sem_post(3) — Linux manual page

sem_post(3)              Library Functions Manual             sem_post(3)

NAME         top

       sem_post - unlock a semaphore

LIBRARY         top

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS         top

       #include <semaphore.h>

       int sem_post(sem_t *sem);

DESCRIPTION         top

       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         top

       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         top

       EINVAL sem is not a valid semaphore.

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                            │ Attribute     │ Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │ sem_post()                           │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS         top

       POSIX.1-2008.

HISTORY         top

       POSIX.1-2001.

NOTES         top

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

EXAMPLES         top

       See sem_wait(3) and shm_open(3).

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.15            2025-05-17                    sem_post(3)

Pages that refer to this page: sem_close(3),  sem_destroy(3),  sem_getvalue(3),  sem_init(3),  sem_open(3),  sem_timedwait(3p),  sem_unlink(3),  sem_wait(3),  sem_overview(7),  signal-safety(7)

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