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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

Multithreading (see pthreads(7) )

The sem_post() function is thread-safe.

Conforming to

POSIX.1-2001.

Notes

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

Example

See sem_wait(3) .

See Also

sem_getvalue(3) , sem_wait(3) , sem_overview(7)

Colophon

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