sem_post(3c) [opensolaris man page]

sem_post(3C)						   Standard C Library Functions 					      sem_post(3C)

NAME

       sem_post - increment the count of a semaphore

SYNOPSIS

       #include <semaphore.h>

       int sem_post(sem_t *sem);

DESCRIPTION

       The sem_post() function unlocks 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 0, then one of the threads   blocked  waiting  for  the  semaphore  will	be
       allowed	to  return  successfully  from	its  call  to sem_wait(3C). If the symbol _POSIX_PRIORITY_SCHEDULING is defined, the thread  to be
       unblocked will 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  will 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 will  be	unblocked.
       If the symbol _POSIX_PRIORITY_SCHEDULING is not defined, the choice of a thread	to unblock is unspecified.

RETURN VALUES

       If successful, sem_post() returns 0; otherwise it returns -1 and sets errno to indicate the error.

ERRORS

       The sem_post() function will fail if:

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

       ENOSYS	     The sem_post() function is not supported by the system.

       EOVERFLOW     The semaphore value exceeds SEM_VALUE_MAX.

USAGE

       The  sem_post() function is reentrant with respect to signals and may be invoked from a signal-catching function. The semaphore functional-
       ity described on this manual page is for the POSIX (see standards(5)) threads implementation. For the documentation of the Solaris  threads
       interface, see semaphore(3C)).

EXAMPLES

       See sem_wait(3C).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Committed 		   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+
       |Standard		     |See standards(5). 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       sched_setscheduler(3C), sem_wait(3C), semaphore(3C), attributes(5), standards(5)

SunOS 5.11							    5 Feb 2008							      sem_post(3C)

Check Out this Related Man Page

sem_post(2)							System Calls Manual						       sem_post(2)

NAME

       sem_post  - unlock a POSIX semaphore

SYNOPSIS

DESCRIPTION

       is  used  to post the semaphore referenced by sem.  The calling thread will not return from its call to unless it can either: increment the
       semaphore value, if there are no blocked threads on this semaphore; give the semaphore to a  blocked  thread,  if  there  are  any  blocked
       threads on this semaphore; or have an error condition.

       If the semaphore value is < 0, the semaphore has blocked threads, waiting for it to become available (the absolute value of the semaphore's
       value indicates the number of waiters at that moment).  If the semaphore value is >= 0, the semaphore has no waiters.

       If the semaphore has no waiters at the time its value is checked, the semaphore's value will be atomically incremented, with respect to the
       checking of its value, up to its maximum value as specified by If the semaphore has waiters at the time its value is checked, the semaphore
       value is not changed.  Instead, the calling thread will attempt to wake up a waiter.  If the semaphore has waiters having realtime  priori-
       ties, the thread must wake up the highest priority waiter.  Otherwise the thread at the head of the channel queue is woken up.

       When  a waiter is successfully woken, the semaphore being posted will be given to the woken waiter.  In other words, the state of the sema-
       phore remains unchanged.  Instead, the semaphore being posted will be inherited by the waiter being woken from this call to

       If the specified semaphore referred to by sem is a named semaphore, then this semaphore must have been opened by the calling  process  with
       The  calling process must have both read and write permissions on the semaphore to perform this operation.  The routine may be called asyn-
       chronously, i.e. from a signal handler.

       To use this function, link in the realtime library by specifying on the compiler or linker command line.

EXAMPLES

       The following call to will post the semaphore sem.

RETURN VALUE

       A successful call to will return 0 and the calling thread would have posted the semaphore.  Otherwise, the call	to  will  return  -1  with
       errno set to the appropriate value of the error condition.

ERRORS

       fails and does not perform the requested operation if any of the following conditions are encountered:

	      [EPERM]	     The calling process does not have the privileges necessary to post the semaphore.

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

SEE ALSO

       <semaphore.h>.

STANDARDS CONFORMANCE

																       sem_post(2)

Linux and UNIX Man Pages

sem_post(3c) [opensolaris man page]

Check Out this Related Man Page