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.
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).
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.
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.
sem_close(3), sem_destroy(3), sem_init(3), sem_getvalue(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7)
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