sem_overview(7) Miscellaneous Information Manual sem_overview(7)
NAME
sem_overview - overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronize their ac-
tions.
A semaphore is an integer whose value is never allowed to fall below
zero. Two operations can be performed on semaphores: increment the sem-
aphore 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 sema-
phores.
Named semaphores
A named semaphore is identified by a name of the form /somename;
that is, a null-terminated string of up to NAME_MAX-4 (i.e., 251)
characters consisting of an initial slash, followed by one or
more characters, none of which are slashes. 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 the threads of a process, for exam-
ple, 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 shmget(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 be-
fore the memory in which it is located is deallocated, the sema-
phore should be destroyed using sem_destroy(3).
The remainder of this section describes some specific details of the
Linux implementation of POSIX semaphores.
Versions
Before Linux 2.6, Linux supported only unnamed, thread-shared sema-
phores. 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
-pthread to link against the real-time library, librt.
Accessing named semaphores via the filesystem
On Linux, named semaphores are created in a virtual filesystem, normally
mounted under /dev/shm, with names of the form sem.somename. (This is
the reason that semaphore names are limited to NAME_MAX-4 rather than
NAME_MAX characters.)
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.
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 available (especially on older systems) than System V semaphores.
EXAMPLES
An example of the use of various POSIX semaphore functions is shown in
sem_wait(3).
SEE ALSO
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), shm_overview(7)
Linux man-pages 6.9.1 2024-05-02 sem_overview(7)
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