1 /*
2   Simple DirectMedia Layer
3   Copyright (C) 1997-2020 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_internal.h"
22 
23 /* An implementation of mutexes using semaphores */
24 
25 #include "SDL_thread.h"
26 #include "SDL_systhread_c.h"
27 
28 
29 struct SDL_mutex
30 {
31     int recursive;
32     SDL_threadID owner;
33     SDL_sem *sem;
34 };
35 
36 /* Create a mutex */
37 SDL_mutex *
SDL_CreateMutex(void)38 SDL_CreateMutex(void)
39 {
40     SDL_mutex *mutex;
41 
42     /* Allocate mutex memory */
43     mutex = (SDL_mutex *) SDL_malloc(sizeof(*mutex));
44     if (mutex) {
45         /* Create the mutex semaphore, with initial value 1 */
46         mutex->sem = SDL_CreateSemaphore(1);
47         mutex->recursive = 0;
48         mutex->owner = 0;
49         if (!mutex->sem) {
50             SDL_free(mutex);
51             mutex = NULL;
52         }
53     } else {
54         SDL_OutOfMemory();
55     }
56     return mutex;
57 }
58 
59 /* Free the mutex */
60 void
SDL_DestroyMutex(SDL_mutex * mutex)61 SDL_DestroyMutex(SDL_mutex * mutex)
62 {
63     if (mutex) {
64         if (mutex->sem) {
65             SDL_DestroySemaphore(mutex->sem);
66         }
67         SDL_free(mutex);
68     }
69 }
70 
71 /* Lock the mutex */
72 int
SDL_LockMutex(SDL_mutex * mutex)73 SDL_LockMutex(SDL_mutex * mutex)
74 {
75 #if SDL_THREADS_DISABLED
76     return 0;
77 #else
78     SDL_threadID this_thread;
79 
80     if (mutex == NULL) {
81         return SDL_SetError("Passed a NULL mutex");
82     }
83 
84     this_thread = SDL_ThreadID();
85     if (mutex->owner == this_thread) {
86         ++mutex->recursive;
87     } else {
88         /* The order of operations is important.
89            We set the locking thread id after we obtain the lock
90            so unlocks from other threads will fail.
91          */
92         SDL_SemWait(mutex->sem);
93         mutex->owner = this_thread;
94         mutex->recursive = 0;
95     }
96 
97     return 0;
98 #endif /* SDL_THREADS_DISABLED */
99 }
100 
101 /* try Lock the mutex */
102 int
SDL_TryLockMutex(SDL_mutex * mutex)103 SDL_TryLockMutex(SDL_mutex * mutex)
104 {
105 #if SDL_THREADS_DISABLED
106     return 0;
107 #else
108     int retval = 0;
109     SDL_threadID this_thread;
110 
111     if (mutex == NULL) {
112         return SDL_SetError("Passed a NULL mutex");
113     }
114 
115     this_thread = SDL_ThreadID();
116     if (mutex->owner == this_thread) {
117         ++mutex->recursive;
118     } else {
119         /* The order of operations is important.
120          We set the locking thread id after we obtain the lock
121          so unlocks from other threads will fail.
122          */
123         retval = SDL_SemWait(mutex->sem);
124         if (retval == 0) {
125             mutex->owner = this_thread;
126             mutex->recursive = 0;
127         }
128     }
129 
130     return retval;
131 #endif /* SDL_THREADS_DISABLED */
132 }
133 
134 /* Unlock the mutex */
135 int
SDL_mutexV(SDL_mutex * mutex)136 SDL_mutexV(SDL_mutex * mutex)
137 {
138 #if SDL_THREADS_DISABLED
139     return 0;
140 #else
141     if (mutex == NULL) {
142         return SDL_SetError("Passed a NULL mutex");
143     }
144 
145     /* If we don't own the mutex, we can't unlock it */
146     if (SDL_ThreadID() != mutex->owner) {
147         return SDL_SetError("mutex not owned by this thread");
148     }
149 
150     if (mutex->recursive) {
151         --mutex->recursive;
152     } else {
153         /* The order of operations is important.
154            First reset the owner so another thread doesn't lock
155            the mutex and set the ownership before we reset it,
156            then release the lock semaphore.
157          */
158         mutex->owner = 0;
159         SDL_SemPost(mutex->sem);
160     }
161     return 0;
162 #endif /* SDL_THREADS_DISABLED */
163 }
164 
165 /* vi: set ts=4 sw=4 expandtab: */
166