3. Library calls (functions within program libraries)

SEM_POST

Section: Linux Programmer's Manual (3)
Updated: 2020-08-13
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NAME

sem_post - unlock a semaphore

SYNOPSIS

#include <semaphore.h>

int sem_post(sem_t *sem);

Link with -pthread.

DESCRIPTION

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 maximum allowable value for a semaphore would be exceeded.

ATTRIBUTES

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

Interface	Attribute	Value
sem_post()	Thread safety	MT-Safe

CONFORMING TO

POSIX.1-2001.

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).

COLOPHON

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