sem_post(3) [ultrix man page]

SEM_POST(3)						     Linux Programmer's Manual						       SEM_POST(3)

NAME

       sem_post - unlock a semaphore

SYNOPSIS

       #include <semaphore.h>

       int sem_post(sem_t *sem);

       Link with -pthread.

DESCRIPTION

       sem_post()  increments  (unlocks)  the  semaphore pointed to by sem.  If the semaphore's value consequently becomes greater than zero, then
       another process or thread blocked in a sem_wait(3) call will be woken up and proceed to lock the semaphore.

RETURN VALUE

       sem_post() returns 0 on success; on error, the value of the semaphore is left unchanged, -1 is returned, and errno is set to  indicate  the
       error.

ERRORS

       EINVAL sem is not a valid semaphore.

       EOVERFLOW
	      The maximum allowable value for a semaphore would be exceeded.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       +-----------+---------------+---------+
       |Interface  | Attribute	   | Value   |
       +-----------+---------------+---------+
       |sem_post() | Thread safety | MT-Safe |
       +-----------+---------------+---------+
CONFORMING TO

       POSIX.1-2001.

NOTES

       sem_post() is async-signal-safe: it may be safely called within a signal handler.

EXAMPLE

       See sem_wait(3).

SEE ALSO

       sem_getvalue(3), sem_wait(3), sem_overview(7), signal-safety(7)

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-03-13							       SEM_POST(3)

sem_post(3)						     Library Functions Manual						       sem_post(3)

NAME

       sem_post - Unlocks a semaphore (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <semaphore.h>

       int sem_post (
       sem_t *sem);

PARAMETERS

       sem	 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 func-
       tion (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 processes 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	priorities
       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:

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

RELATED INFORMATION

       Functions: sem_trywait(3), sem_wait(3)

       Guide to Realtime Programming delim off

																       sem_post(3)