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Version: 2008-06-15 (ubuntu - 08/07/09)

Section: 7 (Divers)


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. 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 by 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 semget(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.


Prior to kernel 2.6, Linux only supported 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.


POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a semaphore will exist until the system is shut down.


Programs using the POSIX semaphores API must be compiled with cc -lrt to link against the real-time library, librt.

Accessing named semaphores via the file system

On Linux, named semaphores are created in a virtual file system, normally mounted under /dev/shm, with names of the form

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)


This page is part of release 3.15 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at