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Man Pages
SEM_POST(3) FreeBSD Library Functions Manual SEM_POST(3)

sem_post
increment (unlock) a semaphore

Standard C Library (libc, -lc)

#include <semaphore.h>
int
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().
The sem_post() function 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.

The sem_post() function will fail if:
[]
The sem argument points to an invalid semaphore.
[]
The semaphore value would exceed SEM_VALUE_MAX.

sem_getvalue(3), sem_trywait(3), sem_wait(3)

The sem_post() function conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).
January 28, 2015 FreeBSD 12.0-RELEASE

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