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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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sem_init(3) Library Functions Manual sem_init(3)
sem_init - initialize an unnamed semaphore
POSIX threads library (libpthread, -lpthread)
#include <semaphore.h>
int sem_init(sem_t *sem, int pshared, unsigned int value);
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), and so on.
Initializing a semaphore that has already been initialized results
in undefined behavior.
sem_init() returns 0 on success; on error, -1 is returned, and
errno is set to indicate the error.
EINVAL value exceeds SEM_VALUE_MAX.
ENOSYS pshared is nonzero, but the system does not support
process-shared semaphores (see sem_overview(7)).
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ sem_init() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
POSIX.1-2001.
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.
See shm_open(3) and sem_wait(3).
sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sem_init(3)
Pages that refer to this page: sem_destroy(3), sem_overview(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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