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sem_overview(7) Miscellaneous Information Manual sem_overview(7)
sem_overview - overview of POSIX semaphores
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 currently 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 initial
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 using 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 semaphore) 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
region (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
Before Linux 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 implementation of
POSIX semaphores is provided.
Persistence
POSIX named semaphores have kernel persistence: if not removed by
sem_unlink(3), a semaphore 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.
System V semaphores (semget(2), semop(2), etc.) are an older
semaphore API. POSIX semaphores 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.
An example of the use of various POSIX semaphore functions is
shown in sem_wait(3).
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sem_overview(7)
Pages that refer to this page: eventfd(2), execve(2), intro(2), semctl(2), semget(2), semop(2), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), tmpfs(5), shm_overview(7), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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