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.br tmpnam (3)
.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 man2/unimplemented.2
.so man3/printf.3
.\" copyright (c) 2006 michael kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%license_start(verbatim)
.\" permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" since the linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. the author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. the author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%license_end
.\"
.th sem_overview 7 2020-06-09 "linux" "linux programmer's manual"
.sh name
sem_overview \- overview of posix semaphores
.sh description
posix semaphores allow processes and threads to synchronize their actions.
.pp
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
.rb ( sem_post (3));
and decrement the semaphore value by one
.rb ( sem_wait (3)).
if the value of a semaphore is currently zero, then a
.br sem_wait (3)
operation will block until the value becomes greater than zero.
.pp
posix semaphores come in two forms: named semaphores and
unnamed semaphores.
.tp
.b named semaphores
a named semaphore is identified by a name of the form
.ir /somename ;
that is, a null-terminated string of up to
.bi name_max \-4
(i.e., 251) characters consisting of an initial slash,
.\" glibc allows the initial slash to be omitted, and makes
.\" multiple initial slashes equivalent to a single slash.
.\" this differs from the implementation of posix message queues.
followed by one or more characters, none of which are slashes.
.\" glibc allows subdirectory components in the name, in which
.\" case the subdirectory tree must exist under /dev/shm, and
.\" the fist subdirectory component must exist as the name
.\" sem.name, and all of the subdirectory components must allow the
.\" required permissions if a user wants to create a semaphore
.\" object in a subdirectory.
two processes can operate on the same named semaphore by passing
the same name to
.br sem_open (3).
.ip
the
.br 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
.br sem_post (3)
and
.br sem_wait (3).
when a process has finished using the semaphore, it can use
.br sem_close (3)
to close the semaphore.
when all processes have finished using the semaphore,
it can be removed from the system using
.br sem_unlink (3).
.tp
.b 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
.ir "thread-shared semaphore" )
or processes (a
.ir "process-shared semaphore" ).