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SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7)
NAME
sem_overview - overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronize their actions.
A semaphore is an integer whose value is never allowed to fall below zero. Two operations
can be performed on semaphores: increment the semaphore value by one (sem_post(3)); and
decrement the semaphore value by one (sem_wait(3)). If the value of a semaphore is cur-
rently zero, then a sem_wait(3) operation will block until the value becomes greater than
zero.
POSIX semaphores come in two forms: named semaphores and unnamed semaphores.
Named semaphores
A named semaphore is identified by a name of the form /somename; that is, a null-
terminated string of up to NAME_MAX-4 (i.e., 251) characters consisting of an ini-
tial slash, followed by one or more characters, none of which are slashes. Two
processes can operate on the same named semaphore by passing the same name to
sem_open(3).
The sem_open(3) function creates a new named semaphore or opens an existing named
semaphore. After the semaphore has been opened, it can be operated on using
sem_post(3) and sem_wait(3). When a process has finished using the semaphore, it
can use sem_close(3) to close the semaphore. When all processes have finished us-
ing the semaphore, it can be removed from the system using sem_unlink(3).
Unnamed semaphores (memory-based semaphores)
An unnamed semaphore does not have a name. Instead the semaphore is placed in a
region of memory that is shared between multiple threads (a thread-shared sema-
phore) or processes (a process-shared semaphore). A thread-shared semaphore is
placed in an area of memory shared between the threads of a process, for example, a
global variable. A process-shared semaphore must be placed in a shared memory re-
gion (e.g., a System V shared memory segment created using shmget(2), or a POSIX
shared memory object built created using shm_open(3)).
Before being used, an unnamed semaphore must be initialized using sem_init(3). It
can then be operated on using sem_post(3) and sem_wait(3). When the semaphore is
no longer required, and before the memory in which it is located is deallocated,
the semaphore should be destroyed using sem_destroy(3).
The remainder of this section describes some specific details of the Linux implementation
of POSIX semaphores.
Versions
Prior to kernel 2.6, Linux supported only unnamed, thread-shared semaphores. On a system
with Linux 2.6 and a glibc that provides the NPTL threading implementation, a complete im-
plementation of POSIX semaphores is provided.
Persistence
POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a sema-
phore will exist until the system is shut down.
Linking
Programs using the POSIX semaphores API must be compiled with cc -pthread to link against
the real-time library, librt.
Accessing named semaphores via the filesystem
On Linux, named semaphores are created in a virtual filesystem, normally mounted under
/dev/shm, with names of the form sem.somename. (This is the reason that semaphore names
are limited to NAME_MAX-4 rather than NAME_MAX characters.)
Since Linux 2.6.19, ACLs can be placed on files under this directory, to control object
permissions on a per-user and per-group basis.
NOTES
System V semaphores (semget(2), semop(2), etc.) are an older semaphore API. POSIX sema-
phores provide a simpler, and better designed interface than System V semaphores; on the
other hand POSIX semaphores are less widely available (especially on older systems) than
System V semaphores.
EXAMPLES
An example of the use of various POSIX semaphore functions is shown in sem_wait(3).
SEE ALSO
sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3),
sem_unlink(3), sem_wait(3), pthreads(7), shm_overview(7)
COLOPHON
This page is part of release 5.10 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2020-06-09 SEM_OVERVIEW(7)
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