From Linux Man Pages
sem_overview - Overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronise 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 vari-
able. A process-shared semaphore must be placed in a shared memory region (e.g., a System V shared memory seg-
ment created using semget(2), or a POSIX shared memory object built created using shm_open(3)).
Before being used, an unnamed semaphore must be initialised 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).
LINUX SPECIFIC DETAILS
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(), 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.
CONFORMING TO
POSIX.1-2001.
NOTES
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 avail-
able (especially on older systems) than System V semaphores.
EXAMPLE
An example of the use of various POSIX semaphore functions is shown in sem_wait(3).
RELATED
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)
CATEGORY