FUNCTIONS

1. SDL_CreateCond
2. SDL_DestroyCond
3. SDL_CondSignal
4. SDL_CondBroadcast
5. SDL_CondWaitTimeout
6. SDL_CondWait

   1 /*
   2     SDL - Simple DirectMedia Layer
   3     Copyright (C) 1997, 1998, 1999, 2000, 2001  Sam Lantinga
   4 
   5     This library is free software; you can redistribute it and/or
   6     modify it under the terms of the GNU Library General Public
   7     License as published by the Free Software Foundation; either
   8     version 2 of the License, or (at your option) any later version.
   9 
  10     This library is distributed in the hope that it will be useful,
  11     but WITHOUT ANY WARRANTY; without even the implied warranty of
  12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13     Library General Public License for more details.
  14 
  15     You should have received a copy of the GNU Library General Public
  16     License along with this library; if not, write to the Free
  17     Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  18 
  19     Sam Lantinga
  20     slouken@devolution.com
  21 */
  23 #ifdef SAVE_RCSID
  24 static char rcsid =
  25  "@(#) $Id: SDL_syscond.c,v 1.1.2.5 2001/02/10 07:20:04 hercules Exp $";
  26 #endif
  28 /* An implementation of condition variables using semaphores and mutexes */
  29 /*
  30    This implementation borrows heavily from the BeOS condition variable
  31    implementation, written by Christopher Tate and Owen Smith.  Thanks!
  32  */
  34 #include <stdio.h>
  35 #include <stdlib.h>
  37 #include "SDL_error.h"
  38 #include "SDL_thread.h"
  40 struct SDL_cond
  41 {
  42         SDL_mutex *lock;
  43         int waiting;
  44         int signals;
  45         SDL_sem *wait_sem;
  46         SDL_sem *wait_done;
  47 };
  49 /* Create a condition variable */
  50 SDL_cond * SDL_CreateCond(void)
     /* [<][>][^][v][top][bottom][index][help] */
  51 {
  52         SDL_cond *cond;
  54         cond = (SDL_cond *) malloc(sizeof(SDL_cond));
  55         if ( cond ) {
  56                 cond->lock = SDL_CreateMutex();
  57                 cond->wait_sem = SDL_CreateSemaphore(0);
  58                 cond->wait_done = SDL_CreateSemaphore(0);
  59                 cond->waiting = cond->signals = 0;
  60                 if ( ! cond->lock || ! cond->wait_sem || ! cond->wait_done ) {
SDL_DestroyCond(cond);
  62                         cond = NULL;
  63                 }
  64         } else {
SDL_OutOfMemory();
  66         }
  67         return(cond);
  68 }
  70 /* Destroy a condition variable */
  71 void SDL_DestroyCond(SDL_cond *cond)
     /* [<][>][^][v][top][bottom][index][help] */
  72 {
  73         if ( cond ) {
  74                 if ( cond->wait_sem ) {
SDL_DestroySemaphore(cond->wait_sem);
  76                 }
  77                 if ( cond->wait_done ) {
SDL_DestroySemaphore(cond->wait_done);
  79                 }
  80                 if ( cond->lock ) {
SDL_DestroyMutex(cond->lock);
  82                 }
  83                 free(cond);
  84         }
  85 }
  87 /* Restart one of the threads that are waiting on the condition variable */
  88 int SDL_CondSignal(SDL_cond *cond)
     /* [<][>][^][v][top][bottom][index][help] */
  89 {
  90         if ( ! cond ) {
SDL_SetError("Passed a NULL condition variable");
  92                 return -1;
  93         }
  95         /* If there are waiting threads not already signalled, then
  96            signal the condition and wait for the thread to respond.
  97         */
SDL_LockMutex(cond->lock);
  99         if ( cond->waiting > cond->signals ) {
 100                 ++cond->signals;
SDL_SemPost(cond->wait_sem);
SDL_UnlockMutex(cond->lock);
SDL_SemWait(cond->wait_done);
 104         } else {
SDL_UnlockMutex(cond->lock);
 106         }
 108         return 0;
 109 }
 111 /* Restart all threads that are waiting on the condition variable */
 112 int SDL_CondBroadcast(SDL_cond *cond)
     /* [<][>][^][v][top][bottom][index][help] */
 113 {
 114         if ( ! cond ) {
SDL_SetError("Passed a NULL condition variable");
 116                 return -1;
 117         }
 119         /* If there are waiting threads not already signalled, then
 120            signal the condition and wait for the thread to respond.
 121         */
SDL_LockMutex(cond->lock);
 123         if ( cond->waiting > cond->signals ) {
 124                 int i, num_waiting;
 126                 num_waiting = (cond->waiting - cond->signals);
 127                 cond->signals = cond->waiting;
 128                 for ( i=0; i<num_waiting; ++i ) {
SDL_SemPost(cond->wait_sem);
 130                 }
 131                 /* Now all released threads are blocked here, waiting for us.
 132                    Collect them all (and win fabulous prizes!) :-)
 133                  */
SDL_UnlockMutex(cond->lock);
 135                 for ( i=0; i<num_waiting; ++i ) {
SDL_SemWait(cond->wait_done);
 137                 }
 138         } else {
SDL_UnlockMutex(cond->lock);
 140         }
 142         return 0;
 143 }
 145 /* Wait on the condition variable for at most 'ms' milliseconds.
 146    The mutex must be locked before entering this function!
 147    The mutex is unlocked during the wait, and locked again after the wait.
 148 
 149 Typical use:
 150 
 151 Thread A:
 152         SDL_LockMutex(lock);
 153         while ( ! condition ) {
 154                 SDL_CondWait(cond);
 155         }
 156         SDL_UnlockMutex(lock);
 157 
 158 Thread B:
 159         SDL_LockMutex(lock);
 160         ...
 161         condition = true;
 162         ...
 163         SDL_UnlockMutex(lock);
 164  */
 165 int SDL_CondWaitTimeout(SDL_cond *cond, SDL_mutex *mutex, Uint32 ms)
     /* [<][>][^][v][top][bottom][index][help] */
 166 {
 167         int retval;
 169         if ( ! cond ) {
SDL_SetError("Passed a NULL condition variable");
 171                 return -1;
 172         }
 174         /* Obtain the protection mutex, and increment the number of waiters.
 175            This allows the signal mechanism to only perform a signal if there
 176            are waiting threads.
 177          */
SDL_LockMutex(cond->lock);
 179         ++cond->waiting;
SDL_UnlockMutex(cond->lock);
 182         /* Unlock the mutex, as is required by condition variable semantics */
SDL_UnlockMutex(mutex);
 185         /* Wait for a signal */
 186         if ( ms == SDL_MUTEX_MAXWAIT ) {
 187                 retval = SDL_SemWait(cond->wait_sem);
 188         } else {
 189                 retval = SDL_SemWaitTimeout(cond->wait_sem, ms);
 190         }
 192         /* Let the signaler know we have completed the wait, otherwise
 193            the signaler can race ahead and get the condition semaphore
 194            if we are stopped between the mutex unlock and semaphore wait,
 195            giving a deadlock.  See the following URL for details:
 196         http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html
 197         */
SDL_LockMutex(cond->lock);
 199         if ( cond->signals > 0 ) {
 200                 /* If we timed out, we need to eat a condition signal */
 201                 if ( retval > 0 ) {
SDL_SemWait(cond->wait_sem);
 203                 }
 204                 /* We always notify the signal thread that we are done */
SDL_SemPost(cond->wait_done);
 207                 /* Signal handshake complete */
 208                 --cond->signals;
 209         }
 210         --cond->waiting;
SDL_UnlockMutex(cond->lock);
 213         /* Lock the mutex, as is required by condition variable semantics */
SDL_LockMutex(mutex);
 216         return retval;
 217 }
 219 /* Wait on the condition variable forever */
 220 int SDL_CondWait(SDL_cond *cond, SDL_mutex *mutex)
     /* [<][>][^][v][top][bottom][index][help] */
 221 {
 222         return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT);
 223 }