NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

       #include <semaphore.h>

       int sem_wait ( sem_t *sem);

       int sem_trywait ( sem_t *sem)

LIBRARY    [Toc]    [Back]

       Realtime Library (librt.so, librt.a)

PARAMETERS    [Toc]    [Back]

       Specifies a pointer to the semaphore to be locked.

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       The sem_wait and sem_trywait functions fail under the following
  conditions:  The  semaphore was already locked and
       cannot be locked by the sem_trywait  operation   (sem_trywait
  only).  A signal interrupted this function.  The sem
       argument does not refer to a valid semaphore.

SEE ALSO    [Toc]    [Back]

       Functions: sem_post(3)

       Guide to Realtime Programming



                                                      sem_wait(3)