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sem_post - unlock a semaphore
#include <semaphore.h> int sem_post(sem_t *sem); Link with -pthread.
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.
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.
EINVAL sem is not a valid semaphore. EOVERFLOW The maximum allowable value for a semaphore would be exceeded.
sem_post() is async-signal-safe: it may be safely called within a signal handler.
sem_getvalue(3), sem_wait(3), sem_overview(7)
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