SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7)
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; 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 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.
Versions
Prior to kernel 2.6, Linux supported only 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
-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.
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_getvalue(3), sem_init(3),
sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7),
shm_overview(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 SEM_OVERVIEW(7)
Pages that refer to this page: eventfd(2), execve(2), intro(2), semctl(2), semget(2), semop(2), sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), tmpfs(5), shm_overview(7)