/* * Copyright (c) 2004, Swedish Institute of Computer Science. * All rights reserved.  * * 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 of the Institute 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 INSTITUTE 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 INSTITUTE 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.  * * This file is part of the protothreads library. *  * Author: Adam Dunkels <adam@sics.se> * * $Id: pt-sem.h,v 1.2 2005/02/24 10:36:59 adam Exp $ *//** * \addtogroup pt * @{ *//** * \defgroup ptsem Protothread semaphores * @{ * * This module implements counting semaphores on top of * protothreads. Semaphores are a synchronization primitive that * provide two operations: "wait" and "signal". The "wait" operation * checks the semaphore counter and blocks the thread if the counter * is zero. The "signal" operation increases the semaphore counter but * does not block. If another thread has blocked waiting for the * semaphore that is signalled, the blocked thread will become * runnable again. * * Semaphores can be used to implement other, more structured, * synchronization primitives such as monitors and message * queues/bounded buffers (see below). * * The following example shows how the producer-consumer problem, also * known as the bounded buffer problem, can be solved using * protothreads and semaphores. Notes on the program follow after the * example. * \code#include "pt-sem.h"#define NUM_ITEMS 32#define BUFSIZE 8static struct pt_sem mutex, full, empty;PT_THREAD(producer(struct pt *pt)){  static int produced;    PT_BEGIN(pt);    for(produced = 0; produced < NUM_ITEMS; ++produced) {      PT_SEM_WAIT(pt, &full);        PT_SEM_WAIT(pt, &mutex);    add_to_buffer(produce_item());        PT_SEM_SIGNAL(pt, &mutex);        PT_SEM_SIGNAL(pt, &empty);  }  PT_END(pt);}PT_THREAD(consumer(struct pt *pt)){  static int consumed;    PT_BEGIN(pt);  for(consumed = 0; consumed < NUM_ITEMS; ++consumed) {        PT_SEM_WAIT(pt, &empty);        PT_SEM_WAIT(pt, &mutex);        consume_item(get_from_buffer());        PT_SEM_SIGNAL(pt, &mutex);        PT_SEM_SIGNAL(pt, &full);  }  PT_END(pt);}PT_THREAD(driver_thread(struct pt *pt)){  static struct pt pt_producer, pt_consumer;  PT_BEGIN(pt);    PT_SEM_INIT(&empty, 0);  PT_SEM_INIT(&full, BUFSIZE);  PT_SEM_INIT(&mutex, 1);  PT_INIT(&pt_producer);  PT_INIT(&pt_consumer);  PT_WAIT_THREAD(pt, producer(&pt_producer) &		     consumer(&pt_consumer));  PT_END(pt);} \endcode * * The program uses three protothreads: one protothread that * implements the consumer, one thread that implements the producer, * and one protothread that drives the two other protothreads. The * program uses three semaphores: "full", "empty" and "mutex". The * "mutex" semaphore is used to provide mutual exclusion for the * buffer, the "empty" semaphore is used to block the consumer is the * buffer is empty, and the "full" semaphore is used to block the * producer is the buffer is full. * * The "driver_thread" holds two protothread state variables, * "pt_producer" and "pt_consumer". It is important to note that both * these variables are declared as <i>static</i>. If the static * keyword is not used, both variables are stored on the stack. Since * protothreads do not store the stack, these variables may be * overwritten during a protothread wait operation. Similarly, both * the "consumer" and "producer" protothreads declare their local * variables as static, to avoid them being stored on the stack. *  * */   /** * \file * Couting semaphores implemented on protothreads * \author * Adam Dunkels <adam@sics.se> * */#ifndef __PT_SEM_H__#define __PT_SEM_H__#include "pt.h"struct pt_sem {  unsigned int count;};/** * Initialize a semaphore * * This macro initializes a semaphore with a value for the * counter. Internally, the semaphores use an "unsigned int" to * represent the counter, and therefore the "count" argument should be * within range of an unsigned int. * * \param s (struct pt_sem *) A pointer to the pt_sem struct * representing the semaphore * * \param c (unsigned int) The initial count of the semaphore. * \hideinitializer */#define PT_SEM_INIT(s, c) (s)->count = c/** * Wait for a semaphore * * This macro carries out the "wait" operation on the semaphore. The * wait operation causes the protothread to block while the counter is * zero. When the counter reaches a value larger than zero, the * protothread will continue. * * \param pt (struct pt *) A pointer to the protothread (struct pt) in * which the operation is executed. * * \param s (struct pt_sem *) A pointer to the pt_sem struct * representing the semaphore * * \hideinitializer */#define PT_SEM_WAIT(pt, s)	\  do {						\    PT_WAIT_UNTIL(pt, (s)->count > 0);		\    --(s)->count;				\  } while(0)/** * Signal a semaphore * * This macro carries out the "signal" operation on the semaphore. The * signal operation increments the counter inside the semaphore, which * eventually will cause waiting protothreads to continue executing. * * \param pt (struct pt *) A pointer to the protothread (struct pt) in * which the operation is executed. * * \param s (struct pt_sem *) A pointer to the pt_sem struct * representing the semaphore * * \hideinitializer */#define PT_SEM_SIGNAL(pt, s) ++(s)->count#endif /* __PT_SEM_H__ *//** @} *//** @} */