Name
sem_init — initialize an unnamed semaphore
Synopsis
#include <semaphore.h>
int
sem_init( |
sem_t *sem, |
| int pshared, | |
unsigned int value); |
![[Note]](../stylesheet/note.png) |
Note |
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DESCRIPTION
sem_init() initializes the
unnamed semaphore at the address pointed to by sem. The value argument specifies the
initial value for the semaphore.
The pshared
argument indicates whether this semaphore is to be shared
between the threads of a process, or between processes.
If pshared has the
value 0, then the semaphore is shared between the threads of
a process, and should be located at some address that is
visible to all threads (e.g., a global variable, or a
variable allocated dynamically on the heap).
If pshared is
nonzero, then the semaphore is shared between processes, and
should be located in a region of shared memory (see shm_open(3), mmap(2), and shmget(2)). (Since a child
created by fork(2) inherits its
parent's memory mappings, it can also access the semaphore.)
Any process that can access the shared memory region can
operate on the semaphore using sem_post(3), sem_wait(3), etc.
Initializing a semaphore that has already been initialized results in undefined behavior.
RETURN VALUE
sem_init() returns 0 on
success; on error, −1 is returned, and errno is set to indicate the error.
ERRORS
- EINVAL
-
valueexceedsSEM_VALUE_MAX. - ENOSYS
-
psharedis nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).
NOTES
Bizarrely, POSIX.1-2001 does not specify the value that
should be returned by a successful call to sem_init(). POSIX.1-2008 rectifies this,
specifying the zero return on success.
COLOPHON
This page is part of release 3.33 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at http://man7.org/linux/man-pages/.
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t Copyright (C) 2006 Michael Kerrisk <mtk.manpagesgmail.com> 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. |