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SEM_POST(3) Library Functions Manual SEM_POST(3)

increment (unlock) a semaphore

#include <semaphore.h>
sem_post(sem_t *sem);

The sem_post() function increments (unlocks) the semaphore pointed to by sem. If there are threads blocked on the semaphore when sem_post() is called, then the highest priority thread that has been blocked the longest on the semaphore will be allowed to return from sem_wait().
sem_post() is signal-reentrant and may be called within signal handlers.

The sem_post() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

sem_post() will fail if:
sem points to an invalid semaphore.

sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_wait(3)

sem_post() conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).
June 5, 2013 OpenBSD-current