| 1 | /* |
| 2 | SDL - Simple DirectMedia Layer |
| 3 | Copyright (C) 1997-2009 Sam Lantinga |
| 4 | |
| 5 | This library is free software; you can redistribute it and/or |
| 6 | modify it under the terms of the GNU Lesser General Public |
| 7 | License as published by the Free Software Foundation; either |
| 8 | version 2.1 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 | Lesser General Public License for more details. |
| 14 | |
| 15 | You should have received a copy of the GNU Lesser General Public |
| 16 | License along with this library; if not, write to the Free Software |
| 17 | Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA |
| 18 | |
| 19 | Sam Lantinga |
| 20 | slouken@libsdl.org |
| 21 | */ |
| 22 | #include "SDL_config.h" |
| 23 | |
| 24 | /* An implementation of condition variables using semaphores and mutexes */ |
| 25 | /* |
| 26 | This implementation borrows heavily from the BeOS condition variable |
| 27 | implementation, written by Christopher Tate and Owen Smith. Thanks! |
| 28 | */ |
| 29 | |
| 30 | #include "SDL_thread.h" |
| 31 | |
| 32 | struct SDL_cond |
| 33 | { |
| 34 | SDL_mutex *lock; |
| 35 | int waiting; |
| 36 | int signals; |
| 37 | SDL_sem *wait_sem; |
| 38 | SDL_sem *wait_done; |
| 39 | }; |
| 40 | |
| 41 | /* Create a condition variable */ |
| 42 | SDL_cond * SDL_CreateCond(void) |
| 43 | { |
| 44 | SDL_cond *cond; |
| 45 | |
| 46 | cond = (SDL_cond *) SDL_malloc(sizeof(SDL_cond)); |
| 47 | if ( cond ) { |
| 48 | cond->lock = SDL_CreateMutex(); |
| 49 | cond->wait_sem = SDL_CreateSemaphore(0); |
| 50 | cond->wait_done = SDL_CreateSemaphore(0); |
| 51 | cond->waiting = cond->signals = 0; |
| 52 | if ( ! cond->lock || ! cond->wait_sem || ! cond->wait_done ) { |
| 53 | SDL_DestroyCond(cond); |
| 54 | cond = NULL; |
| 55 | } |
| 56 | } else { |
| 57 | SDL_OutOfMemory(); |
| 58 | } |
| 59 | return(cond); |
| 60 | } |
| 61 | |
| 62 | /* Destroy a condition variable */ |
| 63 | void SDL_DestroyCond(SDL_cond *cond) |
| 64 | { |
| 65 | if ( cond ) { |
| 66 | if ( cond->wait_sem ) { |
| 67 | SDL_DestroySemaphore(cond->wait_sem); |
| 68 | } |
| 69 | if ( cond->wait_done ) { |
| 70 | SDL_DestroySemaphore(cond->wait_done); |
| 71 | } |
| 72 | if ( cond->lock ) { |
| 73 | SDL_DestroyMutex(cond->lock); |
| 74 | } |
| 75 | SDL_free(cond); |
| 76 | } |
| 77 | } |
| 78 | |
| 79 | /* Restart one of the threads that are waiting on the condition variable */ |
| 80 | int SDL_CondSignal(SDL_cond *cond) |
| 81 | { |
| 82 | if ( ! cond ) { |
| 83 | SDL_SetError("Passed a NULL condition variable"); |
| 84 | return -1; |
| 85 | } |
| 86 | |
| 87 | /* If there are waiting threads not already signalled, then |
| 88 | signal the condition and wait for the thread to respond. |
| 89 | */ |
| 90 | SDL_LockMutex(cond->lock); |
| 91 | if ( cond->waiting > cond->signals ) { |
| 92 | ++cond->signals; |
| 93 | SDL_SemPost(cond->wait_sem); |
| 94 | SDL_UnlockMutex(cond->lock); |
| 95 | SDL_SemWait(cond->wait_done); |
| 96 | } else { |
| 97 | SDL_UnlockMutex(cond->lock); |
| 98 | } |
| 99 | |
| 100 | return 0; |
| 101 | } |
| 102 | |
| 103 | /* Restart all threads that are waiting on the condition variable */ |
| 104 | int SDL_CondBroadcast(SDL_cond *cond) |
| 105 | { |
| 106 | if ( ! cond ) { |
| 107 | SDL_SetError("Passed a NULL condition variable"); |
| 108 | return -1; |
| 109 | } |
| 110 | |
| 111 | /* If there are waiting threads not already signalled, then |
| 112 | signal the condition and wait for the thread to respond. |
| 113 | */ |
| 114 | SDL_LockMutex(cond->lock); |
| 115 | if ( cond->waiting > cond->signals ) { |
| 116 | int i, num_waiting; |
| 117 | |
| 118 | num_waiting = (cond->waiting - cond->signals); |
| 119 | cond->signals = cond->waiting; |
| 120 | for ( i=0; i<num_waiting; ++i ) { |
| 121 | SDL_SemPost(cond->wait_sem); |
| 122 | } |
| 123 | /* Now all released threads are blocked here, waiting for us. |
| 124 | Collect them all (and win fabulous prizes!) :-) |
| 125 | */ |
| 126 | SDL_UnlockMutex(cond->lock); |
| 127 | for ( i=0; i<num_waiting; ++i ) { |
| 128 | SDL_SemWait(cond->wait_done); |
| 129 | } |
| 130 | } else { |
| 131 | SDL_UnlockMutex(cond->lock); |
| 132 | } |
| 133 | |
| 134 | return 0; |
| 135 | } |
| 136 | |
| 137 | /* Wait on the condition variable for at most 'ms' milliseconds. |
| 138 | The mutex must be locked before entering this function! |
| 139 | The mutex is unlocked during the wait, and locked again after the wait. |
| 140 | |
| 141 | Typical use: |
| 142 | |
| 143 | Thread A: |
| 144 | SDL_LockMutex(lock); |
| 145 | while ( ! condition ) { |
| 146 | SDL_CondWait(cond); |
| 147 | } |
| 148 | SDL_UnlockMutex(lock); |
| 149 | |
| 150 | Thread B: |
| 151 | SDL_LockMutex(lock); |
| 152 | ... |
| 153 | condition = true; |
| 154 | ... |
| 155 | SDL_UnlockMutex(lock); |
| 156 | */ |
| 157 | int SDL_CondWaitTimeout(SDL_cond *cond, SDL_mutex *mutex, Uint32 ms) |
| 158 | { |
| 159 | int retval; |
| 160 | |
| 161 | if ( ! cond ) { |
| 162 | SDL_SetError("Passed a NULL condition variable"); |
| 163 | return -1; |
| 164 | } |
| 165 | |
| 166 | /* Obtain the protection mutex, and increment the number of waiters. |
| 167 | This allows the signal mechanism to only perform a signal if there |
| 168 | are waiting threads. |
| 169 | */ |
| 170 | SDL_LockMutex(cond->lock); |
| 171 | ++cond->waiting; |
| 172 | SDL_UnlockMutex(cond->lock); |
| 173 | |
| 174 | /* Unlock the mutex, as is required by condition variable semantics */ |
| 175 | SDL_UnlockMutex(mutex); |
| 176 | |
| 177 | /* Wait for a signal */ |
| 178 | if ( ms == SDL_MUTEX_MAXWAIT ) { |
| 179 | retval = SDL_SemWait(cond->wait_sem); |
| 180 | } else { |
| 181 | retval = SDL_SemWaitTimeout(cond->wait_sem, ms); |
| 182 | } |
| 183 | |
| 184 | /* Let the signaler know we have completed the wait, otherwise |
| 185 | the signaler can race ahead and get the condition semaphore |
| 186 | if we are stopped between the mutex unlock and semaphore wait, |
| 187 | giving a deadlock. See the following URL for details: |
| 188 | http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html |
| 189 | */ |
| 190 | SDL_LockMutex(cond->lock); |
| 191 | if ( cond->signals > 0 ) { |
| 192 | /* If we timed out, we need to eat a condition signal */ |
| 193 | if ( retval > 0 ) { |
| 194 | SDL_SemWait(cond->wait_sem); |
| 195 | } |
| 196 | /* We always notify the signal thread that we are done */ |
| 197 | SDL_SemPost(cond->wait_done); |
| 198 | |
| 199 | /* Signal handshake complete */ |
| 200 | --cond->signals; |
| 201 | } |
| 202 | --cond->waiting; |
| 203 | SDL_UnlockMutex(cond->lock); |
| 204 | |
| 205 | /* Lock the mutex, as is required by condition variable semantics */ |
| 206 | SDL_LockMutex(mutex); |
| 207 | |
| 208 | return retval; |
| 209 | } |
| 210 | |
| 211 | /* Wait on the condition variable forever */ |
| 212 | int SDL_CondWait(SDL_cond *cond, SDL_mutex *mutex) |
| 213 | { |
| 214 | return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT); |
| 215 | } |