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
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 and decrement the semaphore value by one
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 and decrement the semaphore value by one 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 .
On Linux, named semaphores are created in a virtual file system, normally mounted under /dev/shm , with names of the form \fBsem.\fPname .
System V semaphores
System V semaphores 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)
