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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
  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 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 processes
  were blocked waiting for the semaphore to be unlocked; the semaphore value
  is incremented. If the semaphore value after a sem_post function is zero,
  one of the processes blocked waiting for the semaphore 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.

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

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for S

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