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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
.br shmget (2),
or a posix shared memory object built created using
.br shm_open (3)).
.ip
before being used, an unnamed semaphore must be initialized using
.br sem_init (3).
it can then be operated on using
.br sem_post (3)
and
.br 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
.br sem_destroy (3).
.pp
the remainder of this section describes some specific details
of the linux implementation of posix semaphores.
.ss 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.
.ss persistence
posix named semaphores have kernel persistence:
if not removed by
.br sem_unlink (3),
a semaphore will exist until the system is shut down.
.ss linking
programs using the posix semaphores api must be compiled with
.i cc \-pthread
to link against the real-time library,
.ir librt .
.ss accessing named semaphores via the filesystem
on linux, named semaphores are created in a virtual filesystem,
normally mounted under
.ir /dev/shm ,
with names of the form
.ir \fbsem.\fpsomename .
(this is the reason that semaphore names are limited to
.bi name_max \-4
rather than
.b name_max
characters.)
.pp
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.
.sh notes
system v semaphores
.rb ( semget (2),
.br 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.
.sh examples
an example of the use of various posix semaphore functions is shown in
.br sem_wait (3).
.sh see also
.br sem_close (3),
.br sem_destroy (3),
.br sem_getvalue (3),
.br sem_init (3),
.br sem_open (3),
.br sem_post (3),
.br sem_unlink (3),
.br sem_wait (3),
.br pthreads (7),
.br shm_overview (7)
.sh colophon
this page is part of release 5.13 of the linux
.i 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/.
.so man7/iso_8859-11.7
.\" copyright (c) bruno haible <haible@clisp.cons.org>
.\"
.\" %%%license_start(gplv2+_doc_onepara)
.\" this is free documentation; you can redistribute it and/or
.\" modify it under the terms of the gnu general public license as
.\" published by the free software foundation; either version 2 of
.\" the license, or (at your option) any later version.
.\" %%%license_end
.\"
.\" references consulted:
.\" gnu glibc-2 source code and manual
.\" dinkumware c library reference http://www.dinkumware.com/
.\" opengroup's single unix specification http://www.unix-systems.org/online.html
.\" iso/iec 9899:1999