sem_trywait.c

/*
 * -------------------------------------------------------------
 *
 * Module: sem_trywait.c
 *
 * Purpose:
 *	Semaphores aren't actually part of the PThreads standard.
 *	They are defined by the POSIX Standard:
 *
 *		POSIX 1003.1b-1993	(POSIX.1b)
 *
 * -------------------------------------------------------------
 *
 * --------------------------------------------------------------------------
 *
 *      Pthreads-win32 - POSIX Threads Library for Win32
 *      Copyright(C) 1998 John E. Bossom
 *      Copyright(C) 1999,2005 Pthreads-win32 contributors
 * 
 *      Contact Email: rpj@callisto.canberra.edu.au
 * 
 *      The current list of contributors is contained
 *      in the file CONTRIBUTORS included with the source
 *      code distribution. The list can also be seen at the
 *      following World Wide Web location:
 *      http://sources.redhat.com/pthreads-win32/contributors.html
 * 
 *      This library is free software; you can redistribute it and/or
 *      modify it under the terms of the GNU Lesser General Public
 *      License as published by the Free Software Foundation; either
 *      version 2 of the License, or (at your option) any later version.
 * 
 *      This library is distributed in the hope that it will be useful,
 *      but WITHOUT ANY WARRANTY; without even the implied warranty of
 *      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *      Lesser General Public License for more details.
 * 
 *      You should have received a copy of the GNU Lesser General Public
 *      License along with this library in the file COPYING.LIB;
 *      if not, write to the Free Software Foundation, Inc.,
 *      59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */

#include "pthread.h"
#include "semaphore.h"
#include "implement.h"


int
sem_trywait (sem_t * sem)
     /*
      * ------------------------------------------------------
      * DOCPUBLIC
      *      This function tries to wait on a semaphore.
      *
      * PARAMETERS
      *      sem
      *              pointer to an instance of sem_t
      *
      * DESCRIPTION
      *      This function tries to wait on a semaphore. If the
      *      semaphore value is greater than zero, it decreases
      *      its value by one. If the semaphore value is zero, then
      *      this function returns immediately with the error EAGAIN
      *
      * RESULTS
      *              0               successfully decreased semaphore,
      *              -1              failed, error in errno
      * ERRNO
      *              EAGAIN          the semaphore was already locked,
      *              EINVAL          'sem' is not a valid semaphore,
      *              ENOTSUP         sem_trywait is not supported,
      *              EINTR           the function was interrupted by a signal,
      *              EDEADLK         a deadlock condition was detected.
      *
      * ------------------------------------------------------
      */
{
  int result = 0;
  sem_t s = *sem;

  if (s == NULL)
    {
      result = EINVAL;
    }
  else if ((result = pthread_mutex_lock (&s->lock)) == 0)
    {
      /* See sem_destroy.c
       */
     if (*sem == NULL)
        {
          (void) pthread_mutex_unlock (&s->lock);
          errno = EINVAL;
          return -1;
        }

      if (s->value > 0)
	{
	  s->value--;
	}
      else
	{
	  result = EAGAIN;
	}

      (void) pthread_mutex_unlock (&s->lock);
    }

  if (result != 0)
    {
      errno = result;
      return -1;
    }

  return 0;

}				/* sem_trywait */