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sem_overview - Overview of POSIX semaphores
POSIX semaphores allow processes and threads to synchronize their
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
POSIX semaphores come in two forms: named semaphores and unnamed
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
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
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 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 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
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.
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.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.
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
An example of the use of various POSIX semaphore functions is shown in
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)
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