhead.
Here is yet another way to print your environment:
my @keys = keys %ENV;
my @values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach my $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the hash, so modifying them
will not affect the original hash. Compare "values".
To sort a hash by value, you'll need to use a "sort" function. Here's a descending
numeric sort of a hash by its values:
foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
Used as an lvalue, "keys" allows you to increase the number of hash buckets allocated
for the given hash. This can gain you a measure of efficiency if you know the hash is
going to get big. (This is similar to pre-extending an array by assigning a larger
number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them, in fact,
since it rounds up to the next power of two. These buckets will be retained even if
you do "%hash = ()", use "undef %hash" if you want to free the storage while %hash is
still in scope. You can't shrink the number of buckets allocated for the hash using
"keys" in this way (but you needn't worry about doing this by accident, as trying has
no effect). "keys @array" in an lvalue context is a syntax error.
Starting with Perl 5.14, an experimental feature allowed "keys" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
To avoid confusing would-be users of your code who are running earlier versions of
Perl with mysterious syntax errors, put this sort of thing at the top of your file to
signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays
See also "each", "values", and "sort".
kill SIGNAL, LIST
kill SIGNAL
Sends a signal to a list of processes. Returns the number of arguments that were
successfully used to signal (which is not necessarily the same as the number of
processes actually killed, e.g. where a process group is killed).
my $cnt = kill 'HUP', $child1, $child2;
kill 'KILL', @goners;
SIGNAL may be either a signal name (a string) or a signal number. A signal name may
start with a "SIG" prefix, thus "FOO" and "SIGFOO" refer to the same signal. The
string form of SIGNAL is recommended for portability because the same signal may have
different numbers in different operating systems.
A list of signal names supported by the current platform can be found in
$Config{sig_name}, which is provided by the "Config" module. See Config for more
details.
A negative signal name is the same as a negative signal number, killing process groups
instead of processes. For example, "kill '-KILL', $pgrp" and "kill -9, $pgrp" will
send "SIGKILL" to the entire process group specified. That means you usually want to
use positive not negative signals.
If SIGNAL is either the number 0 or the string "ZERO" (or "SIGZERO"), no signal is
sent to the process, but "kill" checks whether it's possible to send a signal to it
(that means, to be brief, that the process is owned by the same user, or we are the
super-user). This is useful to check that a child process is still alive (even if
only as a zombie) and hasn't changed its UID. See perlport for notes on the
portability of this construct.
The behavior of kill when a PROCESS number is zero or negative depends on the
operating system. For example, on POSIX-conforming systems, zero will signal the
current process group, -1 will signal all processes, and any other negative PROCESS
number will act as a negative signal number and kill the entire process group
specified.
If both the SIGNAL and the PROCESS are negative, the results are undefined. A warning
may be produced in a future version.
See "Signals" in perlipc for more details.
On some platforms such as Windows where the fork(2) system call is not available, Perl
can be built to emulate "fork" at the interpreter level. This emulation has
limitations related to kill that have to be considered, for code running on Windows
and in code intended to be portable.
See perlfork for more details.
If there is no LIST of processes, no signal is sent, and the return value is 0. This
form is sometimes used, however, because it causes tainting checks to be run. But see
"Laundering and Detecting Tainted Data" in perlsec.
Portability issues: "kill" in perlport.
last LABEL
last EXPR
last
The "last" command is like the "break" statement in C (as used in loops); it
immediately exits the loop in question. If the LABEL is omitted, the command refers
to the innermost enclosing loop. The "last EXPR" form, available starting in Perl
5.18.0, allows a label name to be computed at run time, and is otherwise identical to
"last LABEL". The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot return a value from a block that typically returns a value, such as
"eval {}", "sub {}", or "do {}". It will perform its flow control behavior, which
precludes any return value. It should not be used to exit a "grep" or "map" operation.
Note that a block by itself is semantically identical to a loop that executes once.
Thus "last" can be used to effect an early exit out of such a block.
See also "continue" for an illustration of how "last", "next", and "redo" work.
Unlike most named operators, this has the same precedence as assignment. It is also
exempt from the looks-like-a-function rule, so "last ("foo")."bar"" will cause "bar"
to be part of the argument to "last".
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal function implementing the
"\L" escape in double-quoted strings.
If EXPR is omitted, uses $_.
What gets returned depends on several factors:
If "use bytes" is in effect:
The results follow ASCII rules. Only the characters "A-Z" change, to "a-z"
respectively.
Otherwise, if "use locale" for "LC_CTYPE" is in effect:
Respects current "LC_CTYPE" locale for code points < 256; and uses Unicode rules
for the remaining code points (this last can only happen if the UTF8 flag is also
set). See perllocale.
Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8.
Otherwise, there is a deficiency in this scheme, which is that case changes that
cross the 255/256 boundary are not well-defined. For example, the lower case of
LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII
platforms). But under "use locale" (prior to v5.20 or not a UTF-8 locale), the
lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S
in the current locale, and Perl has no way of knowing if that character even
exists in the locale, much less what code point it is. Perl returns a result that
is above 255 (almost always the input character unchanged), for all instances (and
there aren't many) where the 255/256 boundary would otherwise be crossed; and
starting in v5.22, it raises a locale warning.
Otherwise, If EXPR has the UTF8 flag set:
Unicode rules are used for the case change.
Otherwise, if "use feature 'unicode_strings'" or "use locale ':not_characters'" is in
effect:
Unicode rules are used for the case change.
Otherwise:
ASCII rules are used for the case change. The lowercase of any character outside
the ASCII range is the character itself.
lcfirst EXPR
lcfirst
Returns the value of EXPR with the first character lowercased. This is the internal
function implementing the "\l" escape in double-quoted strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as in a locale, as "lc"
does.
length EXPR
length
Returns the length in characters of the value of EXPR. If EXPR is omitted, returns
the length of $_. If EXPR is undefined, returns "undef".
This function cannot be used on an entire array or hash to find out how many elements
these have. For that, use "scalar @array" and "scalar keys %hash", respectively.
Like all Perl character operations, "length" normally deals in logical characters, not
physical bytes. For how many bytes a string encoded as UTF-8 would take up, use
"length(Encode::encode('UTF-8', EXPR))" (you'll have to "use Encode" first). See
Encode and perlunicode.
__LINE__
A special token that compiles to the current line number. It can be altered by the
mechanism described at "Plain Old Comments (Not!)" in perlsyn.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false
otherwise.
Portability issues: "link" in perlport.
listen SOCKET,QUEUESIZE
Does the same thing that the listen(2) system call does. Returns true if it
succeeded, false otherwise. See the example in "Sockets: Client/Server Communication"
in perlipc.
local EXPR
You really probably want to be using "my" instead, because "local" isn't what most
people think of as "local". See "Private Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or
eval. If more than one value is listed, the list must be placed in parentheses. See
"Temporary Values via local()" in perlsub for details, including issues with tied
arrays and hashes.
The "delete local EXPR" construct can also be used to localize the deletion of
array/hash elements to the current block. See "Localized deletion of elements of
composite types" in perlsub.
localtime EXPR
localtime
Converts a time as returned by the time function to a 9-element list with the time
analyzed for the local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric and come straight out of the C `struct tm'. $sec, $min,
and $hour are the seconds, minutes, and hours of the specified time.
$mday is the day of the month and $mon the month in the range 0..11, with 0 indicating
January and 11 indicating December. This makes it easy to get a month name from a
list:
my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year contains the number of years since 1900. To get a 4-digit year write:
$year += 1900;
To get the last two digits of the year (e.g., "01" in 2001) do:
$year = sprintf("%02d", $year % 100);
$wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday.
$yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)
$isdst is true if the specified time occurs when Daylight Saving Time is in effect,
false otherwise.
If EXPR is omitted, "localtime" uses the current time (as returned by "time").
In scalar context, "localtime" returns the ctime(3) value:
my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is always in English, and is not locale-dependent. To get similar
but locale-dependent date strings, try for example:
use POSIX qw(strftime);
my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
C$now_string> will be formatted according to the current LC_TIME locale the program or
thread is running in. See perllocale for how to set up and change that locale. Note
that %a and %b, the short forms of the day of the week and the month of the year, may
not necessarily be three characters wide.
The Time::gmtime and Time::localtime modules provide a convenient, by-name access
mechanism to the "gmtime" and "localtime" functions, respectively.
For a comprehensive date and time representation look at the DateTime module on CPAN.
For GMT instead of local time use the "gmtime" builtin.
See also the "Time::Local" module (for converting seconds, minutes, hours, and such
back to the integer value returned by "time"), and the POSIX module's "mktime"
function.
Portability issues: "localtime" in perlport.
lock THING
This function places an advisory lock on a shared variable or referenced object
contained in THING until the lock goes out of scope.
The value returned is the scalar itself, if the argument is a scalar, or a reference,
if the argument is a hash, array or subroutine.
"lock" is a "weak keyword"; this means that if you've defined a function by this name
(before any calls to it), that function will be called instead. If you are not under
"use threads::shared" this does nothing. See threads::shared.
log EXPR
log Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns the log
of $_. To get the log of another base, use basic algebra: The base-N log of a number
is equal to the natural log of that number divided by the natural log of N. For
example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat FILEHANDLE
lstat EXPR
lstat DIRHANDLE
lstat
Does the same thing as the "stat" function (including setting the special "_"
filehandle) but stats a symbolic link instead of the file the symbolic link points to.
If symbolic links are unimplemented on your system, a normal "stat" is done. For much
more detailed information, please see the documentation for "stat".
If EXPR is omitted, stats $_.
Portability issues: "lstat" in perlport.
m// The match operator. See "Regexp Quote-Like Operators" in perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
element) and composes a list of the results of each such evaluation. Each element of
LIST may produce zero, one, or more elements in the generated list, so the number of
elements in the generated list may differ from that in LIST. In scalar context,
returns the total number of elements so generated. In list context, returns the
generated list.
my @chars = map(chr, @numbers);
translates a list of numbers to the corresponding characters.
my @squares = map { $_ * $_ } @numbers;
translates a list of numbers to their squared values.
my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
shows that number of returned elements can differ from the number of input elements.
To omit an element, return an empty list (). This could also be achieved by writing
my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
which makes the intention more clear.
Map always returns a list, which can be assigned to a hash such that the elements
become key/value pairs. See perldata for more details.
my %hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
my %hash;
foreach (@array) {
$hash{get_a_key_for($_)} = $_;
}
Note that $_ is an alias to the list value, so it can be used to modify the elements
of the LIST. While this is useful and supported, it can cause bizarre results if the
elements of LIST are not variables. Using a regular "foreach" loop for this purpose
would be clearer in most cases. See also "grep" for a list composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.
"{" starts both hash references and blocks, so "map { ..." could be either the start
of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing
"}" it has to take a guess at which it's dealing with based on what it finds just
after the "{". Usually it gets it right, but if it doesn't it won't realize something
is wrong until it gets to the "}" and encounters the missing (or unexpected) comma.
The syntax error will be reported close to the "}", but you'll need to change
something near the "{" such as using a unary "+" or semicolon to give Perl some help:
my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
my %hash = map {; "\L$_" => 1 } @array # this also works
my %hash = map { ("\L$_" => 1) } @array # as does this
my %hash = map { lc($_) => 1 } @array # and this.
my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
# comma at end
to get a list of anonymous hashes each with only one entry apiece.
mkdir FILENAME,MODE
mkdir FILENAME
mkdir
Creates the directory specified by FILENAME, with permissions specified by MODE (as
modified by "umask"). If it succeeds it returns true; otherwise it returns false and
sets $! (errno). MODE defaults to 0777 if omitted, and FILENAME defaults to $_ if
omitted.
In general, it is better to create directories with a permissive MODE and let the user
modify that with their "umask" than it is to supply a restrictive MODE and give the
user no way to be more permissive. The exceptions to this rule are when the file or
directory should be kept private (mail files, for instance). The documentation for
"umask" discusses the choice of MODE in more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of
trailing slashes. Some operating and filesystems do not get this right, so Perl
automatically removes all trailing slashes to keep everyone happy.
To recursively create a directory structure, look at the "make_path" function of the
File::Path module.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be
a variable that will hold the returned "msqid_ds" structure. Returns like "ioctl":
the undefined value for error, "0 but true" for zero, or the actual return value
otherwise. See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
"IPC::Semaphore".
Portability issues: "msgctl" in perlport.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the message queue id, or "undef"
on error. See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
"IPC::Msg".
Portability issues: "msgget" in perlport.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from message queue ID into
variable VAR with a maximum message size of SIZE. Note that when a message is
received, the message type as a native long integer will be the first thing in VAR,
followed by the actual message. This packing may be opened with "unpack("l! a*")".
Taints the variable. Returns true if successful, false on error. See also "SysV IPC"
in perlipc and the documentation for "IPC::SysV" and "IPC::Msg".
Portability issues: "msgrcv" in perlport.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the message queue
ID. MSG must begin with the native long integer message type, followed by the message
itself. This kind of packing can be achieved with "pack("l! a*", $type, $message)".
Returns true if successful, false on error. See also "SysV IPC" in perlipc and the
documentation for "IPC::SysV" and "IPC::Msg".
Portability issues: "msgsnd" in perlport.
my VARLIST
my TYPE VARLIST
my VARLIST : ATTRS
my TYPE VARLIST : ATTRS
A "my" declares the listed variables to be local (lexically) to the enclosing block,
file, or "eval". If more than one variable is listed, the list must be placed in
parentheses.
Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for
example to skip assignment of initial values:
my ( undef, $min, $hour ) = localtime;
Redeclaring a variable in the same scope or statement will "shadow" the previous
declaration, creating a new instance and preventing access to the previous one. This
is usually undesired and, if warnings are enabled, will result in a warning in the
"shadow" category.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE may be a
bareword, a constant declared with "use constant", or "__PACKAGE__". It is currently
bound to the use of the fields pragma, and attributes are handled using the attributes
pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module. See
"Private Variables via my()" in perlsub for details.
next LABEL
next EXPR
next
The "next" command is like the "continue" statement in C; it starts the next iteration
of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the above, it would get executed even on
discarded lines. If LABEL is omitted, the command refers to the innermost enclosing
loop. The "next EXPR" form, available as of Perl 5.18.0, allows a label name to be
computed at run time, being otherwise identical to "next LABEL".
"next" cannot return a value from a block that typically returns a value, such as
"eval {}", "sub {}", or "do {}". It will perform its flow control behavior, which
precludes any return value. It should not be used to exit a "grep" or "map" operation.
Note that a block by itself is semantically identical to a loop that executes once.
Thus "next" will exit such a block early.
See also "continue" for an illustration of how "last", "next", and "redo" work.
Unlike most named operators, this has the same precedence as assignment. It is also
exempt from the looks-like-a-function rule, so "next ("foo")."bar"" will cause "bar"
to be part of the argument to "next".
no MODULE VERSION LIST
no MODULE VERSION
no MODULE LIST
no MODULE
no VERSION
See the "use" function, of which "no" is the opposite.
oct EXPR
oct Interprets EXPR as an octal string and returns the corresponding value. An octal
string consists of octal digits and, as of Perl 5.33.5, an optional "0o" or "o"
prefix. Each octal digit may be preceded by a single underscore, which will be
ignored. (If EXPR happens to start off with "0x" or "x", interprets it as a hex
string. If EXPR starts off with "0b" or "b", it is interpreted as a binary string.
Leading whitespace is ignored in all three cases.) The following will handle decimal,
binary, octal, and hex in standard Perl notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use
"sprintf" or "printf":
my $dec_perms = (stat("filename"))[2] & 07777;
my $oct_perm_str = sprintf "%o", $perms;
The "oct" function is commonly used when a string such as 644 needs to be converted
into a file mode, for example. Although Perl automatically converts strings into
numbers as needed, this automatic conversion assumes base 10.
Leading white space is ignored without warning, as too are any trailing non-digits,
such as a decimal point ("oct" only handles non-negative integers, not negative
integers or floating point).
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE,EXPR
open FILEHANDLE
Associates an internal FILEHANDLE with the external file specified by EXPR. That
filehandle will subsequently allow you to perform I/O operations on that file, such as
reading from it or writing to it.
Instead of a filename, you may specify an external command (plus an optional argument
list) or a scalar reference, in order to open filehandles on commands or in-memory
scalars, respectively.
A thorough reference to "open" follows. For a gentler introduction to the basics of
"open", see also the perlopentut manual page.
Working with files
Most often, "open" gets invoked with three arguments: the required FILEHANDLE
(usually an empty scalar variable), followed by MODE (usually a literal describing
the I/O mode the filehandle will use), and then the filename that the new
filehandle will refer to.
Simple examples
Reading from a file:
open(my $fh, "<", "input.txt")
or die "Can't open < input.txt: $!";
# Process every line in input.txt
while (my $line = <$fh>) {
#
# ... do something interesting with $line here ...
#
}
or writing to one:
open(my $fh, ">", "output.txt")
or die "Can't open > output.txt: $!";
print $fh "This line gets printed into output.txt.\n";
For a summary of common filehandle operations such as these, see "Files and
I/O" in perlintro.
About filehandles
The first argument to "open", labeled FILEHANDLE in this reference, is usually
a scalar variable. (Exceptions exist, described in "Other considerations",
below.) If the call to "open" succeeds, then the expression provided as
FILEHANDLE will get assigned an open filehandle. That filehandle provides an
internal reference to the specified external file, conveniently stored in a
Perl variable, and ready for I/O operations such as reading and writing.
About modes
When calling "open" with three or more arguments, the second argument --
labeled MODE here -- defines the open mode. MODE is usually a literal string
comprising special characters that define the intended I/O role of the
filehandle being created: whether it's read-only, or read-and-write, and so
on.
If MODE is "<", the file is opened for input (read-only). If MODE is ">", the
file is opened for output, with existing files first being truncated
("clobbered") and nonexisting files newly created. If MODE is ">>", the file
is opened for appending, again being created if necessary.
You can put a "+" in front of the ">" or "<" to indicate that you want both
read and write access to the file; thus "+<" is almost always preferred for
read/write updates--the "+>" mode would clobber the file first. You can't
usually use either read-write mode for updating textfiles, since they have
variable-length records. See the -i switch in perlrun for a better approach.
The file is created with permissions of 0666 modified by the process's "umask"
value.
These various prefixes correspond to the fopen(3) modes of "r", "r+", "w",
"w+", "a", and "a+".
More examples of different modes in action:
# Open a file for concatenation
open(my $log, ">>", "/usr/spool/news/twitlog")
or warn "Couldn't open log file; discarding input";
# Open a file for reading and writing
open(my $dbase, "+<", "dbase.mine")
or die "Can't open 'dbase.mine' for update: $!";
Checking the return value
Open returns nonzero on success, the undefined value otherwise. If the "open"
involved a pipe, the return value happens to be the pid of the subprocess.
When opening a file, it's seldom a good idea to continue if the request
failed, so "open" is frequently used with "die". Even if you want your code to
do something other than "die" on a failed open, you should still always check
the return value from opening a file.
Specifying I/O layers in MODE
You can use the three-argument form of open to specify I/O layers (sometimes
referred to as "disciplines") to apply to the new filehandle. These affect how the
input and output are processed (see open and PerlIO for more details). For
example:
open(my $fh, "<:encoding(UTF-8)", $filename)
|| die "Can't open UTF-8 encoded $filename: $!";
This opens the UTF8-encoded file containing Unicode characters; see perluniintro.
Note that if layers are specified in the three-argument form, then default layers
stored in "${^OPEN}" (usually set by the open pragma or the switch "-CioD") are
ignored. Those layers will also be ignored if you specify a colon with no name
following it. In that case the default layer for the operating system (:raw on
Unix, :crlf on Windows) is used.
On some systems (in general, DOS- and Windows-based systems) "binmode" is
necessary when you're not working with a text file. For the sake of portability
it is a good idea always to use it when appropriate, and never to use it when it
isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded
Unicode, not bytes.
Using "undef" for temporary files
As a special case the three-argument form with a read/write mode and the third
argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to a newly created empty anonymous temporary file. (This
happens under any mode, which makes "+>" the only useful and sensible mode to
use.) You will need to "seek" to do the reading.
Opening a filehandle into an in-memory scalar
You can open filehandles directly to Perl scalars instead of a file or other
resource external to the program. To do so, provide a reference to that scalar as
the third argument to "open", like so:
open(my $memory, ">", \$var)
or die "Can't open memory file: $!";
print $memory "foo!\n"; # output will appear in $var
To (re)open "STDOUT" or "STDERR" as an in-memory file, close it first:
close STDOUT;
open(STDOUT, ">", \$variable)
or die "Can't open STDOUT: $!";
The scalars for in-memory files are treated as octet strings: unless the file is
being opened with truncation the scalar may not contain any code points over 0xFF.
Opening in-memory files can fail for a variety of reasons. As with any other
"open", check the return value for success.
Technical note: This feature works only when Perl is built with PerlIO -- the
default, except with older (pre-5.16) Perl installations that were configured to
not include it (e.g. via "Configure -Uuseperlio"). You can see whether your Perl
was built with PerlIO by running "perl -V:useperlio". If it says 'define', you
have PerlIO; otherwise you don't.
See perliol for detailed info on PerlIO.
Opening a filehandle into a command
If MODE is "|-", then the filename is interpreted as a command to which output is
to be piped, and if MODE is "-|", the filename is interpreted as a command that
pipes output to us. In the two-argument (and one-argument) form, one should
replace dash ("-") with the command. See "Using open() for IPC" in perlipc for
more examples of this. (You are not allowed to "open" to a command that pipes
both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication
with Another Process" in perlipc for alternatives.)
open(my $article_fh, "-|", "caesar <$article") # decrypt
# article
or die "Can't start caesar: $!";
open(my $article_fh, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
In the form of pipe opens taking three or more arguments, if LIST is specified
(extra arguments after the command name) then LIST becomes arguments to the
command invoked if the platform supports it. The meaning of "open" with more than
three arguments for non-pipe modes is not yet defined, but experimental "layers"
may give extra LIST arguments meaning.
If you open a pipe on the command "-" (that is, specify either "|-" or "-|" with
the one- or two-argument forms of "open"), an implicit "fork" is done, so "open"
returns twice: in the parent process it returns the pid of the child process, and
in the child process it returns (a defined) 0. Use "defined($pid)" or "//" to
determine whether the open was successful.
For example, use either
my $child_pid = open(my $from_kid, "-|")
// die "Can't fork: $!";
or
my $child_pid = open(my $to_kid, "|-")
// die "Can't fork: $!";
followed by
if ($child_pid) {
# am the parent:
# either write $to_kid or else read $from_kid
...
waitpid $child_pid, 0;
} else {
# am the child; use STDIN/STDOUT normally
...
exit;
}
The filehandle behaves normally for the parent, but I/O to that filehandle is
piped from/to the STDOUT/STDIN of the child process. In the child process, the
filehandle isn't opened--I/O happens from/to the new STDOUT/STDIN. Typically this
is used like the normal piped open when you want to exercise more control over
just how the pipe command gets executed, such as when running setuid and you don't
want to have to scan shell commands for metacharacters.
The following blocks are more or less equivalent:
open(my $fh, "|tr '[a-z]' '[A-Z]'");
open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
open(my $fh, "cat -n '$file'|");
open(my $fh, "-|", "cat -n '$file'");
open(my $fh, "-|") || exec "cat", "-n", $file;
open(my $fh, "-|", "cat", "-n", $file);
The last two examples in each block show the pipe as "list form", which is not yet
supported on all platforms. (If your platform has a real "fork", such as Linux and
macOS, you can use the list form; it also works on Windows with Perl 5.22 or
later.) You would want to use the list form of the pipe so you can pass literal
arguments to the command without risk of the shell interpreting any shell
metacharacters in them. However, this also bars you from opening pipes to commands
that intentionally contain shell metacharacters, such as:
open(my $fh, "|cat -n | expand -4 | lpr")
|| die "Can't open pipeline to lpr: $!";
See "Safe Pipe Opens" in perlipc for more examples of this.
Duping filehandles
You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&",
in which case the rest of the string is interpreted as the name of a filehandle
(or file descriptor, if numeric) to be duped (as in dup(2)) and opened. You may
use "&" after ">", ">>", "<", "+>", "+>>", and "+<". The mode you specify should
match the mode of the original filehandle. (Duping a filehandle does not take
into account any existing contents of IO buffers.) If you use the three-argument
form, then you can pass either a number, the name of a filehandle, or the normal
"reference to a glob".
Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using
various methods:
#!/usr/bin/perl
open(my $oldout, ">&STDOUT")
or die "Can't dup STDOUT: $!";
open(OLDERR, ">&", \*STDERR)
or die "Can't dup STDERR: $!";
open(STDOUT, '>', "foo.out")
or die "Can't redirect STDOUT: $!";
open(STDERR, ">&STDOUT")
or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open(STDOUT, ">&", $oldout)
or die "Can't dup \$oldout: $!";
open(STDERR, ">&OLDERR")
or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descriptor number or a filehandle, then
Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call
dup(2)); this is more parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(my $fh, "<&=", $fd)
or
open(my $fh, "<&=$fd")
or
# open for append, using the fileno of $oldfh
open(my $fh, ">>&=", $oldfh)
Being parsimonious on filehandles is also useful (besides being parsimonious) for
example when something is dependent on file descriptors, like for example locking
using "flock". If you do just "open(my $A, ">>&", $B)", the filehandle $A will
not have the same file descriptor as $B, and therefore "flock($A)" will not
"flock($B)" nor vice versa. But with "open(my $A, ">>&=", $B)", the filehandles
will share the same underlying system file descriptor.
Note that under Perls older than 5.8.0, Perl uses the standard C library's'
fdopen(3) to implement the "=" functionality. On many Unix systems, fdopen(3)
fails when file descriptors exceed a certain value, typically 255. For Perls
5.8.0 and later, PerlIO is (most often) the default.
Legacy usage
This section describes ways to call "open" outside of best practices; you may
encounter these uses in older code. Perl does not consider their use deprecated,
exactly, but neither is it recommended in new code, for the sake of clarity and
readability.
Specifying mode and filename as a single argument
In the one- and two-argument forms of the call, the mode and filename should
be concatenated (in that order), preferably separated by white space. You
can--but shouldn't--omit the mode in these forms when that mode is "<". It is
safe to use the two-argument form of "open" if the filename argument is a
known literal.
open(my $dbase, "+<dbase.mine") # ditto
or die "Can't open 'dbase.mine' for update: $!";
In the two-argument (and one-argument) form, opening "<-" or "-" opens STDIN
and opening ">-" opens STDOUT.
New code should favor the three-argument form of "open" over this older form.
Declaring the mode and the filename as two distinct arguments avoids any
confusion between the two.
Calling "open" with one argument via global variables
As a shortcut, a one-argument call takes the filename from the global scalar
variable of the same name as the filehandle:
$ARTICLE = 100;
open(ARTICLE)
or die "Can't find article $ARTICLE: $!\n";
Here $ARTICLE must be a global (package) scalar variable - not one declared
with "my" or "state".
Assigning a filehandle to a bareword
An older style is to use a bareword as the filehandle, as
open(FH, "<", "input.txt")
or die "Can't open < input.txt: $!";
Then you can use "FH" as the filehandle, in "close FH" and "<FH>" and so on.
Note that it's a global variable, so this form is not recommended when dealing
with filehandles other than Perl's built-in ones (e.g. STDOUT and STDIN).
Other considerations
Automatic filehandle closure
The filehandle will be closed when its reference count reaches zero. If it is
a lexically scoped variable declared with "my", that usually means the end of
the enclosing scope. However, this automatic close does not check for errors,
so it is better to explicitly close filehandles, especially those used for
writing:
close($handle)
|| warn "close failed: $!";
Automatic pipe flushing
Perl will attempt to flush all files opened for output before any operation
that may do a fork, but this may not be supported on some platforms (see
perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush" method of "IO::Handle" on any open handles.
On systems that support a close-on-exec flag on files, the flag will be set
for the newly opened file descriptor as determined by the value of $^F. See
"$^F" in perlvar.
Closing any piped filehandle causes the parent process to wait for the child
to finish, then returns the status value in $? and "${^CHILD_ERROR_NATIVE}".
Direct versus by-reference assignment of filehandles
If FILEHANDLE -- the first argument in a call to "open" -- is an undefined
scalar variable (or array or hash element), a new filehandle is autovivified,
meaning that the variable is assigned a reference to a newly allocated
anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is
the real filehandle. (This is considered a symbolic reference, so "use strict
"refs"" should not be in effect.)
Whitespace and special characters in the filename argument
The filename passed to the one- and two-argument forms of "open" will have
leading and trailing whitespace deleted and normal redirection characters
honored. This property, known as "magic open", can often be used to good
effect. A user could specify a filename of "rsh cat file |", or you could
change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(my $fh, $filename)
or die "Can't open $filename: $!";
Use the three-argument form to open a file with arbitrary weird characters in
it,
open(my $fh, "<", $file)
|| die "Can't open $file: $!";
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(my $fh, "< $file\0")
|| die "Can't open $file: $!";
(this may not work on some bizarre filesystems). One should conscientiously
choose between the magic and three-argument form of "open":
open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
will allow the user to specify an argument of the form "rsh cat file |", but
will not work on a filename that happens to have a trailing space, while
open(my $in, "<", $ARGV[0])
|| die "Can't open $ARGV[0]: $!";
will have exactly the opposite restrictions. (However, some shells support the
syntax "perl your_program.pl <( rsh cat file )", which produces a filename
that can be opened normally.)
Invoking C-style "open"
If you want a "real" C open(2), then you should use the "sysopen" function,
which involves no such magic (but uses different filemodes than Perl "open",
which corresponds to C fopen(3)). This is another way to protect your
filenames from interpretation. For example:
use IO::Handle;
sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
or die "Can't open $path: $!";
$fh->autoflush(1);
print $fh "stuff $$\n";
seek($fh, 0, 0);
print "File contains: ", readline($fh);
See "seek" for some details about mixing reading and writing.
Portability issues
See "open" in perlport.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir",
"rewinddir", and "closedir". Returns true if successful. DIRHANDLE may be an
expression whose value can be used as an indirect dirhandle, usually the real
dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash
element), the variable is assigned a reference to a new anonymous dirhandle; that is,
it's autovivified. Dirhandles are the same objects as filehandles; an I/O object can
only be open as one of these handle types at once.
See the example at "readdir".
ord EXPR
ord Returns the numeric value of the first character of EXPR. If EXPR is an empty string,
returns 0. If EXPR is omitted, uses $_. (Note character, not byte.)
For the reverse, see "chr". See perlunicode for more about Unicode.
our VARLIST
our TYPE VARLIST
our VARLIST : ATTRS
our TYPE VARLIST : ATTRS
"our" makes a lexical alias to a package (i.e. global) variable of the same name in
the current package for use within the current lexical scope.
"our" has the same scoping rules as "my" or "state", meaning that it is only valid
within a lexical scope. Unlike "my" and "state", which both declare new (lexical)
variables, "our" only creates an alias to an existing variable: a package variable of
the same name.
This means that when "use strict 'vars'" is in effect, "our" lets you use a package
variable without qualifying it with the package name, but only within the lexical
scope of the "our" declaration. This applies immediately--even within the same
statement.
package Foo;
use strict;
$Foo::foo = 23;
{
our $foo; # alias to $Foo::foo
print $foo; # prints 23
}
print $Foo::foo; # prints 23
print $foo; # ERROR: requires explicit package name
This works even if the package variable has not been used before, as package variables
spring into existence when first used.
package Foo;
use strict;
our $foo = 23; # just like $Foo::foo = 23
print $Foo::foo; # prints 23
Because the variable becomes legal immediately under "use strict 'vars'", so long as
there is no variable with that name is already in scope, you can then reference the
package variable again even within the same statement.
package Foo;
use strict;
my $foo = $foo; # error, undeclared $foo on right-hand side
our $foo = $foo; # no errors
If more than one variable is listed, the list must be placed in parentheses.
our($bar, $baz);
An "our" declaration declares an alias for a package variable that will be visible
across its entire lexical scope, even across package boundaries. The package in which
the variable is entered is determined at the point of the declaration, not at the
point of use. This means the following behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the same lexical scope are allowed
if they are in different packages. If they happen to be in the same package, Perl
will emit warnings if you have asked for them, just like multiple "my" declarations.
Unlike a second "my" declaration, which will bind the name to a fresh variable, a
second "our" declaration in the same package, in the same scope, is merely redundant.
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30
An "our" declaration may also have a list of attributes associated with it.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is
currently bound to the use of the fields pragma, and attributes are handled using the
attributes pragma, or, starting from Perl 5.8.0, also via the Attribute::Handlers
module. See "Private Variables via my()" in perlsub for details.
Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for
example to skip assignment of initial values:
our ( undef, $min, $hour ) = localtime;
"our" differs from "use vars", which allows use of an unqualified name only within the
affected package, but across scopes.
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the rules given by the
TEMPLATE. The resulting string is the concatenation of the converted values.
Typically, each converted value looks like its machine-level representation. For
example, on 32-bit machines an integer may be represented by a sequence of 4 bytes,
which will in Perl be presented as a string that's 4 characters long.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the order and type of values, as
follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte,
like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support
those. Raises an exception otherwise.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in native format.
d A double-precision float in native format.
F A Perl internal floating-point value (NV) in native format
D A float of long-double precision in native format.
(Long doubles are available only if your system supports
long double values. Raises an exception otherwise.
Note that there are different long double formats.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in char-
acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut
for details). Its bytes represent an unsigned integer in
base 128, most significant digit first, with as few digits
as possible. Bit eight (the high bit) is set on each byte
except the last.
x A null byte (a.k.a ASCII NUL, "\000", chr(0))
X Back up a byte.
@ Null-fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null-fill or truncate to absolute position specified by
the value.
( Start of a ()-group.
One or more modifiers below may optionally follow certain letters in the TEMPLATE (the
second column lists letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
! xX Make x and X act as alignment commands.
! nNvV Treat integers as signed instead of unsigned.
! @. Specify position as byte offset in the internal
representation of the packed string. Efficient
but dangerous.
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()" groups to force a particular byte-
order on all components in that group, including all its subgroups.
The following rules apply:
o Each letter may optionally be followed by a number indicating the repeat count. A
numeric repeat count may optionally be enclosed in brackets, as in "pack("C[80]",
@arr)". The repeat count gobbles that many values from the LIST when used with
all format types other than "a", "A", "Z", "b", "B", "h", "H", "@", ".", "x", "X",
and "P", where it means something else, described below. Supplying a "*" for the
repeat count instead of a number means to use however many items are left, except
for:
o "@", "x", and "X", where it is equivalent to 0.
o <.>, where it means relative to the start of the string.
o "u", where it is equivalent to 1 (or 45, which here is equivalent).
One can replace a numeric repeat count with a template letter enclosed in brackets
to use the packed byte length of the bracketed template for the repeat count.
For example, the template "x[L]" skips as many bytes as in a packed long, and the
template "$t X[$t] $t" unpacks twice whatever $t (when variable-expanded) unpacks.
If the template in brackets contains alignment commands (such as "x![d]"), its
packed length is calculated as if the start of the template had the maximal
possible alignment.
When used with "Z", a "*" as the repeat count is guaranteed to add a trailing null
byte, so the resulting string is always one byte longer than the byte length of
the item itself.
When used with "@", the repeat count represents an offset from the start of the
innermost "()" group.
When used with ".", the repeat count determines the starting position to calculate
the value offset as follows:
o If the repeat count is 0, it's relative to the current position.
o If the repeat count is "*", the offset is relative to the start of the packed
string.
o And if it's an integer n, the offset is relative to the start of the nth
innermost "( )" group, or to the start of the string if n is bigger then the
group level.
The repeat count for "u" is interpreted as the maximal number of bytes to encode
per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not
be more than 65.
o The "a", "A", and "Z" types gobble just one value, but pack it as a string of
length count, padding with nulls or spaces as needed. When unpacking, "A" strips
trailing whitespace and nulls, "Z" strips everything after the first null, and "a"
returns data with no stripping at all.
If the value to pack is too long, the result is truncated. If it's too long and
an explicit count is provided, "Z" packs only "$count-1" bytes, followed by a null
byte. Thus "Z" always packs a trailing null, except when the count is 0.
o Likewise, the "b" and "B" formats pack a string that's that many bits long. Each
such format generates 1 bit of the result. These are typically followed by a
repeat count like "B8" or "B64".
Each result bit is based on the least-significant bit of the corresponding input
character, i.e., on "ord($char)%2". In particular, characters "0" and "1"
generate bits 0 and 1, as do characters "\000" and "\001".
Starting from the beginning of the input string, each 8-tuple of characters is
converted to 1 character of output. With format "b", the first character of the
8-tuple determines the least-significant bit of a character; with format "B", it
determines the most-significant bit of a character.
If the length of the input string is not evenly divisible by 8, the remainder is
packed as if the input string were padded by null characters at the end.
Similarly during unpacking, "extra" bits are ignored.
If the input string is longer than needed, remaining characters are ignored.
A "*" for the repeat count uses all characters of the input field. On unpacking,
bits are converted to a string of 0s and 1s.
o The "h" and "H" formats pack a string that many nybbles (4-bit groups,
representable as hexadecimal digits, "0".."9" "a".."f") long.
For each such format, "pack" generates 4 bits of result. With non-alphabetical
characters, the result is based on the 4 least-significant bits of the input
character, i.e., on "ord($char)%16". In particular, characters "0" and "1"
generate nybbles 0 and 1, as do bytes "\000" and "\001". For characters "a".."f"
and "A".."F", the result is compatible with the usual hexadecimal digits, so that
"a" and "A" both generate the nybble "0xA==10". Use only these specific hex
characters with this format.
Starting from the beginning of the template to "pack", each pair of characters is
converted to 1 character of output. With format "h", the first character of the
pair determines the least-significant nybble of the output character; with format
"H", it determines the most-significant nybble.
If the length of the input string is not even, it behaves as if padded by a null
character at the end. Similarly, "extra" nybbles are ignored during unpacking.
If the input string is longer than needed, extra characters are ignored.
A "*" for the repeat count uses all characters of the input field. For "unpack",
nybbles are converted to a string of hexadecimal digits.
o The "p" format packs a pointer to a null-terminated string. You are responsible
for ensuring that the string is not a temporary value, as that could potentially
get deallocated before you got around to using the packed result. The "P" format
packs a pointer to a structure of the size indicated by the length. A null
pointer is created if the corresponding value for "p" or "P" is "undef"; similarly
with "unpack", where a null pointer unpacks into "undef".
If your system has a strange pointer size--meaning a pointer is neither as big as
an int nor as big as a long--it may not be possible to pack or unpack pointers in
big- or little-endian byte order. Attempting to do so raises an exception.
o The "/" template character allows packing and unpacking of a sequence of items
where the packed structure contains a packed item count followed by the packed
items themselves. This is useful when the structure you're unpacking has encoded
the sizes or repeat counts for some of its fields within the structure itself as
separate fields.
For "pack", you write length-item"/"sequence-item, and the length-item describes
how the length value is packed. Formats likely to be of most use are integer-
packing ones like "n" for Java strings, "w" for ASN.1 or SNMP, and "N" for Sun
XDR.
For "pack", sequence-item may have a repeat count, in which case the minimum of
that and the number of available items is used as the argument for length-item.
If it has no repeat count or uses a '*', the number of available items is used.
For "unpack", an internal stack of integer arguments unpacked so far is used. You
write "/"sequence-item and the repeat count is obtained by popping off the last
element from the stack. The sequence-item must not have a repeat count.
If sequence-item refers to a string type ("A", "a", or "Z"), the length-item is
the string length, not the number of strings. With an explicit repeat count for
pack, the packed string is adjusted to that length. For example:
This code: gives this result:
unpack("W/a", "\004Gurusamy") ("Guru")
unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
pack("a/W2", ord("a") .. ord("z")) "2ab"
The length-item is not returned explicitly from "unpack".
Supplying a count to the length-item format letter is only useful with "A", "a",
or "Z". Packing with a length-item of "a" or "Z" may introduce "\000" characters,
which Perl does not regard as legal in numeric strings.
o The integer types "s", "S", "l", and "L" may be followed by a "!" modifier to
specify native shorts or longs. As shown in the example above, a bare "l" means
exactly 32 bits, although the native "long" as seen by the local C compiler may be
larger. This is mainly an issue on 64-bit platforms. You can see whether using
"!" makes any difference this way:
printf "format s is %d, s! is %d\n",
length pack("s"), length pack("s!");
printf "format l is %d, l! is %d\n",
length pack("l"), length pack("l!");
"i!" and "I!" are also allowed, but only for completeness' sake: they are
identical to "i" and "I".
The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the
platform where Perl was built are also available from the command line:
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';
or programmatically via the "Config" module:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
$Config{longlongsize} is undefined on systems without long long support.
o The integer formats "s", "S", "i", "I", "l", "L", "j", and "J" are inherently non-
portable between processors and operating systems because they obey native
byteorder and endianness. For example, a 4-byte integer 0x12345678 (305419896
decimal) would be ordered natively (arranged in and handled by the CPU registers)
into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, Intel and VAX CPUs are little-endian, while everybody else, including
Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian. Alpha and
MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode,
but SGI/Cray uses them in big-endian mode.
The names big-endian and little-endian are comic references to the egg-eating
habits of the little-endian Lilliputians and the big-endian Blefuscudians from the
classic Jonathan Swift satire, Gulliver's Travels. This entered computer lingo
via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137,
April 1, 1980.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
These are called mid-endian, middle-endian, mixed-endian, or just weird.
You can determine your system endianness with this incantation:
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built is also available via Config:
use Config;
print "$Config{byteorder}\n";
or from the command line:
$ perl -V:byteorder
Byteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-
endian. Systems with multiarchitecture binaries will have "ffff", signifying that
static information doesn't work, one must use runtime probing.
For portably packed integers, either use the formats "n", "N", "v", and "V" or
else use the ">" and "<" modifiers described immediately below. See also
perlport.
o Also floating point numbers have endianness. Usually (but not always) this agrees
with the integer endianness. Even though most platforms these days use the IEEE
754 binary format, there are differences, especially if the long doubles are
involved. You can see the "Config" variables "doublekind" and "longdblkind" (also
"doublesize", "longdblsize"): the "kind" values are enums, unlike "byteorder".
Portability-wise the best option is probably to keep to the IEEE 754 64-bit
doubles, and of agreed-upon endianness. Another possibility is the "%a") format
of "printf".
o Starting with Perl 5.10.0, integer and floating-point formats, along with the "p"
and "P" formats and "()" groups, may all be followed by the ">" or "<" endianness
modifiers to respectively enforce big- or little-endian byte-order. These
modifiers are especially useful given how "n", "N", "v", and "V" don't cover
signed integers, 64-bit integers, or floating-point values.
Here are some concerns to keep in mind when using an endianness modifier:
o Exchanging signed integers between different platforms works only when all
platforms store them in the same format. Most platforms store signed integers
in two's-complement notation, so usually this is not an issue.
o The ">" or "<" modifiers can only be used on floating-point formats on big- or
little-endian machines. Otherwise, attempting to use them raises an
exception.
o Forcing big- or little-endian byte-order on floating-point values for data
exchange can work only if all platforms use the same binary representation
such as IEEE floating-point. Even if all platforms are using IEEE, there may
still be subtle differences. Being able to use ">" or "<" on floating-point
values can be useful, but also dangerous if you don't know exactly what you're
doing. It is not a general way to portably store floating-point values.
o When using ">" or "<" on a "()" group, this affects all types inside the group
that accept byte-order modifiers, including all subgroups. It is silently
ignored for all other types. You are not allowed to override the byte-order
within a group that already has a byte-order modifier suffix.
o Real numbers (floats and doubles) are in native machine format only. Due to the
multiplicity of floating-point formats and the lack of a standard "network"
representation for them, no facility for interchange has been made. This means
that packed floating-point data written on one machine may not be readable on
another, even if both use IEEE floating-point arithmetic (because the endianness
of the memory representation is not part of the IEEE spec). See also perlport.
If you know exactly what you're doing, you can use the ">" or "<" modifiers to
force big- or little-endian byte-order on floating-point values.
Because Perl uses doubles (or long doubles, if configured) internally for all
numeric calculation, converting from double into float and thence to double again
loses precision, so "unpack("f", pack("f", $foo)") will not in general equal $foo.
o Pack and unpack can operate in two modes: character mode ("C0" mode) where the
packed string is processed per character, and UTF-8 byte mode ("U0" mode) where
the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte
basis. Character mode is the default unless the format string starts with "U".
You can always switch mode mid-format with an explicit "C0" or "U0" in the format.
This mode remains in effect until the next mode change, or until the end of the
"()" group it (directly) applies to.
Using "C0" to get Unicode characters while using "U0" to get non-Unicode bytes is
not necessarily obvious. Probably only the first of these is what you want:
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89
Those examples also illustrate that you should not try to use "pack"/"unpack" as a
substitute for the Encode module.
o You must yourself do any alignment or padding by inserting, for example, enough
"x"es while packing. There is no way for "pack" and "unpack" to know where
characters are going to or coming from, so they handle their output and input as
flat sequences of characters.
o A "()" group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat
count either as postfix, or for "unpack", also via the "/" template character.
Within each repetition of a group, positioning with "@" starts over at 0.
Therefore, the result of
pack("@1A((@2A)@3A)", qw[X Y Z])
is the string "\0X\0\0YZ".
o "x" and "X" accept the "!" modifier to act as alignment commands: they jump
forward or back to the closest position aligned at a multiple of "count"
characters. For example, to "pack" or "unpack" a C structure like
struct {
char c; /* one signed, 8-bit character */
double d;
char cc[2];
}
one may need to use the template "c x![d] d c[2]". This assumes that doubles must
be aligned to the size of double.
For alignment commands, a "count" of 0 is equivalent to a "count" of 1; both are
no-ops.
o "n", "N", "v" and "V" accept the "!" modifier to represent signed 16-/32-bit
integers in big-/little-endian order. This is portable only when all platforms
sharing packed data use the same binary representation for signed integers; for
example, when all platforms use two's-complement representation.
o Comments can be embedded in a TEMPLATE using "#" through the end of line. White
space can separate pack codes from each other, but modifiers and repeat counts
must follow immediately. Breaking complex templates into individual line-by-line
components, suitably annotated, can do as much to improve legibility and
maintainability of pack/unpack formats as "/x" can for complicated pattern
matches.
o If TEMPLATE requires more arguments than "pack" is given, "pack" assumes
additional "" arguments. If TEMPLATE requires fewer arguments than given, extra
arguments are ignored.
o Attempting to pack the special floating point values "Inf" and "NaN" (infinity,
also in negative, and not-a-number) into packed integer values (like "L") is a
fatal error. The reason for this is that there simply isn't any sensible mapping
for these special values into integers.
Examples:
$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the
# UTF-8 bytes because the U at the start of the format caused
# a switch to U0-mode, so the UTF-8 bytes get joined into
# characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the
# previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. On EBCDIC systems, the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\001\000\002\000" on little-endian
# "\000\001\000\002" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same
The same template may generally also be used in "unpack".
package NAMESPACE
package NAMESPACE VERSION
package NAMESPACE BLOCK
package NAMESPACE VERSION BLOCK
Declares the BLOCK or the rest of the compilation unit as being in the given
namespace. The scope of the package declaration is either the supplied code BLOCK or,
in the absence of a BLOCK, from the declaration itself through the end of current
scope (the enclosing block, file, or "eval"). That is, the forms without a BLOCK are
operative through the end of the current scope, just like the "my", "state", and "our"
operators. All unqualified dynamic identifiers in this scope will be in the given
namespace, except where overridden by another "package" declaration or when they're
one of the special identifiers that qualify into "main::", like "STDOUT", "ARGV",
"ENV", and the punctuation variables.
A package statement affects dynamic variables only, including those you've used
"local" on, but not lexically-scoped variables, which are created with "my", "state",
or "our". Typically it would be the first declaration in a file included by "require"
or "use". You can switch into a package in more than one place, since this only
determines which default symbol table the compiler uses for the rest of that block.
You can refer to identifiers in other packages than the current one by prefixing the
identifier with the package name and a double colon, as in $SomePack::var or
"ThatPack::INPUT_HANDLE". If package name is omitted, the "main" package is assumed.
That is, $::sail is equivalent to $main::sail (as well as to "$main'sail", still seen
in ancient code, mostly from Perl 4).
If VERSION is provided, "package" sets the $VERSION variable in the given namespace to
a version object with the VERSION provided. VERSION must be a "strict" style version
number as defined by the version module: a positive decimal number (integer or
decimal-fraction) without exponentiation or else a dotted-decimal v-string with a
leading 'v' character and at least three components. You should set $VERSION only
once per package.
See "Packages" in perlmod for more information about packages, modules, and classes.
See perlsub for other scoping issues.
__PACKAGE__
A special token that returns the name of the package in which it occurs.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding system call. Note that if you
set up a loop of piped processes, deadlock can occur unless you are very careful. In
addition, note that Perl's pipes use IO buffering, so you may need to set $| to flush
your WRITEHANDLE after each command, depending on the application.
Returns true on success.
See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in
perlipc for examples of such things.
On systems that support a close-on-exec flag on files, that flag is set on all newly
opened file descriptors whose "fileno"s are higher than the current value of $^F (by
default 2 for "STDERR"). See "$^F" in perlvar.
pop ARRAY
pop Pops and returns the last value of the array, shortening the array by one element.
Returns the undefined value if the array is empty, although this may also happen at
other times. If ARRAY is omitted, pops the @ARGV array in the main program, but the
@_ array in subroutines, just like "shift".
Starting with Perl 5.14, an experimental feature allowed "pop" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
pos SCALAR
pos Returns the offset of where the last "m//g" search left off for the variable in
question ($_ is used when the variable is not specified). This offset is in
characters unless the (no-longer-recommended) "use bytes" pragma is in effect, in
which case the offset is in bytes. Note that 0 is a valid match offset. "undef"
indicates that the search position is reset (usually due to match failure, but can
also be because no match has yet been run on the scalar).
"pos" directly accesses the location used by the regexp engine to store the offset, so
assigning to "pos" will change that offset, and so will also influence the "\G" zero-
width assertion in regular expressions. Both of these effects take place for the next
match, so you can't affect the position with "pos" during the current match, such as
in "(?{pos() = 5})" or "s//pos() = 5/e".
Setting "pos" also resets the matched with zero-length flag, described under "Repeated
Patterns Matching a Zero-length Substring" in perlre.
Because a failed "m//gc" match doesn't reset the offset, the return from "pos" won't
change either in this case. See perlre and perlop.
print FILEHANDLE LIST
print FILEHANDLE
print LIST
print
Prints a string or a list of strings. Returns true if successful. FILEHANDLE may be
a scalar variable containing the name of or a reference to the filehandle, thus
introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next
token is a term, it may be misinterpreted as an operator unless you interpose a "+" or
put parentheses around the arguments.) If FILEHANDLE is omitted, prints to the last
selected (see "select") output handle. If LIST is omitted, prints $_ to the currently
selected output handle. To use FILEHANDLE alone to print the content of $_ to it, you
must use a bareword filehandle like "FH", not an indirect one like $fh. To set the
default output handle to something other than STDOUT, use the select operation.
The current value of $, (if any) is printed between each LIST item. The current value
of "$\" (if any) is printed after the entire LIST has been printed. Because print
takes a LIST, anything in the LIST is evaluated in list context, including any
subroutines whose return lists you pass to "print". Be careful not to follow the
print keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print; put parentheses around all
arguments (or interpose a "+", but that doesn't look as good).
If you're storing handles in an array or hash, or in general whenever you're using any
expression more complex than a bareword handle or a plain, unsubscripted scalar
variable to retrieve it, you will have to use a block returning the filehandle value
instead, in which case the LIST may not be omitted:
print { $files[$i] } "stuff\n";
print { $OK ? *STDOUT : *STDERR } "stuff\n";
Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for
more on signal handling.
printf FILEHANDLE FORMAT, LIST
printf FILEHANDLE
printf FORMAT, LIST
printf
Equivalent to "print FILEHANDLE sprintf(FORMAT, LIST)", except that "$\" (the output
record separator) is not appended. The FORMAT and the LIST are actually parsed as a
single list. The first argument of the list will be interpreted as the "printf"
format. This means that "printf(@_)" will use $_[0] as the format. See sprintf for
an explanation of the format argument. If "use locale" (including "use locale
':not_characters'") is in effect and "POSIX::setlocale" has been called, the character
used for the decimal separator in formatted floating-point numbers is affected by the
"LC_NUMERIC" locale setting. See perllocale and POSIX.
For historical reasons, if you omit the list, $_ is used as the format; to use
FILEHANDLE without a list, you must use a bareword filehandle like "FH", not an
indirect one like $fh. However, this will rarely do what you want; if $_ contains
formatting codes, they will be replaced with the empty string and a warning will be
emitted if warnings are enabled. Just use "print" if you want to print the contents
of $_.
Don't fall into the trap of using a "printf" when a simple "print" would do. The
"print" is more efficient and less error prone.
prototype FUNCTION
prototype
Returns the prototype of a function as a string (or "undef" if the function has no
prototype). FUNCTION is a reference to, or the name of, the function whose prototype
you want to retrieve. If FUNCTION is omitted, $_ is used.
If FUNCTION is a string starting with "CORE::", the rest is taken as a name for a Perl
builtin. If the builtin's arguments cannot be adequately expressed by a prototype
(such as "system"), "prototype" returns "undef", because the builtin does not really
behave like a Perl function. Otherwise, the string describing the equivalent
prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack by appending the values of LIST to the end of ARRAY. The
length of ARRAY increases by the length of LIST. Has the same effect as
for my $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the number of elements in the array following the
completed "push".
Starting with Perl 5.14, an experimental feature allowed "push" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
q/STRING/
qq/STRING/
qw/STRING/
qx/STRING/
Generalized quotes. See "Quote-Like Operators" in perlop.
qr/STRING/
Regexp-like quote. See "Regexp Quote-Like Operators" in perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all the ASCII non-"word" characters backslashed. (That
is, all ASCII characters not matching "/[A-Za-z_0-9]/" will be preceded by a backslash
in the returned string, regardless of any locale settings.) This is the internal
function implementing the "\Q" escape in double-quoted strings. (See below for the
behavior on non-ASCII code points.)
If EXPR is omitted, uses $_.
quotemeta (and "\Q" ... "\E") are useful when interpolating strings into regular
expressions, because by default an interpolated variable will be considered a mini-
regular expression. For example:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};
Will cause $sentence to become 'The big bad wolf jumped over...'.
On the other hand:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};
Or:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};
Will both leave the sentence as is. Normally, when accepting literal string input
from the user, "quotemeta" or "\Q" must be used.
Beware that if you put literal backslashes (those not inside interpolated variables)
between "\Q" and "\E", double-quotish backslash interpolation may lead to confusing
results. If you need to use literal backslashes within "\Q...\E", consult "Gory
details of parsing quoted constructs" in perlop.
Because the result of "\QSTRING\E" has all metacharacters quoted, there is no way to
insert a literal "$" or "@" inside a "\Q\E" pair. If protected by "\", "$" will be
quoted to become "\\\$"; if not, it is interpreted as the start of an interpolated
scalar.
In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but
not quoted in UTF-8 strings.
Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII
characters; the quoting of ASCII characters is unchanged.
Also unchanged is the quoting of non-UTF-8 strings when outside the scope of a "use
feature 'unicode_strings'", which is to quote all characters in the upper Latin1
range. This provides complete backwards compatibility for old programs which do not
use Unicode. (Note that "unicode_strings" is automatically enabled within the scope
of a "usev5.12" or greater.)
Within the scope of "use locale", all non-ASCII Latin1 code points are quoted whether
the string is encoded as UTF-8 or not. As mentioned above, locale does not affect the
quoting of ASCII-range characters. This protects against those locales where
characters such as "|" are considered to be word characters.
Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see
<https://www.unicode.org/reports/tr31/>). The only code points that are quoted are
those that have any of the Unicode properties: Pattern_Syntax, Pattern_White_Space,
White_Space, Default_Ignorable_Code_Point, or General_Category=Control.
Of these properties, the two important ones are Pattern_Syntax and
Pattern_White_Space. They have been set up by Unicode for exactly this purpose of
deciding which characters in a regular expression pattern should be quoted. No
character that can be in an identifier has these properties.
Perl promises, that if we ever add regular expression pattern metacharacters to the
dozen already defined ("\ | ( ) [ { ^ $ * + ? ."), that we will only use ones that
have the Pattern_Syntax property. Perl also promises, that if we ever add characters
that are considered to be white space in regular expressions (currently mostly
affected by "/x"), they will all have the Pattern_White_Space property.
Unicode promises that the set of code points that have these two properties will never
change, so something that is not quoted in v5.16 will never need to be quoted in any
future Perl release. (Not all the code points that match Pattern_Syntax have actually
had characters assigned to them; so there is room to grow, but they are quoted whether
assigned or not. Perl, of course, would never use an unassigned code point as an
actual metacharacter.)
Quoting characters that have the other 3 properties is done to enhance the readability
of the regular expression and not because they actually need to be quoted for regular
expression purposes (characters with the White_Space property are likely to be
indistinguishable on the page or screen from those with the Pattern_White_Space
property; and the other two properties contain non-printing characters).
rand EXPR
rand
Returns a random fractional number greater than or equal to 0 and less than the value
of EXPR. (EXPR should be positive.) If EXPR is omitted, the value 1 is used.
Currently EXPR with the value 0 is also special-cased as 1 (this was undocumented
before Perl 5.8.0 and is subject to change in future versions of Perl). Automatically
calls "srand" unless "srand" has already been called. See also "srand".
Apply "int" to the value returned by "rand" if you want random integers instead of
random fractional numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns numbers that are too large or too
small, then your version of Perl was probably compiled with the wrong number of
RANDBITS.)
"rand" is not cryptographically secure. You should not rely on it in security-
sensitive situations. As of this writing, a number of third-party CPAN modules offer
random number generators intended by their authors to be cryptographically secure,
including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR from the specified
FILEHANDLE. Returns the number of characters actually read, 0 at end of file, or
undef if there was an error (in the latter case $! is also set). SCALAR will be grown
or shrunk so that the last character actually read is the last character of the scalar
after the read.
An OFFSET may be specified to place the read data at some place in the string other
than the beginning. A negative OFFSET specifies placement at that many characters
counting backwards from the end of the string. A positive OFFSET greater than the
length of SCALAR results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.
The call is implemented in terms of either Perl's or your system's native fread(3)
library function, via the PerlIO layers applied to the handle. To get a true read(2)
system call, see sysread.
Note the characters: depending on the status of the filehandle, either (8-bit) bytes
or characters are read. By default, all filehandles operate on bytes, but for example
if the filehandle has been opened with the ":utf8" I/O layer (see "open", and the open
pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes.
Similarly for the ":encoding" layer: in that case pretty much any characters can be
read.
readdir DIRHANDLE
Returns the next directory entry for a directory opened by "opendir". If used in list
context, returns all the rest of the entries in the directory. If there are no more
entries, returns the undefined value in scalar context and the empty list in list
context.
If you're planning to filetest the return values out of a "readdir", you'd better
prepend the directory in question. Otherwise, because we didn't "chdir" there, it
would have been testing the wrong file.
opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;
As of Perl 5.12 you can use a bare "readdir" in a "while" loop, which will set $_ on
every iteration. If either a "readdir" expression or an explicit assignment of a
"readdir" expression to a scalar is used as a "while"/"for" condition, then the
condition actually tests for definedness of the expression's value, not for its
regular truth value.
opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
while (readdir $dh) {
print "$some_dir/$_\n";
}
closedir $dh;
To avoid confusing would-be users of your code who are running earlier versions of
Perl with mysterious failures, put this sort of thing at the top of your file to
signal that your code will work only on Perls of a recent vintage:
use 5.012; # so readdir assigns to $_ in a lone while test
readline EXPR
readline
Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR
is not provided). In scalar context, each call reads and returns the next line until
end-of-file is reached, whereupon the subsequent call returns "undef". In list
context, reads until end-of-file is reached and returns a list of lines. Note that
the notion of "line" used here is whatever you may have defined with $/ (or
$INPUT_RECORD_SEPARATOR in English). See "$/" in perlvar.
When $/ is set to "undef", when "readline" is in scalar context (i.e., file slurp
mode), and when an empty file is read, it returns '' the first time, followed by
"undef" subsequently.
This is the internal function implementing the "<EXPR>" operator, but you can use it
directly. The "<EXPR>" operator is discussed in more detail in "I/O Operators" in
perlop.
my $line = <STDIN>;
my $line = readline(STDIN); # same thing
If "readline" encounters an operating system error, $! will be set with the
corresponding error message. It can be helpful to check $! when you are reading from
filehandles you don't trust, such as a tty or a socket. The following example uses
the operator form of "readline" and dies if the result is not defined.
while ( ! eof($fh) ) {
defined( $_ = readline $fh ) or die "readline failed: $!";
...
}
Note that you have can't handle "readline" errors that way with the "ARGV" filehandle.
In that case, you have to open each element of @ARGV yourself since "eof" handles
"ARGV" differently.
foreach my $arg (@ARGV) {
open(my $fh, $arg) or warn "Can't open $arg: $!";
while ( ! eof($fh) ) {
defined( $_ = readline $fh )
or die "readline failed for $arg: $!";
...
}
}
Like the "<EXPR>" operator, if a "readline" expression is used as the condition of a
"while" or "for" loop, then it will be implicitly assigned to $_. If either a
"readline" expression or an explicit assignment of a "readline" expression to a scalar
is used as a "while"/"for" condition, then the condition actually tests for
definedness of the expression's value, not for its regular truth value.
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links are implemented. If not,
raises an exception. If there is a system error, returns the undefined value and sets
$! (errno). If EXPR is omitted, uses $_.
Portability issues: "readlink" in perlport.
readpipe EXPR
readpipe
EXPR is executed as a system command. The collected standard output of the command is
returned. In scalar context, it comes back as a single (potentially multi-line)
string. In list context, returns a list of lines (however you've defined lines with
$/ (or $INPUT_RECORD_SEPARATOR in English)). This is the internal function
implementing the "qx/EXPR/" operator, but you can use it directly. The "qx/EXPR/"
operator is discussed in more detail in ""qx/STRING/"" in perlop. If EXPR is omitted,
uses $_.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive LENGTH characters of data into
variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk
to the length actually read. Takes the same flags as the system call of the same
name. Returns the address of the sender if SOCKET's protocol supports this; returns
an empty string otherwise. If there's an error, returns the undefined value. This
call is actually implemented in terms of the recvfrom(2) system call. See "UDP:
Message Passing" in perlipc for examples.
Note that if the socket has been marked as ":utf8", "recv" will throw an exception.
The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode".
redo LABEL
redo EXPR
redo
The "redo" command restarts the loop block without evaluating the conditional again.
The "continue" block, if any, is not executed. If the LABEL is omitted, the command
refers to the innermost enclosing loop. The "redo EXPR" form, available starting in
Perl 5.18.0, allows a label name to be computed at run time, and is otherwise
identical to "redo LABEL". Programs that want to lie to themselves about what was
just input normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
my $front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot return a value from a block that typically returns a value, such as
"eval {}", "sub {}", or "do {}". It will perform its flow control behavior, which
precludes any return value. It should not be used to exit a "grep" or "map" operation.
Note that a block by itself is semantically identical to a loop that executes once.
Thus "redo" inside such a block will effectively turn it into a looping construct.
See also "continue" for an illustration of how "last", "next", and "redo" work.
Unlike most named operators, this has the same precedence as assignment. It is also
exempt from the looks-like-a-function rule, so "redo ("foo")."bar"" will cause "bar"
to be part of the argument to "redo".
ref EXPR
ref Examines the value of EXPR, expecting it to be a reference, and returns a string
giving information about the reference and the type of referent. If EXPR is not
specified, $_ will be used.
If the operand is not a reference, then the empty string will be returned. An empty
string will only be returned in this situation. "ref" is often useful to just test
whether a value is a reference, which can be done by comparing the result to the empty
string. It is a common mistake to use the result of "ref" directly as a truth value:
this goes wrong because 0 (which is false) can be returned for a reference.
If the operand is a reference to a blessed object, then the name of the class into
which the referent is blessed will be returned. "ref" doesn't care what the physical
type of the referent is; blessing takes precedence over such concerns. Beware that
exact comparison of "ref" results against a class name doesn't perform a class
membership test: a class's members also include objects blessed into subclasses, for
which "ref" will return the name of the subclass. Also beware that class names can
clash with the built-in type names (described below).
If the operand is a reference to an unblessed object, then the return value indicates
the type of object. If the unblessed referent is not a scalar, then the return value
will be one of the strings "ARRAY", "HASH", "CODE", "FORMAT", or "IO", indicating only
which kind of object it is. If the unblessed referent is a scalar, then the return
value will be one of the strings "SCALAR", "VSTRING", "REF", "GLOB", "LVALUE", or
"REGEXP", depending on the kind of value the scalar currently has. But note that
"qr//" scalars are created already blessed, so "ref qr/.../" will likely return
"Regexp". Beware that these built-in type names can also be used as class names, so
"ref" returning one of these names doesn't unambiguously indicate that the referent is
of the kind to which the name refers.
The ambiguity between built-in type names and class names significantly limits the
utility of "ref". For unambiguous information, use "Scalar::Util::blessed()" for
information about blessing, and "Scalar::Util::reftype()" for information about
physical types. Use the "isa" method for class membership tests, though one must be
sure of blessedness before attempting a method call.
See also perlref and perlobj.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME will be clobbered. Returns true
for success; on failure returns false and sets $!.
Behavior of this function varies wildly depending on your system implementation. For
example, it will usually not work across file system boundaries, even though the
system mv command sometimes compensates for this. Other restrictions include whether
it works on directories, open files, or pre-existing files. Check perlport and either
the rename(2) manpage or equivalent system documentation for details.
For a platform independent "move" function look at the File::Copy module.
Portability issues: "rename" in perlport.
require VERSION
require EXPR
require
Demands a version of Perl specified by VERSION, or demands some semantics specified by
EXPR or by $_ if EXPR is not supplied.
VERSION may be either a literal such as v5.24.1, which will be compared to $^V (or
$PERL_VERSION in English), or a numeric argument of the form 5.024001, which will be
compared to $]. An exception is raised if VERSION is greater than the version of the
current Perl interpreter. Compare with "use", which can do a similar check at compile
time.
Specifying VERSION as a numeric argument of the form 5.024001 should generally be
avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 (released
in 2002), the more verbose numeric form was the only supported syntax, which is why
you might see it in older code.
require v5.24.1; # run time version check
require 5.24.1; # ditto
require 5.024_001; # ditto; older syntax compatible
with perl 5.6
Otherwise, "require" demands that a library file be included if it hasn't already been
included. The file is included via the do-FILE mechanism, which is essentially just a
variety of "eval" with the caveat that lexical variables in the invoking script will
be invisible to the included code. If it were implemented in pure Perl, it would have
semantics similar to the following:
use Carp 'croak';
use version;
sub require {
my ($filename) = @_;
if ( my $version = eval { version->parse($filename) } ) {
if ( $version > $^V ) {
my $vn = $version->normal;
croak "Perl $vn required--this is only $^V, stopped";
}
return 1;
}
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
croak "Compilation failed in require";
}
foreach $prefix (@INC) {
if (ref($prefix)) {
#... do other stuff - see text below ....
}
# (see text below about possible appending of .pmc
# suffix to $filename)
my $realfilename = "$prefix/$filename";
next if ! -e $realfilename || -d _ || -b _;
$INC{$filename} = $realfilename;
my $result = do($realfilename);
# but run in caller's namespace
if (!defined $result) {
$INC{$filename} = undef;
croak $@ ? "$@Compilation failed in require"
: "Can't locate $filename: $!\n";
}
if (!$result) {
delete $INC{$filename};
croak "$filename did not return true value";
}
$! = 0;
return $result;
}
croak "Can't locate $filename in \@INC ...";
}
Note that the file will not be included twice under the same specified name.
The file must return true as the last statement to indicate successful execution of
any initialization code, so it's customary to end such a file with "1;" unless you're
sure it'll return true otherwise. But it's better just to put the "1;", in case you
add more statements.
If EXPR is a bareword, "require" assumes a .pm extension and replaces "::" with "/" in
the filename for you, to make it easy to load standard modules. This form of loading
of modules does not risk altering your namespace, however it will autovivify the stash
for the required module.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the Foo/Bar.pm file in the directories
specified in the @INC array, and it will autovivify the "Foo::Bar::" stash at compile
time.
But if you try this:
my $class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the Foo::Bar file in the @INC array and will
complain about not finding Foo::Bar there. In this case you can do:
eval "require $class";
or you could do
require "Foo/Bar.pm";
Neither of these forms will autovivify any stashes at compile time and only have run
time effects.
Now that you understand how "require" looks for files with a bareword argument, there
is a little extra functionality going on behind the scenes. Before "require" looks
for a .pm extension, it will first look for a similar filename with a .pmc extension.
If this file is found, it will be loaded in place of any file ending in a .pm
extension. This applies to both the explicit "require "Foo/Bar.pm";" form and the
"require Foo::Bar;" form.
You can also insert hooks into the import facility by putting Perl code directly into
the @INC array. There are three forms of hooks: subroutine references, array
references, and blessed objects.
Subroutine references are the simplest case. When the inclusion system walks through
@INC and encounters a subroutine, this subroutine gets called with two parameters, the
first a reference to itself, and the second the name of the file to be included (e.g.,
Foo/Bar.pm). The subroutine should return either nothing or else a list of up to four
values in the following order:
1. A reference to a scalar, containing any initial source code to prepend to the file
or generator output.
2. A filehandle, from which the file will be read.
3. A reference to a subroutine. If there is no filehandle (previous item), then this
subroutine is expected to generate one line of source code per call, writing the
line into $_ and returning 1, then finally at end of file returning 0. If there
is a filehandle, then the subroutine will be called to act as a simple source
filter, with the line as read in $_. Again, return 1 for each valid line, and 0
after all lines have been returned. For historical reasons the subroutine will
receive a meaningless argument (in fact always the numeric value zero) as $_[0].
4. Optional state for the subroutine. The state is passed in as $_[1].
If an empty list, "undef", or nothing that matches the first 3 values above is
returned, then "require" looks at the remaining elements of @INC. Note that this
filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob,
whether blessed or unblessed); tied filehandles will be ignored and processing will
stop there.
If the hook is an array reference, its first element must be a subroutine reference.
This subroutine is called as above, but the first parameter is the array reference.
This lets you indirectly pass arguments to the subroutine.
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my (undef, @parameters) = @$arrayref;
...
}
If the hook is an object, it must provide an "INC" method that will be called as
above, the first parameter being the object itself. (Note that you must fully qualify
the sub's name, as unqualified "INC" is always forced into package "main".) Here is a
typical code layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, Foo->new(...);
These hooks are also permitted to set the %INC entry corresponding to the files they
have loaded. See "%INC" in perlvar.
For a yet-more-powerful import facility, see "use" and perlmod.
reset EXPR
reset
Generally used in a "continue" block at the end of a loop to clear variables and reset
"m?pattern?" searches so that they work again. The expression is interpreted as a
list of single characters (hyphens allowed for ranges). All variables (scalars,
arrays, and hashes) in the current package beginning with one of those letters are
reset to their pristine state. If the expression is omitted, one-match searches
("m?pattern?") are reset to match again. Only resets variables or searches in the
current package. Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset m?one-time? searches
Resetting "A-Z" is not recommended because you'll wipe out your @ARGV and @INC arrays
and your %ENV hash.
Resets only package variables; lexical variables are unaffected, but they clean
themselves up on scope exit anyway, so you'll probably want to use them instead. See
"my".
return EXPR
return
Returns from a subroutine, "eval", "do FILE", "sort" block or regex eval block (but
not a "grep", "map", or "do BLOCK" block) with the value given in EXPR. Evaluation of
EXPR may be in list, scalar, or void context, depending on how the return value will
be used, and the context may vary from one execution to the next (see "wantarray").
If no EXPR is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in void context.
(In the absence of an explicit "return", a subroutine, "eval", or "do FILE"
automatically returns the value of the last expression evaluated.)
Unlike most named operators, this is also exempt from the looks-like-a-function rule,
so "return ("foo")."bar"" will cause "bar" to be part of the argument to "return".
reverse LIST
In list context, returns a list value consisting of the elements of LIST in the
opposite order. In scalar context, concatenates the elements of LIST and returns a
string value with all characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world
print scalar reverse "dlrow ,", "olleH"; # Hello, world
Used without arguments in scalar context, "reverse" reverses $_.
$_ = "dlrow ,olleH";
print reverse; # No output, list context
print scalar reverse; # Hello, world
Note that reversing an array to itself (as in "@a = reverse @a") will preserve non-
existent elements whenever possible; i.e., for non-magical arrays or for tied arrays
with "EXISTS" and "DELETE" methods.
This operator is also handy for inverting a hash, although there are some caveats. If
a value is duplicated in the original hash, only one of those can be represented as a
key in the inverted hash. Also, this has to unwind one hash and build a whole new
one, which may take some time on a large hash, such as from a DBM file.
my %by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the directory for the "readdir" routine
on DIRHANDLE.
Portability issues: "rewinddir" in perlport.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like "index" except that it returns the position of the last occurrence of
SUBSTR in STR. If POSITION is specified, returns the last occurrence beginning at or
before that position.
rmdir FILENAME
rmdir
Deletes the directory specified by FILENAME if that directory is empty. If it
succeeds it returns true; otherwise it returns false and sets $! (errno). If FILENAME
is omitted, uses $_.
To remove a directory tree recursively ("rm -rf" on Unix) look at the "rmtree"
function of the File::Path module.
s///
The substitution operator. See "Regexp Quote-Like Operators" in perlop.
say FILEHANDLE LIST
say FILEHANDLE
say LIST
say Just like "print", but implicitly appends a newline at the end of the LIST instead of
any value "$\" might have. To use FILEHANDLE without a LIST to print the contents of
$_ to it, you must use a bareword filehandle like "FH", not an indirect one like $fh.
"say" is available only if the "say" feature is enabled or if it is prefixed with
"CORE::". The "say" feature is enabled automatically with a "use v5.10" (or higher)
declaration in the current scope.
scalar EXPR
Forces EXPR to be interpreted in scalar context and returns the value of EXPR.
my @counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be interpolated in list
context because in practice, this is never needed. If you really wanted to do so,
however, you could use the construction "@{[ (some expression) ]}", but usually a
simple "(some expression)" suffices.
Because "scalar" is a unary operator, if you accidentally use a parenthesized list for
the EXPR, this behaves as a scalar comma expression, evaluating all but the last
element in void context and returning the final element evaluated in scalar context.
This is seldom what you want.
The following single statement:
print uc(scalar(foo(), $bar)), $baz;
is the moral equivalent of these two:
foo();
print(uc($bar), $baz);
See perlop for more details on unary operators and the comma operator, and perldata
for details on evaluating a hash in scalar context.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the fseek(3) call of C "stdio". FILEHANDLE may
be an expression whose value gives the name of the filehandle. The values for WHENCE
are 0 to set the new position in bytes to POSITION; 1 to set it to the current
position plus POSITION; and 2 to set it to EOF plus POSITION, typically negative. For
WHENCE you may use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the
file, current position, end of the file) from the Fcntl module. Returns 1 on success,
false otherwise.
Note the emphasis on bytes: even if the filehandle has been set to operate on
characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
and "sysseek" family of functions use byte offsets, not character offsets, because
seeking to a character offset would be very slow in a UTF-8 file.
If you want to position the file for "sysread" or "syswrite", don't use "seek",
because buffering makes its effect on the file's read-write position unpredictable and
non-portable. Use "sysseek" instead.
Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever
you switch between reading and writing. Amongst other things, this may have the
effect of calling stdio's clearerr(3). A WHENCE of 1 ("SEEK_CUR") is useful for not
moving the file position:
seek($fh, 0, 1);
This is also useful for applications emulating "tail -f". Once you hit EOF on your
read and then sleep for a while, you (probably) have to stick in a dummy "seek" to
reset things. The "seek" doesn't change the position, but it does clear the end-of-
file condition on the handle, so that the next "readline FILE" makes Perl try again to
read something. (We hope.)
If that doesn't work (some I/O implementations are particularly cantankerous), you
might need something like this:
for (;;) {
for ($curpos = tell($fh); $_ = readline($fh);
$curpos = tell($fh)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek($fh, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine on DIRHANDLE. POS must be a value
returned by "telldir". "seekdir" also has the same caveats about possible directory
compaction as the corresponding system library routine.
select FILEHANDLE
select
Returns the currently selected filehandle. If FILEHANDLE is supplied, sets the new
current default filehandle for output. This has two effects: first, a "write" or a
"print" without a filehandle default to this FILEHANDLE. Second, references to
variables related to output will refer to this output channel.
For example, to set the top-of-form format for more than one output channel, you might
do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle.
Thus:
my $oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods,
preferring to write the last example as:
STDERR->autoflush(1);
(Prior to Perl version 5.14, you have to "use IO::Handle;" explicitly first.)
Portability issues: "select" in perlport.
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) syscall with the bit masks specified, which can be
constructed using "fileno" and "vec", along these lines:
my $rin = my $win = my $ein = '';
vec($rin, fileno(STDIN), 1) = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles, you may wish to write a subroutine like
this:
sub fhbits {
my @fhlist = @_;
my $bits = "";
for my $fh (@fhlist) {
vec($bits, fileno($fh), 1) = 1;
}
return $bits;
}
my $rin = fhbits(\*STDIN, $tty, $mysock);
The usual idiom is:
my ($nfound, $timeleft) =
select(my $rout = $rin, my $wout = $win, my $eout = $ein,
$timeout);
or to block until something becomes ready just do this
my $nfound =
select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);
Most systems do not bother to return anything useful in $timeleft, so calling "select"
in scalar context just returns $nfound.
Any of the bit masks can also be "undef". The timeout, if specified, is in seconds,
which may be fractional. Note: not all implementations are capable of returning the
$timeleft. If not, they always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after signals (say, SIGALRM) is
implementation-dependent. See also perlport for notes on the portability of "select".
On error, "select" behaves just like select(2): it returns "-1" and sets $!.
On some Unixes, select(2) may report a socket file descriptor as "ready for reading"
even when no data is available, and thus any subsequent "read" would block. This can
be avoided if you always use "O_NONBLOCK" on the socket. See select(2) and fcntl(2)
for further details.
The standard "IO::Select" module provides a user-friendlier interface to "select",
mostly because it does all the bit-mask work for you.
WARNING: One should not attempt to mix buffered I/O (like "read" or "readline") with
"select", except as permitted by POSIX, and even then only on POSIX systems. You have
to use "sysread" instead.
Portability issues: "select" in perlport.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl(2). You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG
must be a variable that will hold the returned semid_ds structure or semaphore value
array. Returns like "ioctl": the undefined value for error, ""0 but true"" for zero,
or the actual return value otherwise. The ARG must consist of a vector of native
short integers, which may be created with "pack("s!",(0)x$nsem)". See also "SysV IPC"
in perlipc and the documentation for "IPC::SysV" and "IPC::Semaphore".
Portability issues: "semctl" in perlport.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined
value on error. See also "SysV IPC" in perlipc and the documentation for "IPC::SysV"
and "IPC::Semaphore".
Portability issues: "semget" in perlport.
semop KEY,OPSTRING
Calls the System V IPC function semop(2) for semaphore operations such as signalling
and waiting. OPSTRING must be a packed array of semop structures. Each semop
structure can be generated with "pack("s!3", $semnum, $semop, $semflag)". The length
of OPSTRING implies the number of semaphore operations. Returns true if successful,
false on error. As an example, the following code waits on semaphore $semnum of
semaphore id $semid:
my $semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See also "SysV IPC" in perlipc and the
documentation for "IPC::SysV" and "IPC::Semaphore".
Portability issues: "semop" in perlport.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
filehandle. Takes the same flags as the system call of the same name. On unconnected
sockets, you must specify a destination to send to, in which case it does a sendto(2)
syscall. Returns the number of characters sent, or the undefined value on error. The
sendmsg(2) syscall is currently unimplemented. See "UDP: Message Passing" in perlipc
for examples.
Note that if the socket has been marked as ":utf8", "send" will throw an exception.
The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See "binmode".
setpgrp PID,PGRP
Sets the current process group for the specified PID, 0 for the current process.
Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or
BSD setpgrp(2). If the arguments are omitted, it defaults to "0,0". Note that the
BSD 4.2 version of "setpgrp" does not accept any arguments, so only "setpgrp(0,0)" is
portable. See also "POSIX::setsid()".
Portability issues: "setpgrp" in perlport.
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user. (See
setpriority(2).) Raises an exception when used on a machine that doesn't implement
setpriority(2).
"WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER" imported from
"RESOURCE CONSTANTS" in POSIX.
Portability issues: "setpriority" in perlport.
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns "undef" on error. Use integer constants
provided by the "Socket" module for LEVEL and OPNAME. Values for LEVEL can also be
obtained from getprotobyname. OPTVAL might either be a packed string or an integer.
An integer OPTVAL is shorthand for pack("i", OPTVAL).
An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
Portability issues: "setsockopt" in perlport.
shift ARRAY
shift
Shifts the first value of the array off and returns it, shortening the array by 1 and
moving everything down. If there are no elements in the array, returns the undefined
value. If ARRAY is omitted, shifts the @_ array within the lexical scope of
subroutines and formats, and the @ARGV array outside a subroutine and also within the
lexical scopes established by the "eval STRING", "BEGIN {}", "INIT {}", "CHECK {}",
"UNITCHECK {}", and "END {}" constructs.
Starting with Perl 5.14, an experimental feature allowed "shift" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
See also "unshift", "push", and "pop". "shift" and "unshift" do the same thing to the
left end of an array that "pop" and "push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be
a variable that will hold the returned "shmid_ds" structure. Returns like ioctl:
"undef" for error; "0 but true" for zero; and the actual return value otherwise. See
also "SysV IPC" in perlipc and the documentation for "IPC::SysV".
Portability issues: "shmctl" in perlport.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory segment id, or
"undef" on error. See also "SysV IPC" in perlipc and the documentation for
"IPC::SysV".
Portability issues: "shmget" in perlport.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at position POS for
size SIZE by attaching to it, copying in/out, and detaching from it. When reading,
VAR must be a variable that will hold the data read. When writing, if STRING is too
long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes. Return true if successful, false on error. "shmread" taints the
variable. See also "SysV IPC" in perlipc and the documentation for "IPC::SysV" and
the "IPC::Shareable" module from CPAN.
Portability issues: "shmread" in perlport and "shmwrite" in perlport.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which has the same
interpretation as in the syscall of the same name.
shutdown($socket, 0); # I/we have stopped reading data
shutdown($socket, 1); # I/we have stopped writing data
shutdown($socket, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other side you're done writing
but not done reading, or vice versa. It's also a more insistent form of close because
it also disables the file descriptor in any forked copies in other processes.
Returns 1 for success; on error, returns "undef" if the first argument is not a valid
filehandle, or returns 0 and sets $! for any other failure.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of
$_.
For the inverse sine operation, you may use the "Math::Trig::asin" function, or use
this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep
Causes the script to sleep for (integer) EXPR seconds, or forever if no argument is
given. Returns the integer number of seconds actually slept.
EXPR should be a positive integer. If called with a negative integer, "sleep" does not
sleep but instead emits a warning, sets $! ("errno"), and returns zero.
"sleep 0" is permitted, but a function call to the underlying platform implementation
still occurs, with any side effects that may have. "sleep 0" is therefore not exactly
identical to not sleeping at all.
May be interrupted if the process receives a signal such as "SIGALRM".
eval {
local $SIG{ALRM} = sub { die "Alarm!\n" };
sleep;
};
die $@ unless $@ eq "Alarm!\n";
You probably cannot mix "alarm" and "sleep" calls, because "sleep" is often
implemented using "alarm".
On some older systems, it may sleep up to a full second less than what you requested,
depending on how it counts seconds. Most modern systems always sleep the full amount.
They may appear to sleep longer than that, however, because your process might not be
scheduled right away in a busy multitasking system.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN,
and starting from Perl 5.8 part of the standard distribution) provides "usleep". You
may also use Perl's four-argument version of "select" leaving the first three
arguments undefined, or you might be able to use the "syscall" interface to access
setitimer(2) if your system supports it. See perlfaq8 for details.
See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN,
TYPE, and PROTOCOL are specified the same as for the syscall of the same name. You
should "use Socket" first to get the proper definitions imported. See the examples in
"Sockets: Client/Server Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the
newly opened file descriptor, as determined by the value of $^F. See "$^F" in
perlvar.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified domain, of the specified type.
DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name.
If unimplemented, raises an exception. Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will be set for the
newly opened file descriptors, as determined by the value of $^F. See "$^F" in
perlvar.
Some systems define "pipe" in terms of "socketpair", in which a call to "pipe($rdr,
$wtr)" is essentially:
use Socket;
socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown($rdr, 1); # no more writing for reader
shutdown($wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl 5.8 and later will emulate
socketpair using IP sockets to localhost if your system implements sockets but not
socketpair.
Portability issues: "socketpair" in perlport.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the sorted list value. In scalar
context, the behaviour of "sort" is undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard string comparison order. If
SUBNAME is specified, it gives the name of a subroutine that returns an integer less
than, equal to, or greater than 0, depending on how the elements of the list are to be
ordered. (The "<=>" and "cmp" operators are extremely useful in such routines.)
SUBNAME may be a scalar variable name (unsubscripted), in which case the value
provides the name of (or a reference to) the actual subroutine to use. In place of a
SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.
If the subroutine's prototype is "($$)", the elements to be compared are passed by
reference in @_, as for a normal subroutine. This is slower than unprototyped
subroutines, where the elements to be compared are passed into the subroutine as the
package global variables $a and $b (see example below).
If the subroutine is an XSUB, the elements to be compared are pushed on to the stack,
the way arguments are usually passed to XSUBs. $a and $b are not set.
The values to be compared are always passed by reference and should not be modified.
You also cannot exit out of the sort block or subroutine using any of the loop control
operators described in perlsyn or with "goto".
When "use locale" (but not "use locale ':not_characters'") is in effect, "sort LIST"
sorts LIST according to the current collation locale. See perllocale.
"sort" returns aliases into the original list, much as a for loop's index variable
aliases the list elements. That is, modifying an element of a list returned by "sort"
(for example, in a "foreach", "map" or "grep") actually modifies the element in the
original list. This is usually something to be avoided when writing clear code.
Historically Perl has varied in whether sorting is stable by default. If stability
matters, it can be controlled explicitly by using the sort pragma.
Examples:
# sort lexically
my @articles = sort @files;
# same thing, but with explicit sort routine
my @articles = sort {$a cmp $b} @files;
# now case-insensitively
my @articles = sort {fc($a) cmp fc($b)} @files;
# same thing in reversed order
my @articles = sort {$b cmp $a} @files;
# sort numerically ascending
my @articles = sort {$a <=> $b} @files;
# sort numerically descending
my @articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
my @sortedclass = sort byage @class;
sub backwards { $b cmp $a }
my @harry = qw(dog cat x Cain Abel);
my @george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
my @new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
fc($a) cmp fc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my (@nums, @caps);
for (@old) {
push @nums, ( /=(\d+)/ ? $1 : undef );
push @caps, fc($_);
}
my @new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
my @new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, fc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package Other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
# not set here
package main;
my @new = sort Other::backwards @old;
# guarantee stability
use sort 'stable';
my @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
Warning: syntactical care is required when sorting the list returned from a function.
If you want to sort the list returned by the function call "find_records(@key)", you
can use:
my @contact = sort { $a cmp $b } find_records @key;
my @contact = sort +find_records(@key);
my @contact = sort &find_records(@key);
my @contact = sort(find_records(@key));
If instead you want to sort the array @key with the comparison routine
"find_records()" then you can use:
my @contact = sort { find_records() } @key;
my @contact = sort find_records(@key);
my @contact = sort(find_records @key);
my @contact = sort(find_records (@key));
$a and $b are set as package globals in the package the sort() is called from. That
means $main::a and $main::b (or $::a and $::b) in the "main" package, $FooPack::a and
$FooPack::b in the "FooPack" package, etc. If the sort block is in scope of a "my" or
"state" declaration of $a and/or $b, you must spell out the full name of the variables
in the sort block :
package main;
my $a = "C"; # DANGER, Will Robinson, DANGER !!!
print sort { $a cmp $b } qw(A C E G B D F H);
# WRONG
sub badlexi { $a cmp $b }
print sort badlexi qw(A C E G B D F H);
# WRONG
# the above prints BACFEDGH or some other incorrect ordering
print sort { $::a cmp $::b } qw(A C E G B D F H);
# OK
print sort { our $a cmp our $b } qw(A C E G B D F H);
# also OK
print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
# also OK
sub lexi { our $a cmp our $b }
print sort lexi qw(A C E G B D F H);
# also OK
# the above print ABCDEFGH
With proper care you may mix package and my (or state) $a and/or $b:
my $a = {
tiny => -2,
small => -1,
normal => 0,
big => 1,
huge => 2
};
say sort { $a->{our $a} <=> $a->{our $b} }
qw{ huge normal tiny small big};
# prints tinysmallnormalbighuge
$a and $b are implicitly local to the sort() execution and regain their former values
upon completing the sort.
Sort subroutines written using $a and $b are bound to their calling package. It is
possible, but of limited interest, to define them in a different package, since the
subroutine must still refer to the calling package's $a and $b :
package Foo;
sub lexi { $Bar::a cmp $Bar::b }
package Bar;
... sort Foo::lexi ...
Use the prototyped versions (see above) for a more generic alternative.
The comparison function is required to behave. If it returns inconsistent results
(sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for
example) the results are not well-defined.
Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), be careful
when sorting with a comparison function like "$a <=> $b" any lists that might contain
a "NaN". The following example takes advantage that "NaN != NaN" to eliminate any
"NaN"s from the input list.
my @result = sort { $a <=> $b } grep { $_ == $_ } @input;
In this version of perl, the "sort" function is implemented via the mergesort
algorithm.
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them
with the elements of LIST, if any. In list context, returns the elements removed from
the array. In scalar context, returns the last element removed, or "undef" if no
elements are removed. The array grows or shrinks as necessary. If OFFSET is negative
then it starts that far from the end of the array. If LENGTH is omitted, removes
everything from OFFSET onward. If LENGTH is negative, removes the elements from
OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and
LENGTH are omitted, removes everything. If OFFSET is past the end of the array and a
LENGTH was provided, Perl issues a warning, and splices at the end of the array.
The following equivalences hold (assuming "$#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
"splice" can be used, for example, to implement n-ary queue processing:
sub nary_print {
my $n = shift;
while (my @next_n = splice @_, 0, $n) {
say join q{ -- }, @next_n;
}
}
nary_print(3, qw(a b c d e f g h));
# prints:
# a -- b -- c
# d -- e -- f
# g -- h
Starting with Perl 5.14, an experimental feature allowed "splice" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split
Splits the string EXPR into a list of strings and returns the list in list context, or
the size of the list in scalar context. (Prior to Perl 5.11, it also overwrote @_
with the list in void and scalar context. If you target old perls, beware.)
If only PATTERN is given, EXPR defaults to $_.
Anything in EXPR that matches PATTERN is taken to be a separator that separates the
EXPR into substrings (called "fields") that do not include the separator. Note that a
separator may be longer than one character or even have no characters at all (the
empty string, which is a zero-width match).
The PATTERN need not be constant; an expression may be used to specify a pattern that
varies at runtime.
If PATTERN matches the empty string, the EXPR is split at the match position (between
characters). As an example, the following:
my @x = split(/b/, "abc"); # ("a", "c")
uses the "b" in 'abc' as a separator to produce the list ("a", "c"). However, this:
my @x = split(//, "abc"); # ("a", "b", "c")
uses empty string matches as separators; thus, the empty string may be used to split
EXPR into a list of its component characters.
As a special case for "split", the empty pattern given in match operator syntax ("//")
specifically matches the empty string, which is contrary to its usual interpretation
as the last successful match.
If PATTERN is "/^/", then it is treated as if it used the multiline modifier ("/^/m"),
since it isn't much use otherwise.
"/m" and any of the other pattern modifiers valid for "qr" (summarized in
"qr/STRING/msixpodualn" in perlop) may be specified explicitly.
As another special case, "split" emulates the default behavior of the command line
tool awk when the PATTERN is either omitted or a string composed of a single space
character (such as '' or "\x20", but not e.g. "//"). In this case, any leading
whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead
treated as if it were "/\s+/"; in particular, this means that any contiguous
whitespace (not just a single space character) is used as a separator.
my @x = split(" ", " Quick brown fox\n");
# ("Quick", "brown", "fox")
my @x = split(" ", "RED\tGREEN\tBLUE");
# ("RED", "GREEN", "BLUE")
Using split in this fashion is very similar to how "qw//" works.
However, this special treatment can be avoided by specifying the pattern "//" instead
of the string "", thereby allowing only a single space character to be a separator.
In earlier Perls this special case was restricted to the use of a plain "" as the
pattern argument to split; in Perl 5.18.0 and later this special case is triggered by
any expression which evaluates to the simple string "".
As of Perl 5.28, this special-cased whitespace splitting works as expected in the
scope of "usefeature'unicode_strings'". In previous versions, and outside the scope of
that feature, it exhibits "The "Unicode Bug"" in perlunicode: characters that are
whitespace according to Unicode rules but not according to ASCII rules can be treated
as part of fields rather than as field separators, depending on the string's internal
encoding.
If omitted, PATTERN defaults to a single space, "", triggering the previously
described awk emulation.
If LIMIT is specified and positive, it represents the maximum number of fields into
which the EXPR may be split; in other words, LIMIT is one greater than the maximum
number of times EXPR may be split. Thus, the LIMIT value 1 means that EXPR may be
split a maximum of zero times, producing a maximum of one field (namely, the entire
value of EXPR). For instance:
my @x = split(//, "abc", 1); # ("abc")
my @x = split(//, "abc", 2); # ("a", "bc")
my @x = split(//, "abc", 3); # ("a", "b", "c")
my @x = split(//, "abc", 4); # ("a", "b", "c")
If LIMIT is negative, it is treated as if it were instead arbitrarily large; as many
fields as possible are produced.
If LIMIT is omitted (or, equivalently, zero), then it is usually treated as if it were
instead negative but with the exception that trailing empty fields are stripped (empty
leading fields are always preserved); if all fields are empty, then all fields are
considered to be trailing (and are thus stripped in this case). Thus, the following:
my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c")
produces only a three element list.
my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "")
produces a six element list.
In time-critical applications, it is worthwhile to avoid splitting into more fields
than necessary. Thus, when assigning to a list, if LIMIT is omitted (or zero), then
LIMIT is treated as though it were one larger than the number of variables in the
list; for the following, LIMIT is implicitly 3:
my ($login, $passwd) = split(/:/);
Note that splitting an EXPR that evaluates to the empty string always produces zero
fields, regardless of the LIMIT specified.
An empty leading field is produced when there is a positive-width match at the
beginning of EXPR. For instance:
my @x = split(/ /, " abc"); # ("", "abc")
splits into two elements. However, a zero-width match at the beginning of EXPR never
produces an empty field, so that:
my @x = split(//, " abc"); # (" ", "a", "b", "c")
splits into four elements instead of five.
An empty trailing field, on the other hand, is produced when there is a match at the
end of EXPR, regardless of the length of the match (of course, unless a non-zero LIMIT
is given explicitly, such fields are removed, as in the last example). Thus:
my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "")
If the PATTERN contains capturing groups, then for each separator, an additional field
is produced for each substring captured by a group (in the order in which the groups
are specified, as per backreferences); if any group does not match, then it captures
the "undef" value instead of a substring. Also, note that any such additional field
is produced whenever there is a separator (that is, whenever a split occurs), and such
an additional field does not count towards the LIMIT. Consider the following
expressions evaluated in list context (each returned list is provided in the
associated comment):
my @x = split(/-|,/ , "1-10,20", 3);
# ("1", "10", "20")
my @x = split(/(-|,)/ , "1-10,20", 3);
# ("1", "-", "10", ",", "20")
my @x = split(/-|(,)/ , "1-10,20", 3);
# ("1", undef, "10", ",", "20")
my @x = split(/(-)|,/ , "1-10,20", 3);
# ("1", "-", "10", undef, "20")
my @x = split(/(-)|(,)/, "1-10,20", 3);
# ("1", "-", undef, "10", undef, ",", "20")
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf" conventions of the C library function
"sprintf". See below for more details and see sprintf(3) or printf(3) on your system
for an explanation of the general principles.
For example:
# Format number with up to 8 leading zeroes
my $result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
my $rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting: it emulates the C function sprintf(3), but
doesn't use it except for floating-point numbers, and even then only standard
modifiers are allowed. Non-standard extensions in your local sprintf(3) are therefore
unavailable from Perl.
Unlike "printf", "sprintf" does not do what you probably mean when you pass it an
array as your first argument. The array is given scalar context, and instead of using
the 0th element of the array as the format, Perl will use the count of elements in the
array as the format, which is almost never useful.
Perl's "sprintf" permits the following universally-known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next argument in the parameter list
%a hexadecimal floating point
%A like %a, but using upper-case letters
Finally, for backward (and we do mean "backward") compatibility, Perl permits these
unnecessary but widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the scientific notation produced by %e, %E,
%g and %G for numbers with the modulus of the exponent less than 100 is system-
dependent: it may be three or less (zero-padded as necessary). In other words, 1.23
times ten to the 99th may be either "1.23e99" or "1.23e099". Similarly for %a and %A:
the exponent or the hexadecimal digits may float: especially the "long doubles" Perl
configuration option may cause surprises.
Between the "%" and the format letter, you may specify several additional attributes
controlling the interpretation of the format. In order, these are:
format parameter index
An explicit format parameter index, such as "2$". By default sprintf will format
the next unused argument in the list, but this allows you to take the arguments
out of order:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix non-negative number with a space
+ prefix non-negative number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# ensure the leading "0" for any octal,
prefix non-zero hexadecimal with "0x" or "0X",
prefix non-zero binary with "0b" or "0B"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<% d>', 0; # prints "< 0>"
printf '<% d>', -12; # prints "<-12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%+d>', 0; # prints "<+0>"
printf '<%+d>', -12; # prints "<-12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#o>', 12; # prints "<014>"
printf '<%#x>', 12; # prints "<0xc>"
printf '<%#X>', 12; # prints "<0XC>"
printf '<%#b>', 12; # prints "<0b1100>"
printf '<%#B>', 12; # prints "<0B1100>"
When a space and a plus sign are given as the flags at once, the space is ignored.
printf '<%+ d>', 12; # prints "<+12>"
printf '<% +d>', 12; # prints "<+12>"
When the # flag and a precision are given in the %o conversion, the precision is
incremented if it's necessary for the leading "0".
printf '<%#.5o>', 012; # prints "<00012>"
printf '<%#.5o>', 012345; # prints "<012345>"
printf '<%#.0o>', 0; # prints "<0>"
vector flag
This flag tells Perl to interpret the supplied string as a vector of integers, one
for each character in the string. Perl applies the format to each integer in
turn, then joins the resulting strings with a separator (a dot "." by default).
This can be useful for displaying ordinal values of characters in arbitrary
strings:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override the string to use to separate the
numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument number to use for the join string
using something like "*2$v"; for example:
printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
@addr[1..3], ":";
(minimum) width
Arguments are usually formatted to be only as wide as required to display the
given value. You can override the width by putting a number here, or get the
width from the next argument (with "*") or from a specified argument (e.g., with
"*2$"):
printf "<%s>", "a"; # prints "<a>"
printf "<%6s>", "a"; # prints "< a>"
printf "<%*s>", 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf "<%2s>", "long"; # prints "<long>" (does not truncate)
If a field width obtained through "*" is negative, it has the same effect as the
"-" flag: left-justification.
precision, or maximum width
You can specify a precision (for numeric conversions) or a maximum width (for
string conversions) by specifying a "." followed by a number. For floating-point
formats except "g" and "G", this specifies how many places right of the decimal
point to show (the default being 6). For example:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For "g" and "G", this specifies the maximum number of significant digits to show;
for example:
# These examples are subject to system-specific variation.
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
printf '<%.1g>', 0.0111; # prints "<0.01>"
printf '<%.2g>', 0.0111; # prints "<0.011>"
printf '<%.3g>', 0.0111; # prints "<0.0111>"
For integer conversions, specifying a precision implies that the output of the
number itself should be zero-padded to this width, where the 0 flag is ignored:
printf '<%.6d>', 1; # prints "<000001>"
printf '<%+.6d>', 1; # prints "<+000001>"
printf '<%-10.6d>', 1; # prints "<000001 >"
printf '<%10.6d>', 1; # prints "< 000001>"
printf '<%010.6d>', 1; # prints "< 000001>"
printf '<%+10.6d>', 1; # prints "< +000001>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
printf '<%10.6x>', 1; # prints "< 000001>"
printf '<%010.6x>', 1; # prints "< 000001>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"
For string conversions, specifying a precision truncates the string to fit the
specified width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next argument using ".*", or from a
specified argument (e.g., with ".*2$"):
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
printf '<%.*2$x>', 1, 6; # prints "<000001>"
printf '<%6.*2$x>', 1, 4; # prints "< 0001>"
If a precision obtained through "*" is negative, it counts as having no precision
at all.
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*s>', 0, "string"; # prints "<>"
printf '<%.*s>', -1, "string"; # prints "<string>"
printf '<%.*d>', 1, 0; # prints "<0>"
printf '<%.*d>', 0, 0; # prints "<>"
printf '<%.*d>', -1, 0; # prints "<0>"
size
For numeric conversions, you can specify the size to interpret the number as using
"l", "h", "V", "q", "L", or "ll". For integer conversions ("d u o x X b i D U
O"), numbers are usually assumed to be whatever the default integer size is on
your platform (usually 32 or 64 bits), but you can override this to use instead
one of the standard C types, as supported by the compiler used to build Perl:
hh interpret integer as C type "char" or "unsigned
char" on Perl 5.14 or later
h interpret integer as C type "short" or
"unsigned short"
j interpret integer as C type "intmax_t" on Perl
5.14 or later; and prior to Perl 5.30, only with
a C99 compiler (unportable)
l interpret integer as C type "long" or
"unsigned long"
q, L, or ll interpret integer as C type "long long",
"unsigned long long", or "quad" (typically
64-bit integers)
t interpret integer as C type "ptrdiff_t" on Perl
5.14 or later
z interpret integer as C types "size_t" or
"ssize_t" on Perl 5.14 or later
Note that, in general, using the "l" modifier (for example, when writing "%ld" or
"%lu" instead of "%d" and "%u") is unnecessary when used from Perl code.
Moreover, it may be harmful, for example on Windows 64-bit where a long is
32-bits.
As of 5.14, none of these raises an exception if they are not supported on your
platform. However, if warnings are enabled, a warning of the "printf" warning
class is issued on an unsupported conversion flag. Should you instead prefer an
exception, do this:
use warnings FATAL => "printf";
If you would like to know about a version dependency before you start running the
program, put something like this at its top:
use 5.014; # for hh/j/t/z/ printf modifiers
You can find out whether your Perl supports quads via Config:
use Config;
if ($Config{use64bitint} eq "define"
|| $Config{longsize} >= 8) {
print "Nice quads!\n";
}
For floating-point conversions ("e f g E F G"), numbers are usually assumed to be
the default floating-point size on your platform (double or long double), but you
can force "long double" with "q", "L", or "ll" if your platform supports them.
You can find out whether your Perl supports long doubles via Config:
use Config;
print "long doubles\n" if $Config{d_longdbl} eq "define";
You can find out whether Perl considers "long double" to be the default floating-
point size to use on your platform via Config:
use Config;
if ($Config{uselongdouble} eq "define") {
print "long doubles by default\n";
}
It can also be that long doubles and doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl code, but is supported for
compatibility with XS code. It means "use the standard size for a Perl integer or
floating-point number", which is the default.
order of arguments
Normally, "sprintf" takes the next unused argument as the value to format for each
format specification. If the format specification uses "*" to require additional
arguments, these are consumed from the argument list in the order they appear in
the format specification before the value to format. Where an argument is
specified by an explicit index, this does not affect the normal order for the
arguments, even when the explicitly specified index would have been the next
argument.
So:
printf "<%*.*s>", $a, $b, $c;
uses $a for the width, $b for the precision, and $c as the value to format; while:
printf '<%*1$.*s>', $a, $b;
would use $a for the width and precision, and $b as the value to format.
Here are some more examples; be aware that when using an explicit index, the "$"
may need escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"
If "use locale" (including "use locale ':not_characters'") is in effect and
"POSIX::setlocale" has been called, the character used for the decimal separator in
formatted floating-point numbers is affected by the "LC_NUMERIC" locale. See
perllocale and POSIX.
sqrt EXPR
sqrt
Return the positive square root of EXPR. If EXPR is omitted, uses $_. Works only for
non-negative operands unless you've loaded the "Math::Complex" module.
use Math::Complex;
print sqrt(-4); # prints 2i
srand EXPR
srand
Sets and returns the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand" function so that "rand" can produce
a different sequence each time you run your program. When called with a parameter,
"srand" uses that for the seed; otherwise it (semi-)randomly chooses a seed. In
either case, starting with Perl 5.14, it returns the seed. To signal that your code
will work only on Perls of a recent vintage:
use 5.014; # so srand returns the seed
If "srand" is not called explicitly, it is called implicitly without a parameter at
the first use of the "rand" operator. However, there are a few situations where
programs are likely to want to call "srand". One is for generating predictable
results, generally for testing or debugging. There, you use "srand($seed)", with the
same $seed each time. Another case is that you may want to call "srand" after a
"fork" to avoid child processes sharing the same seed value as the parent (and
consequently each other).
Do not call "srand()" (i.e., without an argument) more than once per process. The
internal state of the random number generator should contain more entropy than can be
provided by any seed, so calling "srand" again actually loses randomness.
Most implementations of "srand" take an integer and will silently truncate decimal
numbers. This means "srand(42)" will usually produce the same results as
"srand(42.1)". To be safe, always pass "srand" an integer.
A typical use of the returned seed is for a test program which has too many
combinations to test comprehensively in the time available to it each run. It can
test a random subset each time, and should there be a failure, log the seed used for
that run so that it can later be used to reproduce the same results.
"rand" is not cryptographically secure. You should not rely on it in security-
sensitive situations. As of this writing, a number of third-party CPAN modules offer
random number generators intended by their authors to be cryptographically secure,
including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.
stat FILEHANDLE
stat EXPR
stat DIRHANDLE
stat
Returns a 13-element list giving the status info for a file, either the file opened
via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats $_ (not
"_"!). Returns the empty list if "stat" fails. Typically used as follows:
my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are the meanings of the
fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred I/O size in bytes for interacting with the
file (may vary from file to file)
12 blocks actual number of system-specific blocks allocated
on disk (often, but not always, 512 bytes each)
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem types. Notably, the ctime field is
non-portable. In particular, you cannot expect it to be a "creation time"; see "Files
and Filesystems" in perlport for details.
If "stat" is passed the special filehandle consisting of an underline, no stat is
done, but the current contents of the stat structure from the last "stat", "lstat", or
filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is negative under NFS.)
On some platforms inode numbers are of a type larger than perl knows how to handle as
integer numerical values. If necessary, an inode number will be returned as a decimal
string in order to preserve the entire value. If used in a numeric context, this will
be converted to a floating-point numerical value, with rounding, a fate that is best
avoided. Therefore, you should prefer to compare inode numbers using "eq" rather than
"==". "eq" will work fine on inode numbers that are represented numerically, as well
as those represented as strings.
Because the mode contains both the file type and its permissions, you should mask off
the file type portion and (s)printf using a "%o" if you want to see the real
permissions.
my $mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value indicating success or failure, and,
if successful, sets the information associated with the special filehandle "_".
The File::stat module provides a convenient, by-name access mechanism:
use File::stat;
my $sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and functions ("S_IS*") from the
Fcntl module:
use Fcntl ':mode';
my $mode = (stat($filename))[2];
my $user_rwx = ($mode & S_IRWXU) >> 6;
my $group_read = ($mode & S_IRGRP) >> 3;
my $other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
my $is_setuid = $mode & S_ISUID;
my $is_directory = S_ISDIR($mode);
You could write the last two using the "-u" and "-d" operators. Commonly available
"S_IF*" constants are:
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system-dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not all are necessarily available on
# your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR,
# S_IWUSR, and S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the "S_IF*" functions are
S_IMODE($mode) the part of $mode containing the permission
bits and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with (for example)
S_IFREG or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details about the "S_*"
constants. To get status info for a symbolic link instead of the target file behind
the link, use the "lstat" function.
Portability issues: "stat" in perlport.
state VARLIST
state TYPE VARLIST
state VARLIST : ATTRS
state TYPE VARLIST : ATTRS
"state" declares a lexically scoped variable, just like "my". However, those
variables will never be reinitialized, contrary to lexical variables that are
reinitialized each time their enclosing block is entered. See "Persistent Private
Variables" in perlsub for details.
If more than one variable is listed, the list must be placed in parentheses. With a
parenthesised list, "undef" can be used as a dummy placeholder. However, since
initialization of state variables in such lists is currently not possible this would
serve no purpose.
Redeclaring a variable in the same scope or statement will "shadow" the previous
declaration, creating a new instance and preventing access to the previous one. This
is usually undesired and, if warnings are enabled, will result in a warning in the
"shadow" category.
"state" is available only if the "state" feature is enabled or if it is prefixed with
"CORE::". The "state" feature is enabled automatically with a "use v5.10" (or higher)
declaration in the current scope.
study SCALAR
study
At this time, "study" does nothing. This may change in the future.
Prior to Perl version 5.16, it would create an inverted index of all characters that
occurred in the given SCALAR (or $_ if unspecified). When matching a pattern, the
rarest character from the pattern would be looked up in this index. Rarity was based
on some static frequency tables constructed from some C programs and English text.
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function per se. Without a BLOCK it's just
a forward declaration. Without a NAME, it's an anonymous function declaration, so
does return a value: the CODE ref of the closure just created.
See perlsub and perlref for details about subroutines and references; see attributes
and Attribute::Handlers for more information about attributes.
__SUB__
A special token that returns a reference to the current subroutine, or "undef" outside
of a subroutine.
The behaviour of "__SUB__" within a regex code block (such as "/(?{...})/") is subject
to change.
This token is only available under "use v5.16" or the "current_sub" feature. See
feature.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First character is at offset zero.
If OFFSET is negative, starts that far back from the end of the string. If LENGTH is
omitted, returns everything through the end of the string. If LENGTH is negative,
leaves that many characters off the end of the string.
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # tr
You can use the "substr" function as an lvalue, in which case EXPR must itself be an
lvalue. If you assign something shorter than LENGTH, the string will shrink, and if
you assign something longer than LENGTH, the string will grow to accommodate it. To
keep the string the same length, you may need to pad or chop your value using
"sprintf".
If OFFSET and LENGTH specify a substring that is partly outside the string, only the
part within the string is returned. If the substring is beyond either end of the
string, "substr" returns the undefined value and produces a warning. When used as an
lvalue, specifying a substring that is entirely outside the string raises an
exception. Here's an example showing the behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exception
An alternative to using "substr" as an lvalue is to specify the replacement string as
the 4th argument. This allows you to replace parts of the EXPR and return what was
there before in one operation, just as you can with "splice".
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"
Note that the lvalue returned by the three-argument version of "substr" acts as a
'magic bullet'; each time it is assigned to, it remembers which part of the original
string is being modified; for example:
my $x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}
With negative offsets, it remembers its position from the end of the string when the
target string is modified:
my $x = '1234';
for (substr($x, -3, 2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4, as above
$x = 'abcdefg';
print $_,"\n"; # prints f
}
Prior to Perl version 5.10, the result of using an lvalue multiple times was
unspecified. Prior to 5.16, the result with negative offsets was unspecified.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename. Returns 1 for
success, 0 otherwise. On systems that don't support symbolic links, raises an
exception. To check for that, use eval:
my $symlink_exists = eval { symlink("",""); 1 };
Portability issues: "symlink" in perlport.
syscall NUMBER, LIST
Calls the system call specified as the first element of the list, passing the
remaining elements as arguments to the system call. If unimplemented, raises an
exception. The arguments are interpreted as follows: if a given argument is numeric,
the argument is passed as an int. If not, the pointer to the string value is passed.
You are responsible to make sure a string is pre-extended long enough to receive any
result that might be written into a string. You can't use a string literal (or other
read-only string) as an argument to "syscall" because Perl has to assume that any
string pointer might be written through. If your integer arguments are not literals
and have never been interpreted in a numeric context, you may need to add 0 to them to
force them to look like numbers. This emulates the "syswrite" function (or vice
versa):
require 'syscall.ph'; # may need to run h2ph
my $s = "hi there\n";
syscall(SYS_write(), fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your syscall, which in
practice should (usually) suffice.
Syscall returns whatever value returned by the system call it calls. If the system
call fails, "syscall" returns "-1" and sets $! (errno). Note that some system calls
can legitimately return "-1". The proper way to handle such calls is to assign "$! =
0" before the call, then check the value of $! if "syscall" returns "-1".
There's a problem with "syscall(SYS_pipe())": it returns the file number of the read
end of the pipe it creates, but there is no way to retrieve the file number of the
other end. You can avoid this problem by using "pipe" instead.
Portability issues: "syscall" in perlport.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE.
If FILEHANDLE is an expression, its value is used as the real filehandle wanted; an
undefined scalar will be suitably autovivified. This function calls the underlying
operating system's open(2) function with the parameters FILENAME, MODE, and PERMS.
Returns true on success and "undef" otherwise.
PerlIO layers will be applied to the handle the same way they would in an "open" call
that does not specify layers. That is, the current value of "${^OPEN}" as set by the
open pragma in a lexical scope, or the "-C" commandline option or "PERL_UNICODE"
environment variable in the main program scope, falling back to the platform defaults
as described in "Defaults and how to override them" in PerlIO. If you want to remove
any layers that may transform the byte stream, use "binmode" after opening it.
The possible values and flag bits of the MODE parameter are system-dependent; they are
available via the standard module "Fcntl". See the documentation of your operating
system's open(2) syscall to see which values and flag bits are available. You may
combine several flags using the "|"-operator.
Some of the most common values are "O_RDONLY" for opening the file in read-only mode,
"O_WRONLY" for opening the file in write-only mode, and "O_RDWR" for opening the file
in read-write mode.
For historical reasons, some values work on almost every system supported by Perl: 0
means read-only, 1 means write-only, and 2 means read/write. We know that these
values do not work under OS/390 and on the Macintosh; you probably don't want to use
them in new code.
If the file named by FILENAME does not exist and the "open" call creates it (typically
because MODE includes the "O_CREAT" flag), then the value of PERMS specifies the
permissions of the newly created file. If you omit the PERMS argument to "sysopen",
Perl uses the octal value 0666. These permission values need to be in octal, and are
modified by your process's current "umask".
In many systems the "O_EXCL" flag is available for opening files in exclusive mode.
This is not locking: exclusiveness means here that if the file already exists,
"sysopen" fails. "O_EXCL" may not work on network filesystems, and has no effect
unless the "O_CREAT" flag is set as well. Setting "O_CREAT|O_EXCL" prevents the file
from being opened if it is a symbolic link. It does not protect against symbolic
links in the file's path.
Sometimes you may want to truncate an already-existing file. This can be done using
the "O_TRUNC" flag. The behavior of "O_TRUNC" with "O_RDONLY" is undefined.
You should seldom if ever use 0644 as argument to "sysopen", because that takes away
the user's option to have a more permissive umask. Better to omit it. See "umask"
for more on this.
This function has no direct relation to the usage of "sysread", "syswrite", or
"sysseek". A handle opened with this function can be used with buffered IO just as
one opened with "open" can be used with unbuffered IO.
Note that under Perls older than 5.8.0, "sysopen" depends on the fdopen(3) C library
function. On many Unix systems, fdopen(3) is known to fail when file descriptors
exceed a certain value, typically 255. If you need more file descriptors than that,
consider using the "POSIX::open" function. For Perls 5.8.0 and later, PerlIO is (most
often) the default.
See perlopentut for a kinder, gentler explanation of opening files.
Portability issues: "sysopen" in perlport.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from the specified
FILEHANDLE, using read(2). It bypasses any PerlIO layers including buffered IO (but
is affected by the presence of the ":utf8" layer as described later), so mixing this
with other kinds of reads, "print", "write", "seek", "tell", or "eof" can cause
confusion because the ":perlio" or ":crlf" layers usually buffer data. Returns the
number of bytes actually read, 0 at end of file, or undef if there was an error (in
the latter case $! is also set). SCALAR will be grown or shrunk so that the last byte
actually read is the last byte of the scalar after the read.
An OFFSET may be specified to place the read data at some place in the string other
than the beginning. A negative OFFSET specifies placement at that many characters
counting backwards from the end of the string. A positive OFFSET greater than the
length of SCALAR results in the string being padded to the required size with "\0"
bytes before the result of the read is appended.
There is no syseof() function, which is ok, since "eof" doesn't work well on device
files (like ttys) anyway. Use "sysread" and check for a return value of 0 to decide
whether you're done.
Note that if the filehandle has been marked as ":utf8", "sysread" will throw an
exception. The ":encoding(...)" layer implicitly introduces the ":utf8" layer. See
"binmode", "open", and the open pragma.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may be an
expression whose value gives the name of the filehandle. The values for WHENCE are 0
to set the new position to POSITION; 1 to set it to the current position plus
POSITION; and 2 to set it to EOF plus POSITION, typically negative.
Note the emphasis on bytes: even if the filehandle has been set to operate on
characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
and "sysseek" family of functions use byte offsets, not character offsets, because
seeking to a character offset would be very slow in a UTF-8 file.
"sysseek" bypasses normal buffered IO, so mixing it with reads other than "sysread"
(for example "readline" or "read"), "print", "write", "seek", "tell", or "eof" may
cause confusion.
For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END"
(start of the file, current position, end of the file) from the Fcntl module. Use of
the constants is also more portable than relying on 0, 1, and 2. For example to
define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on failure. A position of zero is
returned as the string "0 but true"; thus "sysseek" returns true on success and false
on failure, yet you can still easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec", except that a fork is done first and the parent
process waits for the child process to exit. Note that argument processing varies
depending on the number of arguments. If there is more than one argument in LIST, or
if LIST is an array with more than one value, starts the program given by the first
element of the list with arguments given by the rest of the list. If there is only
one scalar argument, the argument is checked for shell metacharacters, and if there
are any, the entire argument is passed to the system's command shell for parsing (this
is "/bin/sh -c" on Unix platforms, but varies on other platforms). If there are no
shell metacharacters in the argument, it is split into words and passed directly to
"execvp", which is more efficient. On Windows, only the "system PROGRAM LIST" syntax
will reliably avoid using the shell; "system LIST", even with more than one element,
will fall back to the shell if the first spawn fails.
Perl will attempt to flush all files opened for output before any operation that may
do a fork, but this may not be supported on some platforms (see perlport). To be
safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of
"IO::Handle" on any open handles.
The return value is the exit status of the program as returned by the "wait" call. To
get the actual exit value, shift right by eight (see below). See also "exec". This
is not what you want to use to capture the output from a command; for that you should
use merely backticks or "qx//", as described in "`STRING`" in perlop. Return value of
-1 indicates a failure to start the program or an error of the wait(2) system call
(inspect $! for the reason).
If you'd like to make "system" (and many other bits of Perl) die on error, have a look
at the autodie pragma.
Like "exec", "system" allows you to lie to a program about its name if you use the
"system PROGRAM LIST" syntax. Again, see "exec".
Since "SIGINT" and "SIGQUIT" are ignored during the execution of "system", if you
expect your program to terminate on receipt of these signals you will need to arrange
to do so yourself based on the return value.
my @args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?";
If you'd like to manually inspect "system"'s failure, you can check all possible
failure modes by inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
Alternatively, you may inspect the value of "${^CHILD_ERROR_NATIVE}" with the "W*()"
calls from the POSIX module.
When "system"'s arguments are executed indirectly by the shell, results and return
codes are subject to its quirks. See "`STRING`" in perlop and "exec" for details.
Since "system" does a "fork" and "wait" it may affect a "SIGCHLD" handler. See
perlipc for details.
Portability issues: "system" in perlport.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable SCALAR to the specified
FILEHANDLE, using write(2). If LENGTH is not specified, writes whole SCALAR. It
bypasses any PerlIO layers including buffered IO (but is affected by the presence of
the ":utf8" layer as described later), so mixing this with reads (other than
"sysread)"), "print", "write", "seek", "tell", or "eof" may cause confusion because
the ":perlio" and ":crlf" layers usually buffer data. Returns the number of bytes
actually written, or "undef" if there was an error (in this case the errno variable $!
is also set). If the LENGTH is greater than the data available in the SCALAR after
the OFFSET, only as much data as is available will be written.
An OFFSET may be specified to write the data from some part of the string other than
the beginning. A negative OFFSET specifies writing that many characters counting
backwards from the end of the string. If SCALAR is of length zero, you can only use
an OFFSET of 0.
WARNING: If the filehandle is marked ":utf8", "syswrite" will raise an exception. The
":encoding(...)" layer implicitly introduces the ":utf8" layer. Alternately, if the
handle is not marked with an encoding but you attempt to write characters with code
points over 255, raises an exception. See "binmode", "open", and the open pragma.
tell FILEHANDLE
tell
Returns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may
be an expression whose value gives the name of the actual filehandle. If FILEHANDLE
is omitted, assumes the file last read.
Note the emphasis on bytes: even if the filehandle has been set to operate on
characters (for example using the ":encoding(UTF-8)" I/O layer), the "seek", "tell",
and "sysseek" family of functions use byte offsets, not character offsets, because
seeking to a character offset would be very slow in a UTF-8 file.
The return value of "tell" for the standard streams like the STDIN depends on the
operating system: it may return -1 or something else. "tell" on pipes, fifos, and
sockets usually returns -1.
There is no "systell" function. Use "sysseek($fh, 0, 1)" for that.
Do not use "tell" (or other buffered I/O operations) on a filehandle that has been
manipulated by "sysread", "syswrite", or "sysseek". Those functions ignore the
buffering, while "tell" does not.
telldir DIRHANDLE
Returns the current position of the "readdir" routines on DIRHANDLE. Value may be
given to "seekdir" to access a particular location in a directory. "telldir" has the
same caveats about possible directory compaction as the corresponding system library
routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that will provide the implementation
for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is
the name of a class implementing objects of correct type. Any additional arguments
are passed to the appropriate constructor method of the class (meaning "TIESCALAR",
"TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically these are arguments such as might
be passed to the dbm_open(3) function of C. The object returned by the constructor is
also returned by the "tie" function, which would be useful if you want to access other
methods in CLASSNAME.
Note that functions such as "keys" and "values" may return huge lists when used on
large objects, like DBM files. You may prefer to use the "each" function to iterate
over such. Example:
# print out history file offsets
use NDBM_File;
tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (my ($key,$val) = each %HIST) {
print $key, ' = ', unpack('L', $val), "\n";
}
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DELETE this, key
EXISTS this, key
DESTROY this
UNTIE this
A class implementing a filehandle should have the following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented. See perltie, Tie::Hash,
Tie::Array, Tie::Scalar, and Tie::Handle.
Unlike "dbmopen", the "tie" function will not "use" or "require" a module for you; you
need to do that explicitly yourself. See DB_File or the Config module for interesting
"tie" implementations.
For further details see perltie, "tied".
tied VARIABLE
Returns a reference to the object underlying VARIABLE (the same value that was
originally returned by the "tie" call that bound the variable to a package.) Returns
the undefined value if VARIABLE isn't tied to a package.
time
Returns the number of non-leap seconds since whatever time the system considers to be
the epoch, suitable for feeding to "gmtime" and "localtime". On most systems the
epoch is 00:00:00 UTC, January 1, 1970; a prominent exception being Mac OS Classic
which uses 00:00:00, January 1, 1904 in the current local time zone for its epoch.
For measuring time in better granularity than one second, use the Time::HiRes module
from Perl 5.8 onwards (or from CPAN before then), or, if you have gettimeofday(2), you
may be able to use the "syscall" interface of Perl. See perlfaq8 for details.
For date and time processing look at the many related modules on CPAN. For a
comprehensive date and time representation look at the DateTime module.
times
Returns a four-element list giving the user and system times in seconds for this
process and any exited children of this process.
my ($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
Children's times are only included for terminated children.
Portability issues: "times" in perlport.
tr///
The transliteration operator. Same as "y///". See "Quote-Like Operators" in perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length.
Raises an exception if truncate isn't implemented on your system. Returns true if
successful, "undef" on error.
The behavior is undefined if LENGTH is greater than the length of the file.
The position in the file of FILEHANDLE is left unchanged. You may want to call seek
before writing to the file.
Portability issues: "truncate" in perlport.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal function implementing the
"\U" escape in double-quoted strings. It does not attempt to do titlecase mapping on
initial letters. See "ucfirst" for that.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as in a locale, as "lc"
does.
ucfirst EXPR
ucfirst
Returns the value of EXPR with the first character in uppercase (titlecase in
Unicode). This is the internal function implementing the "\u" escape in double-quoted
strings.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as in a locale, as "lc"
does.
umask EXPR
umask
Sets the umask for the process to EXPR and returns the previous value. If EXPR is
omitted, merely returns the current umask.
The Unix permission "rwxr-x---" is represented as three sets of three bits, or three
octal digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The
"umask" value is such a number representing disabled permissions bits. The permission
(or "mode") values you pass "mkdir" or "sysopen" are modified by your umask, so even
if you tell "sysopen" to create a file with permissions 0777, if your umask is 0022,
then the file will actually be created with permissions 0755. If your "umask" were
0027 (group can't write; others can't read, write, or execute), then passing "sysopen"
0666 would create a file with mode 0640 (because "0666 &~ 027" is 0640).
Here's some advice: supply a creation mode of 0666 for regular files (in "sysopen")
and one of 0777 for directories (in "mkdir") and executable files. This gives users
the freedom of choice: if they want protected files, they might choose process umasks
of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely
if ever make policy decisions better left to the user. The exception to this is when
writing files that should be kept private: mail files, web browser cookies, .rhosts
files, and so on.
If umask(2) is not implemented on your system and you are trying to restrict access
for yourself (i.e., "(EXPR & 0700) > 0"), raises an exception. If umask(2) is not
implemented and you are not trying to restrict access for yourself, returns "undef".
Remember that a umask is a number, usually given in octal; it is not a string of octal
digits. See also "oct", if all you have is a string.
Portability issues: "umask" in perlport.
undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an
array (using "@"), a hash (using "%"), a subroutine (using "&"), or a typeglob (using
"*"). Saying "undef $hash{$key}" will probably not do what you expect on most
predefined variables or DBM list values, so don't do that; see "delete". Always
returns the undefined value. You can omit the EXPR, in which case nothing is
undefined, but you still get an undefined value that you could, for instance, return
from a subroutine, assign to a variable, or pass as a parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
my ($x, $y, undef, $z) = foo(); # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink
Deletes a list of files. On success, it returns the number of files it successfully
deleted. On failure, it returns false and sets $! (errno):
my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";
On error, "unlink" will not tell you which files it could not remove. If you want to
know which files you could not remove, try them one at a time:
foreach my $file ( @goners ) {
unlink $file or warn "Could not unlink $file: $!";
}
Note: "unlink" will not attempt to delete directories unless you are superuser and the
-U flag is supplied to Perl. Even if these conditions are met, be warned that
unlinking a directory can inflict damage on your filesystem. Finally, using "unlink"
on directories is not supported on many operating systems. Use "rmdir" instead.
If LIST is omitted, "unlink" uses $_.
unpack TEMPLATE,EXPR
unpack TEMPLATE
"unpack" does the reverse of "pack": it takes a string and expands it out into a list
of values. (In scalar context, it returns merely the first value produced.)
If EXPR is omitted, unpacks the $_ string. See perlpacktut for an introduction to
this function.
The string is broken into chunks described by the TEMPLATE. Each chunk is converted
separately to a value. Typically, either the string is a result of "pack", or the
characters of the string represent a C structure of some kind.
The TEMPLATE has the same format as in the "pack" function. Here's a subroutine that
does substring:
sub substr {
my ($what, $where, $howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in "pack", you may prefix a field with a %<number> to
indicate that you want a <number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. The checksum is calculated by summing
numeric values of expanded values (for string fields the sum of "ord($char)" is taken;
for bit fields the sum of zeroes and ones).
For example, the following computes the same number as the System V sum program:
my $checksum = do {
local $/; # slurp!
unpack("%32W*", readline) % 65535;
};
The following efficiently counts the number of set bits in a bit vector:
my $setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since Perl has no way of checking
whether the value passed to "unpack" corresponds to a valid memory location, passing a
pointer value that's not known to be valid is likely to have disastrous consequences.
If there are more pack codes or if the repeat count of a field or a group is larger
than what the remainder of the input string allows, the result is not well defined:
the repeat count may be decreased, or "unpack" may produce empty strings or zeros, or
it may raise an exception. If the input string is longer than one described by the
TEMPLATE, the remainder of that input string is ignored.
See "pack" for more examples and notes.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of a "push", depending on how you
look at it. Prepends list to the front of the array and returns the new number of
elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the prepended elements
stay in the same order. Use "reverse" to do the reverse.
Starting with Perl 5.14, an experimental feature allowed "unshift" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
untie VARIABLE
Breaks the binding between a variable and a package. (See tie.) Has no effect if the
variable is not tied.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from the named module, generally by
aliasing certain subroutine or variable names into your package. It is exactly
equivalent to
BEGIN { require Module; Module->import( LIST ); }
except that Module must be a bareword. The importation can be made conditional by
using the if module.
In the "use VERSION" form, VERSION may be either a v-string such as v5.24.1, which
will be compared to $^V (aka $PERL_VERSION), or a numeric argument of the form
5.024001, which will be compared to $]. An exception is raised if VERSION is greater
than the version of the current Perl interpreter; Perl will not attempt to parse the
rest of the file. Compare with "require", which can do a similar check at run time.
Symmetrically, "no VERSION" allows you to specify that you want a version of Perl
older than the specified one.
Specifying VERSION as a numeric argument of the form 5.024001 should generally be
avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 released
in 2002 the more verbose numeric form was the only supported syntax, which is why you
might see it in
use v5.24.1; # compile time version check
use 5.24.1; # ditto
use 5.024_001; # ditto; older syntax compatible with perl 5.6
This is often useful if you need to check the current Perl version before "use"ing
library modules that won't work with older versions of Perl. (We try not to do this
more than we have to.)
"use VERSION" also lexically enables all features available in the requested version
as defined by the feature pragma, disabling any features not in the requested
version's feature bundle. See feature. Similarly, if the specified Perl version is
greater than or equal to 5.12.0, strictures are enabled lexically as with "use
strict". Any explicit use of "use strict" or "no strict" overrides "use VERSION",
even if it comes before it. Later use of "use VERSION" will override all behavior of
a previous "use VERSION", possibly removing the "strict" and "feature" added by "use
VERSION". "use VERSION" does not load the feature.pm or strict.pm files.
The "BEGIN" forces the "require" and "import" to happen at compile time. The
"require" makes sure the module is loaded into memory if it hasn't been yet. The
"import" is not a builtin; it's just an ordinary static method call into the "Module"
package to tell the module to import the list of features back into the current
package. The module can implement its "import" method any way it likes, though most
modules just choose to derive their "import" method via inheritance from the
"Exporter" class that is defined in the "Exporter" module. See Exporter. If no
"import" method can be found, then the call is skipped, even if there is an AUTOLOAD
method.
If you do not want to call the package's "import" method (for instance, to stop your
namespace from being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the "use" will call
the "VERSION" method in class Module with the given version as an argument:
use Module 12.34;
is equivalent to:
BEGIN { require Module; Module->VERSION(12.34) }
The default "VERSION" method, inherited from the "UNIVERSAL" class, croaks if the
given version is larger than the value of the variable $Module::VERSION.
The VERSION argument cannot be an arbitrary expression. It only counts as a VERSION
argument if it is a version number literal, starting with either a digit or "v"
followed by a digit. Anything that doesn't look like a version literal will be parsed
as the start of the LIST. Nevertheless, many attempts to use an arbitrary expression
as a VERSION argument will appear to work, because Exporter's "import" method handles
numeric arguments specially, performing version checks rather than treating them as
things to export.
Again, there is a distinction between omitting LIST ("import" called with no
arguments) and an explicit empty LIST "()" ("import" not called). Note that there is
no comma after VERSION!
Because this is a wide-open interface, pragmas (compiler directives) are also
implemented this way. Some of the currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable);
Some of these pseudo-modules import semantics into the current block scope (like
"strict" or "integer", unlike ordinary modules, which import symbols into the current
package (which are effective through the end of the file).
Because "use" takes effect at compile time, it doesn't respect the ordinary flow
control of the code being compiled. In particular, putting a "use" inside the false
branch of a conditional doesn't prevent it from being processed. If a module or
pragma only needs to be loaded conditionally, this can be done using the if pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);
There's a corresponding "no" declaration that unimports meanings imported by "use",
i.e., it calls "Module->unimport(LIST)" instead of "import". It behaves just as
"import" does with VERSION, an omitted or empty LIST, or no unimport method being
found.
no integer;
no strict 'refs';
no warnings;
Care should be taken when using the "no VERSION" form of "no". It is only meant to be
used to assert that the running Perl is of a earlier version than its argument and not
to undo the feature-enabling side effects of "use VERSION".
See perlmodlib for a list of standard modules and pragmas. See perlrun for the "-M"
and "-m" command-line options to Perl that give "use" functionality from the command-
line.
utime LIST
Changes the access and modification times on each file of a list of files. The first
two elements of the list must be the NUMERIC access and modification times, in that
order. Returns the number of files successfully changed. The inode change time of
each file is set to the current time. For example, this code has the same effect as
the Unix touch(1) command when the files already exist and belong to the user running
the program:
#!/usr/bin/perl
my $atime = my $mtime = time;
utime $atime, $mtime, @ARGV;
Since Perl 5.8.0, if the first two elements of the list are "undef", the utime(2)
syscall from your C library is called with a null second argument. On most systems,
this will set the file's access and modification times to the current time (i.e.,
equivalent to the example above) and will work even on files you don't own provided
you have write permission:
for my $file (@ARGV) {
utime(undef, undef, $file)
|| warn "Couldn't touch $file: $!";
}
Under NFS this will use the time of the NFS server, not the time of the local machine.
If there is a time synchronization problem, the NFS server and local machine will have
different times. The Unix touch(1) command will in fact normally use this form
instead of the one shown in the first example.
Passing only one of the first two elements as "undef" is equivalent to passing a 0 and
will not have the effect described when both are "undef". This also triggers an
uninitialized warning.
On systems that support futimes(2), you may pass filehandles among the files. On
systems that don't support futimes(2), passing filehandles raises an exception.
Filehandles must be passed as globs or glob references to be recognized; barewords are
considered filenames.
Portability issues: "utime" in perlport.
values HASH
values ARRAY
In list context, returns a list consisting of all the values of the named hash. In
Perl 5.12 or later only, will also return a list of the values of an array; prior to
that release, attempting to use an array argument will produce a syntax error. In
scalar context, returns the number of values.
Hash entries are returned in an apparently random order. The actual random order is
specific to a given hash; the exact same series of operations on two hashes may result
in a different order for each hash. Any insertion into the hash may change the order,
as will any deletion, with the exception that the most recent key returned by "each"
or "keys" may be deleted without changing the order. So long as a given hash is
unmodified you may rely on "keys", "values" and "each" to repeatedly return the same
order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on
why hash order is randomized. Aside from the guarantees provided here the exact
details of Perl's hash algorithm and the hash traversal order are subject to change in
any release of Perl. Tied hashes may behave differently to Perl's hashes with respect
to changes in order on insertion and deletion of items.
As a side effect, calling "values" resets the HASH or ARRAY's internal iterator (see
"each") before yielding the values. In particular, calling "values" in void context
resets the iterator with no other overhead.
Apart from resetting the iterator, "values @array" in list context is the same as
plain @array. (We recommend that you use void context "keys @array" for this, but
reasoned that taking "values @array" out would require more documentation than leaving
it in.)
Note that the values are not copied, which means modifying them will modify the
contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
Starting with Perl 5.14, an experimental feature allowed "values" to take a scalar
expression. This experiment has been deemed unsuccessful, and was removed as of Perl
5.24.
To avoid confusing would-be users of your code who are running earlier versions of
Perl with mysterious syntax errors, put this sort of thing at the top of your file to
signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays
See also "keys", "each", and "sort".
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of elements of width BITS and
returns the value of the element specified by OFFSET as an unsigned integer. BITS
therefore specifies the number of bits that are reserved for each element in the bit
vector. This must be a power of two from 1 to 32 (or 64, if your platform supports
that).
If BITS is 8, "elements" coincide with bytes of the input string.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size
BITS/8, and each group is converted to a number as with "pack"/"unpack" with big-
endian formats "n"/"N" (and analogously for BITS==64). See "pack" for details.
If bits is 4 or less, the string is broken into bytes, then the bits of each byte are
broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as
in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single
input byte "chr(0x36)" into two groups gives a list "(0x6, 0x3)"; breaking it into 4
groups gives "(0x2, 0x1, 0x3, 0x0)".
"vec" may also be assigned to, in which case parentheses are needed to give the
expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the value 0 is returned. If an element
off the end of the string is written to, Perl will first extend the string with
sufficiently many zero bytes. It is an error to try to write off the beginning of
the string (i.e., negative OFFSET).
If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag
set), "vec" tries to convert it to use a one-byte-per-character internal
representation. However, if the string contains characters with values of 256 or
higher, a fatal error will occur.
Strings created with "vec" can also be manipulated with the logical operators "|",
"&", "^", and "~". These operators will assume a bit vector operation is desired when
both operands are strings. See "Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments
show the string after each step. Note that this code works in the same way on big-
endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of 0's and 1's, use these:
my $bits = unpack("b*", $vector);
my @bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the "*".
Here is an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it runs, the example above should
print the following table:
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait
Behaves like wait(2) on your system: it waits for a child process to terminate and
returns the pid of the deceased process, or "-1" if there are no child processes. The
status is returned in $? and "${^CHILD_ERROR_NATIVE}". Note that a return value of
"-1" could mean that child processes are being automatically reaped, as described in
perlipc.
If you use "wait" in your handler for $SIG{CHLD}, it may accidentally wait for the
child created by "qx" or "system". See perlipc for details.
Portability issues: "wait" in perlport.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the pid of the deceased
process, or "-1" if there is no such child process. A non-blocking wait (with WNOHANG
in FLAGS) can return 0 if there are child processes matching PID but none have
terminated yet. The status is returned in $? and "${^CHILD_ERROR_NATIVE}".
A PID of 0 indicates to wait for any child process whose process group ID is equal to
that of the current process. A PID of less than "-1" indicates to wait for any child
process whose process group ID is equal to -PID. A PID of "-1" indicates to wait for
any child process.
If you say
use POSIX ":sys_wait_h";
my $kid;
do {
$kid = waitpid(-1, WNOHANG);
} while $kid > 0;
or
1 while waitpid(-1, WNOHANG) > 0;
then you can do a non-blocking wait for all pending zombie processes (see "WAIT" in
POSIX). Non-blocking wait is available on machines supporting either the waitpid(2)
or wait4(2) syscalls. However, waiting for a particular pid with FLAGS of 0 is
implemented everywhere. (Perl emulates the system call by remembering the status
values of processes that have exited but have not been harvested by the Perl script
yet.)
Note that on some systems, a return value of "-1" could mean that child processes are
being automatically reaped. See perlipc for details, and for other examples.
Portability issues: "waitpid" in perlport.
wantarray
Returns true if the context of the currently executing subroutine or "eval" is looking
for a list value. Returns false if the context is looking for a scalar. Returns the
undefined value if the context is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
"wantarray"'s result is unspecified in the top level of a file, in a "BEGIN",
"UNITCHECK", "CHECK", "INIT" or "END" block, or in a "DESTROY" method.
This function should have been named wantlist() instead.
warn LIST
Emits a warning, usually by printing it to "STDERR". "warn" interprets its operand
LIST in the same way as "die", but is slightly different in what it defaults to when
LIST is empty or makes an empty string. If it is empty and $@ already contains an
exception value then that value is used after appending "\t...caught". If it is empty
and $@ is also empty then the string "Warning: Something's wrong" is used.
By default, the exception derived from the operand LIST is stringified and printed to
"STDERR". This behaviour can be altered by installing a $SIG{__WARN__} handler. If
there is such a handler then no message is automatically printed; it is the handler's
responsibility to deal with the exception as it sees fit (like, for instance,
converting it into a "die"). Most handlers must therefore arrange to actually display
the warnings that they are not prepared to deal with, by calling "warn" again in the
handler. Note that this is quite safe and will not produce an endless loop, since
"__WARN__" hooks are not called from inside one.
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers
(which don't suppress the error text, but can instead call "die" again to change it).
Using a "__WARN__" handler provides a powerful way to silence all warnings (even the
so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries and for more examples. See the Carp
module for other kinds of warnings using its "carp" and "cluck" functions.
write FILEHANDLE
write EXPR
write
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the
format associated with that file. By default the format for a file is the one having
the same name as the filehandle, but the format for the current output channel (see
the "select" function) may be set explicitly by assigning the name of the format to
the $~ variable.
Top of form processing is handled automatically: if there is insufficient room on the
current page for the formatted record, the page is advanced by writing a form feed and
a special top-of-page format is used to format the new page header before the record
is written. By default, the top-of-page format is the name of the filehandle with
"_TOP" appended, or "top" in the current package if the former does not exist. This
would be a problem with autovivified filehandles, but it may be dynamically set to the
format of your choice by assigning the name to the $^ variable while that filehandle
is selected. The number of lines remaining on the current page is in variable "$-",
which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output channel, which
starts out as STDOUT but may be changed by the "select" operator. If the FILEHANDLE
is an EXPR, then the expression is evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more on formats, see perlform.
Note that write is not the opposite of "read". Unfortunately.
y///
The transliteration operator. Same as "tr///". See "Quote-Like Operators" in perlop.
Non-function Keywords by Cross-reference
perldata
__DATA__
__END__
These keywords are documented in "Special Literals" in perldata.
perlmod
BEGIN
CHECK
END
INIT
UNITCHECK
These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END"
in perlmod.
perlobj
DESTROY
This method keyword is documented in "Destructors" in perlobj.
perlop
and
cmp
eq
ge
gt
le
lt
ne
not
or
x
xor These operators are documented in perlop.
perlsub
AUTOLOAD
This keyword is documented in "Autoloading" in perlsub.
perlsyn
else
elsif
for
foreach
if
unless
until
while
These flow-control keywords are documented in "Compound Statements" in perlsyn.
elseif
The "else if" keyword is spelled "elsif" in Perl. There's no "elif" or "else if"
either. It does parse "elseif", but only to warn you about not using it.
See the documentation for flow-control keywords in "Compound Statements" in perlsyn.
default
given
when
These flow-control keywords related to the experimental switch feature are documented
in "Switch Statements" in perlsyn.
perl v5.34.0 2023-11-23 PERLFUNC(1)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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