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).Unnamed semaphores (memory-based semaphores)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.
Versions
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.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 -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 sem.name.
POSIX.1-2001.
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_init(3), sem_getvalue(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7)
This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.
