nuttx/sched/sem_trywait.c

/****************************************************************************
 * sched/sem_trywait.c
 *
 *   Copyright (C) 2007, 2008 Gregory Nutt. All rights reserved.
 *   Author: Gregory Nutt <spudmonkey@racsa.co.cr>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name NuttX nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <sys/types.h>
#include <semaphore.h>
#include <sched.h>
#include <errno.h>
#include <nuttx/arch.h>
#include "os_internal.h"
#include "sem_internal.h"

/****************************************************************************
 * Compilation Switches
 ****************************************************************************/

/****************************************************************************
 * Definitions
 ****************************************************************************/

/****************************************************************************
 * Private Type Declarations
 ****************************************************************************/

/****************************************************************************
 * Global Variables
 ****************************************************************************/

/****************************************************************************
 * Private Variables
 ****************************************************************************/

/****************************************************************************
 * Private Functions
 ****************************************************************************/

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Function: sem_trywait
 *
 * Description:
 *   This function locks the specified semaphore only if the
 *   semaphore is currently not locked.  Otherwise, it locks
 *   the semaphore.  In either case, the call returns without
 *   blocking.
 *
 * Parameters:
 *   sem - the semaphore descriptor
 *
 * Return Value:
 *   0 (OK) or -1 (ERROR) if unsuccessful
 *   If this function returns -1 (ERROR), then the cause
 *   of the failure will be reported in "errno" as:
 *   - EINVAL:  Invalid attempt to get the semaphore
 *   - EAGAIN:  The semaphore is not available.
 *
 * Assumptions:
 *
 ****************************************************************************/

int sem_trywait(FAR sem_t *sem)
{
  FAR _TCB  *rtcb = (FAR _TCB*)g_readytorun.head;
  irqstate_t saved_state;
  int        ret = ERROR;

  /* This API should not be called from interrupt handlers */

  DEBUGASSERT(up_interrupt_context() == FALSE)

  /* Assume any errors reported are due to invalid arguments. */

  *get_errno_ptr() = EINVAL;

  if (sem)
    {
      /* The following operations must be performed with interrupts
       * disabled because sem_post() may be called from an interrupt
       * handler.
       */

      saved_state = irqsave();

      /* Any further errors could only be occurred because the semaphore
       * is not available.
       */

      *get_errno_ptr() = EAGAIN;

      /* If the semaphore is available, give it to the requesting task */

      if (sem->semcount > 0)
        {
          /* It is, let the task take the semaphore */

          sem->semcount--;
          rtcb->waitsem = NULL;
          ret = OK;
        }

      /* Interrupts may now be enabled. */

      irqrestore(saved_state);
    }

  return ret;
}