sem_post man page on DigitalUNIX

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sem_post(3)							   sem_post(3)

NAME
       sem_post - Unlocks a semaphore (P1003.1b)

SYNOPSIS
       #include <semaphore.h>

       int sem_post ( sem_t *sem);

LIBRARY
       Realtime Library (librt.so, librt.a)

PARAMETERS
       Pointer to the semaphore to be unlocked.

DESCRIPTION
       The sem_post function unlocks the specified semaphore by performing the
       semaphore unlock operation on that semaphore. The appropriate  function
       (sem_open for named semaphores or sem_init for unnamed semaphores) must
       be called for a semaphore before you can call the locking and unlocking
       functions, sem_wait, sem_trywait, and sem_post.

       If  the	semaphore value after a sem_post function is positive, no pro‐
       cesses were blocked waiting for the semaphore to be unlocked; the sema‐
       phore  value  is	 incremented.  If the semaphore value after a sem_post
       function is zero, one of the processes blocked waiting  for  the	 sema‐
       phore is allowed to return successfully from its call to sem_wait.

       If  more	 than  one process is blocked while waiting for the semaphore,
       only one process is unblocked and the state of  the  semaphore  remains
       unchanged  when	the  sem_post  function	 returns.  The	process	 to be
       unblocked is selected according to the scheduling policies and  priori‐
       ties  of	 all blocked processes. If the scheduling policy is SCHED_FIFO
       or SCHED_RR, the highest-priority waiting process is unblocked. If more
       than one process of that priority is blocked, then the process that has
       waited the longest is unblocked.

       The sem_post function can be called from a signal-catching function.

RETURN VALUES
       On successful completion, the sem_post function	returns	 the  value  0
       (zero) and performs a semaphore unlock operation, unblocking a process.

       Otherwise, the function returns the value -1 and sets errno to indicate
       the error.  The state of the semaphore remains unchanged.

ERRORS
       The sem_post function fails under the following condition: The sem does
       not refer to a valid semaphore.

SEE ALSO
       Functions: sem_trywait(3), sem_wait(3)

       Guide to Realtime Programming

								   sem_post(3)
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