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SDL_syscond.c
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1 /*
2  Simple DirectMedia Layer
3  Copyright (C) 1997-2013 Sam Lantinga <slouken@libsdl.org>
4 
5  This software is provided 'as-is', without any express or implied
6  warranty. In no event will the authors be held liable for any damages
7  arising from the use of this software.
8 
9  Permission is granted to anyone to use this software for any purpose,
10  including commercial applications, and to alter it and redistribute it
11  freely, subject to the following restrictions:
12 
13  1. The origin of this software must not be misrepresented; you must not
14  claim that you wrote the original software. If you use this software
15  in a product, an acknowledgment in the product documentation would be
16  appreciated but is not required.
17  2. Altered source versions must be plainly marked as such, and must not be
18  misrepresented as being the original software.
19  3. This notice may not be removed or altered from any source distribution.
20 */
21 #include "SDL_config.h"
22 
23 /* An implementation of condition variables using semaphores and mutexes */
24 /*
25  This implementation borrows heavily from the BeOS condition variable
26  implementation, written by Christopher Tate and Owen Smith. Thanks!
27  */
28 
29 #include "SDL_thread.h"
30 
31 struct SDL_cond
32 {
33  SDL_mutex *lock;
34  int waiting;
35  int signals;
36  SDL_sem *wait_sem;
37  SDL_sem *wait_done;
38 };
39 
40 /* Create a condition variable */
41 SDL_cond *
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 {
58  }
59  return (cond);
60 }
61 
62 /* Destroy a condition variable */
63 void
65 {
66  if (cond) {
67  if (cond->wait_sem) {
68  SDL_DestroySemaphore(cond->wait_sem);
69  }
70  if (cond->wait_done) {
71  SDL_DestroySemaphore(cond->wait_done);
72  }
73  if (cond->lock) {
74  SDL_DestroyMutex(cond->lock);
75  }
76  SDL_free(cond);
77  }
78 }
79 
80 /* Restart one of the threads that are waiting on the condition variable */
81 int
83 {
84  if (!cond) {
85  return SDL_SetError("Passed a NULL condition variable");
86  }
87 
88  /* If there are waiting threads not already signalled, then
89  signal the condition and wait for the thread to respond.
90  */
91  SDL_LockMutex(cond->lock);
92  if (cond->waiting > cond->signals) {
93  ++cond->signals;
94  SDL_SemPost(cond->wait_sem);
95  SDL_UnlockMutex(cond->lock);
96  SDL_SemWait(cond->wait_done);
97  } else {
98  SDL_UnlockMutex(cond->lock);
99  }
100 
101  return 0;
102 }
103 
104 /* Restart all threads that are waiting on the condition variable */
105 int
107 {
108  if (!cond) {
109  return SDL_SetError("Passed a NULL condition variable");
110  }
111 
112  /* If there are waiting threads not already signalled, then
113  signal the condition and wait for the thread to respond.
114  */
115  SDL_LockMutex(cond->lock);
116  if (cond->waiting > cond->signals) {
117  int i, num_waiting;
118 
119  num_waiting = (cond->waiting - cond->signals);
120  cond->signals = cond->waiting;
121  for (i = 0; i < num_waiting; ++i) {
122  SDL_SemPost(cond->wait_sem);
123  }
124  /* Now all released threads are blocked here, waiting for us.
125  Collect them all (and win fabulous prizes!) :-)
126  */
127  SDL_UnlockMutex(cond->lock);
128  for (i = 0; i < num_waiting; ++i) {
129  SDL_SemWait(cond->wait_done);
130  }
131  } else {
132  SDL_UnlockMutex(cond->lock);
133  }
134 
135  return 0;
136 }
137 
138 /* Wait on the condition variable for at most 'ms' milliseconds.
139  The mutex must be locked before entering this function!
140  The mutex is unlocked during the wait, and locked again after the wait.
141 
142 Typical use:
143 
144 Thread A:
145  SDL_LockMutex(lock);
146  while ( ! condition ) {
147  SDL_CondWait(cond, lock);
148  }
149  SDL_UnlockMutex(lock);
150 
151 Thread B:
152  SDL_LockMutex(lock);
153  ...
154  condition = true;
155  ...
156  SDL_CondSignal(cond);
157  SDL_UnlockMutex(lock);
158  */
159 int
161 {
162  int retval;
163 
164  if (!cond) {
165  return SDL_SetError("Passed a NULL condition variable");
166  }
167 
168  /* Obtain the protection mutex, and increment the number of waiters.
169  This allows the signal mechanism to only perform a signal if there
170  are waiting threads.
171  */
172  SDL_LockMutex(cond->lock);
173  ++cond->waiting;
174  SDL_UnlockMutex(cond->lock);
175 
176  /* Unlock the mutex, as is required by condition variable semantics */
177  SDL_UnlockMutex(mutex);
178 
179  /* Wait for a signal */
180  if (ms == SDL_MUTEX_MAXWAIT) {
181  retval = SDL_SemWait(cond->wait_sem);
182  } else {
183  retval = SDL_SemWaitTimeout(cond->wait_sem, ms);
184  }
185 
186  /* Let the signaler know we have completed the wait, otherwise
187  the signaler can race ahead and get the condition semaphore
188  if we are stopped between the mutex unlock and semaphore wait,
189  giving a deadlock. See the following URL for details:
190  http://web.archive.org/web/20010914175514/http://www-classic.be.com/aboutbe/benewsletter/volume_III/Issue40.html#Workshop
191  */
192  SDL_LockMutex(cond->lock);
193  if (cond->signals > 0) {
194  /* If we timed out, we need to eat a condition signal */
195  if (retval > 0) {
196  SDL_SemWait(cond->wait_sem);
197  }
198  /* We always notify the signal thread that we are done */
199  SDL_SemPost(cond->wait_done);
200 
201  /* Signal handshake complete */
202  --cond->signals;
203  }
204  --cond->waiting;
205  SDL_UnlockMutex(cond->lock);
206 
207  /* Lock the mutex, as is required by condition variable semantics */
208  SDL_LockMutex(mutex);
209 
210  return retval;
211 }
212 
213 /* Wait on the condition variable forever */
214 int
216 {
217  return SDL_CondWaitTimeout(cond, mutex, SDL_MUTEX_MAXWAIT);
218 }
219 
220 /* vi: set ts=4 sw=4 expandtab: */
DECLSPEC SDL_sem *SDLCALL SDL_CreateSemaphore(Uint32 initial_value)
Definition: SDL_syssem.c:85
DECLSPEC void SDLCALL SDL_DestroyCond(SDL_cond *cond)
Definition: SDL_syscond.c:64
#define SDL_MUTEX_MAXWAIT
Definition: SDL_mutex.h:49
DECLSPEC int SDLCALL SDL_CondWaitTimeout(SDL_cond *cond, SDL_mutex *mutex, Uint32 ms)
Definition: SDL_syscond.c:160
struct SDL_semaphore SDL_sem
Definition: SDL_mutex.h:107
#define NULL
Definition: ftobjs.h:61
DECLSPEC int SDLCALL SDL_SemWaitTimeout(SDL_sem *sem, Uint32 ms)
Definition: SDL_syssem.c:150
DECLSPEC int SDLCALL SDL_LockMutex(SDL_mutex *mutex)
Definition: SDL_sysmutex.c:73
DECLSPEC int SDLCALL SDL_SemWait(SDL_sem *sem)
Definition: SDL_syssem.c:180
DECLSPEC SDL_mutex *SDLCALL SDL_CreateMutex(void)
Definition: SDL_sysmutex.c:38
DECLSPEC void SDLCALL SDL_free(void *mem)
DECLSPEC int SDLCALL SDL_CondBroadcast(SDL_cond *cond)
Definition: SDL_syscond.c:106
DECLSPEC int SDLCALL SDL_UnlockMutex(SDL_mutex *mutex)
Definition: SDL_sysmutex.c:160
uint32_t Uint32
An unsigned 32-bit integer type.
Definition: SDL_stdinc.h:145
DECLSPEC int SDLCALL SDL_CondSignal(SDL_cond *cond)
Definition: SDL_syscond.c:82
DECLSPEC int SDLCALL SDL_SetError(const char *fmt,...)
Definition: SDL_error.c:53
DECLSPEC void *SDLCALL SDL_malloc(size_t size)
DECLSPEC SDL_cond *SDLCALL SDL_CreateCond(void)
Definition: SDL_syscond.c:42
DECLSPEC void SDLCALL SDL_DestroySemaphore(SDL_sem *sem)
Definition: SDL_syssem.c:111
#define SDL_OutOfMemory()
Definition: SDL_error.h:52
struct SDL_mutex SDL_mutex
Definition: SDL_mutex.h:59
SDL_mutex * lock
Definition: SDL_events.c:75
DECLSPEC void SDLCALL SDL_DestroyMutex(SDL_mutex *mutex)
Definition: SDL_sysmutex.c:61
int i
Definition: pngrutil.c:1377
DECLSPEC int SDLCALL SDL_SemPost(SDL_sem *sem)
Definition: SDL_syssem.c:200
DECLSPEC int SDLCALL SDL_CondWait(SDL_cond *cond, SDL_mutex *mutex)
Definition: SDL_syscond.c:215
struct SDL_cond SDL_cond
Definition: SDL_mutex.h:167