NAME

       sem_post - unlock a semaphore (REALTIME)

SYNOPSIS

       #include <semaphore.h>

       int sem_post(sem_t *sem);

DESCRIPTION

       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 reentrant with respect to signals 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.

ERRORS

       The sem_post() function shall fail if:

       EINVAL The sem argument does not refer to a valid semaphore.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The  sem_post() function is part of the Semaphores option and need not be available on all
       implementations.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       semctl() , semget() , semop() , sem_timedwait() , sem_trywait() , sem_wait()  ,  the  Base
       Definitions volume of IEEE Std 1003.1-2001, <semaphore.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .