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.
Example
See sem_wait(3).
See Also
sem_getvalue(3), sem_wait(3), sem_overview(7), signal-safety(7)
Colophon
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Referenced By
sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_overview(7), sem_timedwait(3p), sem_unlink(3), sem_wait(3), signal-safety(7).