Index for Section 3

Alphabetical listing for S

Bottom of page

sem_post(3)
NAME
  sem_post - Unlock a semaphore (P1003.1b)

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 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 amount of
  time 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 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)

Index for Section 3

Alphabetical listing for S

Top of page