SEM_OVERVIEW(7) manual page
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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.
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 sem_unlink(3)
, 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 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.
POSIX.1-2001.
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)
This page
is part of release 3.78 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 http://www.kernel.org/doc/man-pages/.
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