sem_post man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

sem_post — unlock a semaphore


#include <semaphore.h>

int sem_post(sem_t *sem);


The sem_post() function shall unlock the semaphore referenced by sem by performing a semaphore unlock operation on that semaphore.

If the semaphore value resulting from this operation is positive, then no threads were blocked waiting for the semaphore to become unlocked; the semaphore value is simply incremented.

If the value of the semaphore resulting from this operation is zero, then one of the threads blocked waiting for the semaphore shall be allowed to return successfully from its call to sem_wait(). If the Process Scheduling option is supported, the thread to be unblocked shall be chosen in a manner appropriate to the scheduling policies and parameters in effect for the blocked threads. In the case of the schedulers SCHED_FIFO and SCHED_RR, the highest priority waiting thread shall be unblocked, and if there is more than one highest priority thread blocked waiting for the semaphore, then the highest priority thread that has been waiting the longest shall be unblocked. If the Process Scheduling option is not defined, the choice of a thread to unblock is unspecified.

If the Process Sporadic Server option is supported, and the scheduling policy is SCHED_SPORADIC, the semantics are as per SCHED_FIFO above.

The sem_post() function shall be async-signal-safe and may be invoked from a signal-catching function.

Return Value

If successful, the sem_post() function shall return zero; otherwise, the function shall return -1 and set errno to indicate the error.


The sem_post() function may fail if:

The sem argument does not refer to a valid semaphore.

The following sections are informative.


See sem_timedwait().

See Also

semctl(), semget(), semop(), sem_timedwait(), sem_trywait()

The Base Definitions volume of POSIX.1‐2008, Section 4.11, Memory Synchronization, <semaphore.h>