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sem_wait(3)
NAME
  sem_wait, sem_trywait - Performs (or conditionally performs) a semaphore
  lock (P1003.1b)

SYNOPSIS
  #include <semaphore.h>

  int sem_wait (
  sem_t *sem);

  int sem_trywait (
  sem_t *sem)

LIBRARY
  Realtime Library (librt.so, librt.a)

PARAMETERS
  sem Specifies a pointer to the semaphore to be locked.

DESCRIPTION
  The sem_wait function locks the semaphore referenced by sem by performing a
  semaphore lock operation on it. If the semaphore value is zero, the
  sem_wait function blocks until it either locks the semaphore or is
  interrupted by a signal.

  The sem_trywait function locks a semaphore only if the semaphore is
  currently not locked. If the semaphore value is zero, the sem_trywait
  function returns without locking the semaphore.

  These functions help ensure that the resource associated with the semaphore
  cannot be accessed by other processes. The semaphore remains locked until
  the process unlocks it with a call to the sem_post function.

  Use the sem_wait function instead of the sem_trywait function if the
  process should wait for access to the semaphore.

RETURN VALUES
  If the sem_wait or sem_trywait function returns successfully, a value of 0
  (zero) is returned and the function executes the semaphore lock operation.

  On an unsuccessful call, a value of -1 is returned and errno is set to
  indicate that an error occurred. The state of the semaphore remains
  unchanged.

ERRORS
  The sem_wait and sem_trywait functions fail under the following conditions:

  [EAGAIN]
      The semaphore was already locked and cannot be locked by the
      sem_trywait operation  (sem_trywait only).

  [EINTR]
      A signal interrupted this function.

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

SEE ALSO
  Functions: sem_post(3)

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for S

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