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
); and decrement the semaphore value by one
). If the value of a semaphore is currently zero, then a
operation will block until the value becomes greater than
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
- 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 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.
POSIX named semaphores have kernel persistence: if not removed by
, a semaphore will exist until the system is shut down.
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
, with names of the form sem.somename
(This is the reason that semaphore names are limited to
rather than NAME_MAX
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)
, 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
This page is part of release 3.74 of the Linux man-pages
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