Atlas - SDL_sysmutex.c
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[FILE BEGIN]
1/*
2  Simple DirectMedia Layer
3  Copyright (C) 1997-2025 Sam Lantinga <[email protected]>
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_systhread_c.h"
26
27struct SDL_Mutex
28{
29    int recursive;
30    SDL_ThreadID owner;
31    SDL_Semaphore *sem;
32};
33
34SDL_Mutex *SDL_CreateMutex(void)
35{
36    SDL_Mutex *mutex = (SDL_Mutex *)SDL_calloc(1, sizeof(*mutex));
37
38#ifndef SDL_THREADS_DISABLED
39    if (mutex) {
40        // Create the mutex semaphore, with initial value 1
41        mutex->sem = SDL_CreateSemaphore(1);
42        mutex->recursive = 0;
43        mutex->owner = 0;
44        if (!mutex->sem) {
45            SDL_free(mutex);
46            mutex = NULL;
47        }
48    }
49#endif // !SDL_THREADS_DISABLED
50
51    return mutex;
52}
53
54void SDL_DestroyMutex(SDL_Mutex *mutex)
55{
56    if (mutex) {
57        if (mutex->sem) {
58            SDL_DestroySemaphore(mutex->sem);
59        }
60        SDL_free(mutex);
61    }
62}
63
64void SDL_LockMutex(SDL_Mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
65{
66#ifndef SDL_THREADS_DISABLED
67    if (mutex != NULL) {
68        SDL_ThreadID this_thread = SDL_GetCurrentThreadID();
69        if (mutex->owner == this_thread) {
70            ++mutex->recursive;
71        } else {
72            /* The order of operations is important.
73               We set the locking thread id after we obtain the lock
74               so unlocks from other threads will fail.
75             */
76            SDL_WaitSemaphore(mutex->sem);
77            mutex->owner = this_thread;
78            mutex->recursive = 0;
79        }
80    }
81#endif // SDL_THREADS_DISABLED
82}
83
84bool SDL_TryLockMutex(SDL_Mutex *mutex)
85{
86    bool result = true;
87#ifndef SDL_THREADS_DISABLED
88    if (mutex) {
89        SDL_ThreadID this_thread = SDL_GetCurrentThreadID();
90        if (mutex->owner == this_thread) {
91            ++mutex->recursive;
92        } else {
93            /* The order of operations is important.
94               We set the locking thread id after we obtain the lock
95               so unlocks from other threads will fail.
96             */
97            result = SDL_TryWaitSemaphore(mutex->sem);
98            if (result) {
99                mutex->owner = this_thread;
100                mutex->recursive = 0;
101            }
102        }
103    }
104#endif // SDL_THREADS_DISABLED
105    return result;
106}
107
108void SDL_UnlockMutex(SDL_Mutex *mutex) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
109{
110#ifndef SDL_THREADS_DISABLED
111    if (mutex != NULL) {
112        // If we don't own the mutex, we can't unlock it
113        if (SDL_GetCurrentThreadID() != mutex->owner) {
114            SDL_assert(!"Tried to unlock a mutex we don't own!");
115            return; // (undefined behavior!) SDL_SetError("mutex not owned by this thread");
116        }
117
118        if (mutex->recursive) {
119            --mutex->recursive;
120        } else {
121            /* The order of operations is important.
122               First reset the owner so another thread doesn't lock
123               the mutex and set the ownership before we reset it,
124               then release the lock semaphore.
125             */
126            mutex->owner = 0;
127            SDL_SignalSemaphore(mutex->sem);
128        }
129    }
130#endif // SDL_THREADS_DISABLED
131}
132
133
[FILE END]
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