NAME¶

sem_post - unlock a semaphore

LIBRARY¶

POSIX threads library (libpthread, -lpthread)

SYNOPSIS¶

#include <semaphore.h>

int sem_post(sem_t *sem);

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).

STANDARDS¶

POSIX.1-2008.

HISTORY¶

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).

SEE ALSO¶

sem_getvalue(3), sem_wait(3), sem_overview(7), signal-safety(7)