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
- 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
- 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
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 4.16 of the Linux man-pages
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