sem_post(3)

Usage

#include <semaphore.h>

int sem_post(sem_t *sem);

Library

POSIX threads library (libpthread, -lpthread)

Overview

sem_post() increments (unlocks) the semaphore pointed to by sem. If the semaphore's value consequently becomes greater than zero, then another process or thread blocked in a sem_wait(3) call will be woken up and proceed to lock the semaphore.

Return value

sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to indicate the error.

Errors

`EINVAL`	sem is not a valid semaphore.
`EOVERFLOW`	The value would exceed `SEM_VALUE_MAX`.

Attributes

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
sem_post()	Thread safety	MT-Safe

Notes

sem_post() is async-signal-safe: it may be safely called within a signal handler.

Examples

See sem_wait(3) and shm_open(3).

Releases

12 releases since Debian 3.1 (Sarge); the text changed 11 times.

3.1
4.0
5.0
6.0
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13
Arch

Referred to by

sem_init(3) initialize an unnamed semaphore sem_open(3) initialize and open a named semaphore sem_wait(3) lock a semaphore sem_close(3) close a named semaphore sem_unlink(3) remove a named semaphore sem_destroy(3) destroy an unnamed semaphore sem_trywait(3) lock a semaphore sem_getvalue(3) get the value of a semaphore sem_timedwait(3) lock a semaphore sem(4) POSIX semaphores q_push(3) push a job onto a smolq queue q_wait(3) return the next job from the queue, when available sem_timedwait(3p) lock a semaphore sem_clockwait_np(3) lock a semaphore sem_overview(7) overview of POSIX semaphores signal-safety(7) async-signal-safe functions sem(4freebsd) POSIX semaphores rights(4) Capsicum capability rights for file descriptors rights(4freebsd) Capsicum capability rights for file descriptors