contiki/core/sys/pt-sem.h
2014-06-11 18:44:59 -04:00

234 lines
6.7 KiB
C

/*
* 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 Contiki operating system.
*
* Author: Adam Dunkels <adam@sics.se>
*
*/
/**
* \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 signaled, 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 8
static 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
* Counting semaphores implemented on protothreads
* \author
* Adam Dunkels <adam@sics.se>
*
*/
#ifndef PT_SEM_H_
#define PT_SEM_H_
#include "sys/pt.h"
struct pt_sem {
unsigned int head, tail;
};
#define PT_SEM_COUNT(s) ((s)->head - (s)->tail)
/**
* 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) \
do { \
(s)->tail = 0; \
(s)->head = (c); \
} while(0)
/**
* 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, PT_SEM_COUNT(s) > 0); \
++(s)->tail; \
} 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)->head)
#endif /* PT_SEM_H_ */
/** @} */
/** @} */