Atlas - SDL_sysrwlock.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 rwlocks using mutexes, condition variables, and atomics.
24
25#include "SDL_systhread_c.h"
26
27#include "../generic/SDL_sysrwlock_c.h"
28
29/* If two implementations are to be compiled into SDL (the active one
30 * will be chosen at runtime), the function names need to be
31 * suffixed
32 */
33// !!! FIXME: this is quite a tapdance with macros and the build system, maybe we can simplify how we do this. --ryan.
34#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
35#define SDL_CreateRWLock_generic SDL_CreateRWLock
36#define SDL_DestroyRWLock_generic SDL_DestroyRWLock
37#define SDL_LockRWLockForReading_generic SDL_LockRWLockForReading
38#define SDL_LockRWLockForWriting_generic SDL_LockRWLockForWriting
39#define SDL_UnlockRWLock_generic SDL_UnlockRWLock
40#endif
41
42struct SDL_RWLock
43{
44#ifdef SDL_THREADS_DISABLED
45    int unused;
46#else
47    SDL_Mutex *lock;
48    SDL_Condition *condition;
49    SDL_ThreadID writer_thread;
50    SDL_AtomicInt reader_count;
51    SDL_AtomicInt writer_count;
52#endif
53};
54
55SDL_RWLock *SDL_CreateRWLock_generic(void)
56{
57    SDL_RWLock *rwlock = (SDL_RWLock *) SDL_calloc(1, sizeof (*rwlock));
58
59    if (!rwlock) {
60        return NULL;
61    }
62
63#ifndef SDL_THREADS_DISABLED
64    rwlock->lock = SDL_CreateMutex();
65    if (!rwlock->lock) {
66        SDL_free(rwlock);
67        return NULL;
68    }
69
70    rwlock->condition = SDL_CreateCondition();
71    if (!rwlock->condition) {
72        SDL_DestroyMutex(rwlock->lock);
73        SDL_free(rwlock);
74        return NULL;
75    }
76
77    SDL_SetAtomicInt(&rwlock->reader_count, 0);
78    SDL_SetAtomicInt(&rwlock->writer_count, 0);
79#endif
80
81    return rwlock;
82}
83
84void SDL_DestroyRWLock_generic(SDL_RWLock *rwlock)
85{
86    if (rwlock) {
87#ifndef SDL_THREADS_DISABLED
88        SDL_DestroyMutex(rwlock->lock);
89        SDL_DestroyCondition(rwlock->condition);
90#endif
91        SDL_free(rwlock);
92    }
93}
94
95void SDL_LockRWLockForReading_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
96{
97#ifndef SDL_THREADS_DISABLED
98    if (rwlock) {
99        // !!! FIXME: these don't have to be atomic, we always gate them behind a mutex.
100        SDL_LockMutex(rwlock->lock);
101        SDL_assert(SDL_GetAtomicInt(&rwlock->writer_count) == 0);  // shouldn't be able to grab lock if there's a writer!
102        SDL_AddAtomicInt(&rwlock->reader_count, 1);
103        SDL_UnlockMutex(rwlock->lock);   // other readers can attempt to share the lock.
104    }
105#endif
106}
107
108void SDL_LockRWLockForWriting_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
109{
110#ifndef SDL_THREADS_DISABLED
111    if (rwlock) {
112        SDL_LockMutex(rwlock->lock);
113        while (SDL_GetAtomicInt(&rwlock->reader_count) > 0) {  // while something is holding the shared lock, keep waiting.
114            SDL_WaitCondition(rwlock->condition, rwlock->lock);  // release the lock and wait for readers holding the shared lock to release it, regrab the lock.
115        }
116
117        // we hold the lock!
118        SDL_AddAtomicInt(&rwlock->writer_count, 1);  // we let these be recursive, but the API doesn't require this. It _does_ trust you unlock correctly!
119    }
120#endif
121}
122
123bool SDL_TryLockRWLockForReading_generic(SDL_RWLock *rwlock)
124{
125#ifndef SDL_THREADS_DISABLED
126    if (rwlock) {
127        if (!SDL_TryLockMutex(rwlock->lock)) {
128            // !!! FIXME: there is a small window where a reader has to lock the mutex, and if we hit that, we will return SDL_RWLOCK_TIMEDOUT even though we could have shared the lock.
129            return false;
130        }
131
132        SDL_assert(SDL_GetAtomicInt(&rwlock->writer_count) == 0);  // shouldn't be able to grab lock if there's a writer!
133        SDL_AddAtomicInt(&rwlock->reader_count, 1);
134        SDL_UnlockMutex(rwlock->lock);   // other readers can attempt to share the lock.
135    }
136#endif
137
138    return true;
139}
140
141#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
142bool SDL_TryLockRWLockForReading(SDL_RWLock *rwlock)
143{
144    return SDL_TryLockRWLockForReading_generic(rwlock);
145}
146#endif
147
148bool SDL_TryLockRWLockForWriting_generic(SDL_RWLock *rwlock)
149{
150#ifndef SDL_THREADS_DISABLED
151    if (rwlock) {
152        if (!SDL_TryLockMutex(rwlock->lock)) {
153            return false;
154        }
155
156        if (SDL_GetAtomicInt(&rwlock->reader_count) > 0) {  // a reader is using the shared lock, treat it as unavailable.
157            SDL_UnlockMutex(rwlock->lock);
158            return false;
159        }
160
161        // we hold the lock!
162        SDL_AddAtomicInt(&rwlock->writer_count, 1);  // we let these be recursive, but the API doesn't require this. It _does_ trust you unlock correctly!
163    }
164#endif
165
166    return true;
167}
168
169#ifndef SDL_THREAD_GENERIC_RWLOCK_SUFFIX
170bool SDL_TryLockRWLockForWriting(SDL_RWLock *rwlock)
171{
172    return SDL_TryLockRWLockForWriting_generic(rwlock);
173}
174#endif
175
176void SDL_UnlockRWLock_generic(SDL_RWLock *rwlock) SDL_NO_THREAD_SAFETY_ANALYSIS  // clang doesn't know about NULL mutexes
177{
178#ifndef SDL_THREADS_DISABLED
179    if (rwlock) {
180        SDL_LockMutex(rwlock->lock);  // recursive lock for writers, readers grab lock to make sure things are sane.
181
182        if (SDL_GetAtomicInt(&rwlock->reader_count) > 0) {  // we're a reader
183            SDL_AddAtomicInt(&rwlock->reader_count, -1);
184            SDL_BroadcastCondition(rwlock->condition);  // alert any pending writers to attempt to try to grab the lock again.
185        } else if (SDL_GetAtomicInt(&rwlock->writer_count) > 0) {  // we're a writer
186            SDL_AddAtomicInt(&rwlock->writer_count, -1);
187            SDL_UnlockMutex(rwlock->lock);  // recursive unlock.
188        }
189
190        SDL_UnlockMutex(rwlock->lock);
191    }
192#endif
193}
194
195
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