src/thread/generic/SDL_syscond.c

/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2012 Sam Lantinga

    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 2.1 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; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Sam Lantinga
    slouken@libsdl.org
*/
#include "SDL_config.h"

/* An implementation of condition variables using semaphores and mutexes */
/*
   This implementation borrows heavily from the BeOS condition variable
   implementation, written by Christopher Tate and Owen Smith.  Thanks!
 */

#include "SDL_thread.h"

struct SDL_cond
{
	SDL_mutex *lock;
	int waiting;
	int signals;
	SDL_sem *wait_sem;
	SDL_sem *wait_done;
};

/* Create a condition variable */
SDL_cond * SDL_CreateCond(void)
{
	SDL_cond *cond;

	cond = (SDL_cond *) SDL_malloc(sizeof(SDL_cond));
	if ( cond ) {
		cond->lock = SDL_CreateMutex();
		cond->wait_sem = SDL_CreateSemaphore(0);
		cond->wait_done = SDL_CreateSemaphore(0);
		cond->waiting = cond->signals = 0;
		if ( ! cond->lock || ! cond->wait_sem || ! cond->wait_done ) {
			SDL_DestroyCond(cond);
			cond = NULL;
		}
	} else {
		SDL_OutOfMemory();
	}
	return(cond);
}

/* Destroy a condition variable */
void SDL_DestroyCond(SDL_cond *cond)
{
	if ( cond ) {
		if ( cond->wait_sem ) {
			SDL_DestroySemaphore(cond->wait_sem);
		}
		if ( cond->wait_done ) {
			SDL_DestroySemaphore(cond->wait_done);
		}
		if ( cond->lock ) {
			SDL_DestroyMutex(cond->lock);
		}
		SDL_free(cond);
	}
}

/* Restart one of the threads that are waiting on the condition variable */
int SDL_CondSignal(SDL_cond *cond)
{
	if ( ! cond ) {
		SDL_SetError("Passed a NULL condition variable");
		return -1;
	}

	/* If there are waiting threads not already signalled, then
	   signal the condition and wait for the thread to respond.
	*/
	SDL_LockMutex(cond->lock);
	if ( cond->waiting > cond->signals ) {
		++cond->signals;
		SDL_SemPost(cond->wait_sem);
		SDL_UnlockMutex(cond->lock);
		SDL_SemWait(cond->wait_done);
	} else {
		SDL_UnlockMutex(cond->lock);
	}

	return 0;
}

/* Restart all threads that are waiting on the condition variable */
int SDL_CondBroadcast(SDL_cond *cond)
{
	if ( ! cond ) {
		SDL_SetError("Passed a NULL condition variable");
		return -1;
	}

	/* If there are waiting threads not already signalled, then
	   signal the condition and wait for the thread to respond.
	*/
	SDL_LockMutex(cond->lock);
	if ( cond->waiting > cond->signals ) {
		int i, num_waiting;

		num_waiting = (cond->waiting - cond->signals);
		cond->signals = cond->waiting;
		for ( i=0; i<num_waiting; ++i ) {
			SDL_SemPost(cond->wait_sem);
		}
		/* Now all released threads are blocked here, waiting for us.
		   Collect them all (and win fabulous prizes!) :-)
		 */
		SDL_UnlockMutex(cond->lock);
		for ( i=0; i<num_waiting; ++i ) {
			SDL_SemWait(cond->wait_done);
		}
	} else {
		SDL_UnlockMutex(cond->lock);
	}

	return 0;
}

/* Wait on the condition variable for at most 'ms' milliseconds.
   The mutex must be locked before entering this function!
   The mutex is unlocked during the wait, and locked again after the wait.

Typical use:

Thread A:
	SDL_LockMutex(lock);
	while ( ! condition ) {
		SDL_CondWait(cond);
	}
	SDL_UnlockMutex(lock);

Thread B:
	SDL_LockMutex(lock);
	...
	condition = true;
	...
	SDL_UnlockMutex(lock);
 */
int SDL_CondWaitTimeout(SDL_cond *cond, SDL_mutex *mutex, Uint32 ms)
{
	int retval;

	if ( ! cond ) {
		SDL_SetError("Passed a NULL condition variable");
		return -1;
	}

	/* Obtain the protection mutex, and increment the number of waiters.
	   This allows the signal mechanism to only perform a signal if there
	   are waiting threads.
	 */
	SDL_LockMutex(cond->lock);
	++cond->waiting;
	SDL_UnlockMutex(cond->lock);

	/* Unlock the mutex, as is required by condition variable semantics */
	SDL_UnlockMutex(mutex);

	/* Wait for a signal */
	if ( ms == SDL_MUTEX_MAXWAIT ) {
		retval = SDL_SemWait(cond->wait_sem);
	} else {
		retval = SDL_SemWaitTimeout(cond->wait_sem, ms);
	}

	/* Let the signaler know we have completed the wait, otherwise
           the signaler can race ahead and get the condition semaphore
           if we are stopped between the mutex unlock and semaphore wait,
           giving a deadlock.  See the following URL for details:
        http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html
	*/
	SDL_LockMutex(cond->lock);
	if ( cond->signals > 0 ) {
		/* If we timed out, we need to eat a condition signal */
		if ( retval > 0 ) {
			SDL_SemWait(cond->wait_sem);
		}
		/* We always notify the signal thread that we are done */
		SDL_SemPost(cond->wait_done);

		/* Signal handshake complete */
		--cond->signals;
	}
	--cond->waiting;
	SDL_UnlockMutex(cond->lock);

	/* Lock the mutex, as is required by condition variable semantics */
	SDL_LockMutex(mutex);

	return retval;
}

/* Wait on the condition variable forever */
int SDL_CondWait(SDL_cond *cond, SDL_mutex *mutex)
{
	return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT);
}