1. SDL_semaphore
代码：src\thread\windows\SDL_syssem.c
别名：SDL_sem
1 typedef struct SDL_semaphore SDL_sem ;
基于 WinAPI 匿名 Semaphore 封装。MaximumCount 硬编码为 32 * 1024。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 struct SDL_semaphore { HANDLE id; LONG count; }; SDL_sem * SDL_CreateSemaphore (Uint32 initial_value) { SDL_sem *sem; sem = (SDL_sem *) SDL_malloc(sizeof (*sem)); if (sem) { #if __WINRT__ sem->id = CreateSemaphoreEx(NULL , initial_value, 32 * 1024 , NULL , 0 , SEMAPHORE_ALL_ACCESS); #else sem->id = CreateSemaphore(NULL , initial_value, 32 * 1024 , NULL ); #endif sem->count = initial_value; if (!sem->id) { SDL_SetError("Couldn't create semaphore" ); SDL_free(sem); sem = NULL ; } } else { SDL_OutOfMemory(); } return (sem); }
2. SDL_mutex
代码：src\thread\windows\SDL_sysmutex.c
基于 WinAPI CriticalSection 封装。SpinCount 硬编码为 2000，即在多处理器系统上，如果无法立刻进入临界区，则会自旋最多 2000 次，然后等待 CriticalSection 内部关联的信号量。只要在自旋过程中其它线程退出临界区，则无需进入等待状态。这么做是提高效率，自旋时当前线程还占着 CPU，如果进入等待状态，就是交出 CPU 时间片了，而 CPU 调度是个消耗型操作。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 struct SDL_mutex { CRITICAL_SECTION cs; }; SDL_mutex * SDL_CreateMutex (void ) { SDL_mutex *mutex; mutex = (SDL_mutex *) SDL_malloc(sizeof (*mutex)); if (mutex) { #if __WINRT__ InitializeCriticalSectionEx(&mutex->cs, 2000 , 0 ); #else InitializeCriticalSectionAndSpinCount(&mutex->cs, 2000 ); #endif } else { SDL_OutOfMemory(); } return (mutex); }
3. SDL_cond
代码：src\thread\windows\SDL_syscond.c
基于 SDL_mutex 和 SDL_sem 封装。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 struct SDL_cond { SDL_mutex *lock; int waiting; int signals; SDL_sem *wait_sem; SDL_sem *wait_done; }; SDL_cond * SDL_CreateCond (void ) { SDL_cond *cond; cond = (SDL_cond *) SDL_malloc(sizeof (SDL_cond)); if (cond) { cond->lock = SDL_CreateMutex(); cond->wait_sem = SDL_CreateSemaphore(0 ); cond->wait_done = SDL_CreateSemaphore(0 ); cond->waiting = cond->signals = 0 ; if (!cond->lock || !cond->wait_sem || !cond->wait_done) { SDL_DestroyCond(cond); cond = NULL ; } } else { SDL_OutOfMemory(); } return (cond); }
SDL_CondWaitTimeout 实现较长，本文忽略。重点是：为了避免死锁，它进入等待前，会先解锁第二个参数 mutex。如果不这么做，其它线程也要 Lock 这个 mutex 就会发生死锁。
以下代码是典型用法，线程 A 先 进入临界区后，SDL_CondWait（内部调用 SDL_CondWaitTimeout）会调用 SDL_UnlockMutex(lock); 使得线程 B 可以进入临界区调用 SDL_CondSignal(cond);。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 SDL_LockMutex(lock); while (!condition) { SDL_CondWait(cond, lock); } SDL_UnlockMutex(lock); SDL_LockMutex(lock); condition = true ; SDL_CondSignal(cond); SDL_UnlockMutex(lock);
4. SDL_Atomic
代码：src\atomic\SDL_atomic.c
基于 _Interlocked API 封装。此类原子操作一般底层实现都是相应平台的汇编指令（比如 x86 平台是 lock cmpxchg 之类），但在不同平台下会有不同的封装集，所以 SDL_atomic.c 里有很多平台相关的宏判断。
5. SDL_MemoryBarrier
代码：src\atomic\SDL_atomic.h
内存屏障。参考文章：Acquire and Release Semantics 。
在 Windows x86 环境下等价于 _ReadWriteBarrier：
1 2 3 4 5 6 7 void _ReadWriteBarrier(void );#pragma intrinsic(_ReadWriteBarrier) #define SDL_CompilerBarrier() _ReadWriteBarrier() #define SDL_MemoryBarrierRelease() SDL_CompilerBarrier() #define SDL_MemoryBarrierAcquire() SDL_CompilerBarrier()
Acquire semantics is a property that can only apply to operations that read from shared memory, whether they are read-modify-write operations or plain loads. The operation is then considered a read-acquire . Acquire semantics prevent memory reordering of the read-acquire with any read or write operation that follows it in program order.
Release semantics is a property that can only apply to operations that write to shared memory, whether they are read-modify-write operations or plain stores. The operation is then considered a write-release . Release semantics prevent memory reordering of the write-release with any read or write operation that precedes it in program order.
生硬的翻译如下：
以 x86 内存模型为例说明：
Loads are not reordered with other loads.
Stores are not reordered with other stores.
Stores are not reordered with older loads.
Loads may be reordered with older stores to different locations.
因为 store-load 可以被重排，所以 x86 不是顺序一致。但是其他三种读写顺序不能被重排，所以 x86 是 acquire/release 语义。
aquire 语义：load 之后的读写操作无法被重排至 load 之前。即 load-load, load-store 不能被重排。
release 语义：store 之前的读写操作无法被重排至 store 之后。即 load-store, store-store 不能被重排。
6. SDL_TLSData
意义：TLS，即 Thread Local Storage（线程局部存储）。
代码：src\thread\SDL_thread_c.h 和 src\thread\windows\SDL_systls.c
基于 SDL_Atomic、SDL_MemoryBarrier 和 WinAPI Tls API 封装。
以下结构体包含一个析构函数的指针，非空时，SDL_TLSCleanup() 会调用它。
1 2 3 4 5 6 7 8 typedef struct { unsigned int limit; struct { void *data; void (SDLCALL *destructor)(void *); } array [1 ]; } SDL_TLSData;
7. SDL_Thread
代码：src\thread\SDL_thread.c 和 src\thread\windows\SDL_systhread.c
创建线程的 API 是 SDL_CreateThread 和 SDL_CreateThreadWithStackSize，导出函数 SDL_CreateThread 的定义如下，记为【X】：
1 SDL_DYNAPI_PROC(SDL_Thread*,SDL_CreateThread,(SDL_ThreadFunction a, const char *b, void *c, pfnSDL_CurrentBeginThread d, pfnSDL_CurrentEndThread e),(a,b,c,d,e),return )
下面会有递归展开宏的过程。首先，用 SDL_DYNAPI_PROC 的定义：
1 2 3 4 5 #define SDL_DYNAPI_PROC(rc,fn,params,args,ret) \ static rc SDLCALL fn##_DEFAULT params { \ SDL_InitDynamicAPI(); \ ret jump_table.fn args; \ }
展开【X】得到：
1 2 3 4 static SDL_Thread* __cdecl SDL_CreateThread (SDL_ThreadFunction a, const char *b, void *c, pfnSDL_CurrentBeginThread d, pfnSDL_CurrentEndThread e) { SDL_InitDynamicAPI(); return jump_table.SDL_CreateThread(a,b,c,d,e); }
其中 jump_table.SDL_CreateThread 是【X】被 SDL_dynapi_procs.h 的：
1 2 3 4 5 6 7 8 9 10 11 12 13 typedef struct { #define SDL_DYNAPI_PROC(rc,fn,params,args,ret) SDL_DYNAPIFN_##fn fn; #include "SDL_dynapi_procs.h" #undef SDL_DYNAPI_PROC } SDL_DYNAPI_jump_table; static SDL_DYNAPI_jump_table jump_table = { #define SDL_DYNAPI_PROC(rc,fn,params,args,ret) fn##_DEFAULT, #include "SDL_dynapi_procs.h" #undef SDL_DYNAPI_PROC };
展开得到，为：
1 2 3 4 5 6 7 8 9 10 11 typedef struct { SDL_DYNAPIFN_SDL_CreateThread SDL_CreateThread; } SDL_DYNAPI_jump_table; static SDL_DYNAPI_jump_table jump_table = { SDL_CreateThread_DEFAULT, };
【X】又被 SDL_dynapi.c 的 initialize_jumptable 函数的：
1 2 3 4 #define SDL_DYNAPI_PROC(rc,fn,params,args,ret) jump_table.fn = fn##_REAL; #include "SDL_dynapi_procs.h" #undef SDL_DYNAPI_PROC
展开为：
1 2 3 jump_table.SDL_CreateThread = SDL_CreateThread_REAL;
所以，调用 SDL_CreateThread 最终调用的就是 SDL_CreateThread_REAL，又由于 src\dynapi\SDL_dynapi_overrides.h 中的：
1 #define SDL_CreateThread SDL_CreateThread_REAL
所以调用的是 src\thread\SDL_thread.c 中的：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 #ifdef SDL_PASSED_BEGINTHREAD_ENDTHREAD DECLSPEC SDL_Thread *SDLCALL SDL_CreateThread (int (SDLCALL * fn) (void *), const char *name, void *data, pfnSDL_CurrentBeginThread pfnBeginThread, pfnSDL_CurrentEndThread pfnEndThread) #else DECLSPEC SDL_Thread *SDLCALL SDL_CreateThread (int (SDLCALL * fn) (void *), const char *name, void *data) #endif { const char *stackhint = SDL_GetHint(SDL_HINT_THREAD_STACK_SIZE); size_t stacksize = 0 ; if (stackhint != NULL ) { char *endp = NULL ; const Sint64 hintval = SDL_strtoll(stackhint, &endp, 10 ); if ((*stackhint != '\0' ) && (*endp == '\0' )) { if (hintval > 0 ) { stacksize = (size_t ) hintval; } } } #ifdef SDL_PASSED_BEGINTHREAD_ENDTHREAD return SDL_CreateThreadWithStackSize(fn, name, stacksize, data, pfnBeginThread, pfnEndThread); #else return SDL_CreateThreadWithStackSize(fn, name, stacksize, data); #endif }
可见 SDL_CreateThread 调用了 SDL_CreateThreadWithStackSize，而 SDL_CreateThreadWithStackSize 又调用 src\thread\windows\SDL_systhread.c 中的 SDL_SYS_CreateThread，因为 Windows 平台有 _beginthreadex 和 _endthreadex，所以最后是调用 _beginthreadex：
1 2 3 4 5 6 7 8 9 10 11 12 13 if (pfnBeginThread) { unsigned threadid = 0 ; thread->handle = (SYS_ThreadHandle) ((size_t ) pfnBeginThread(NULL , (unsigned int ) thread->stacksize, RunThreadViaBeginThreadEx, pThreadParms, flags, &threadid)); } else { DWORD threadid = 0 ; thread->handle = CreateThread(NULL , thread->stacksize, RunThreadViaCreateThread, pThreadParms, flags, &threadid); }
其中 RunThreadViaBeginThreadEx 实际上是调用 RunThread:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 static DWORDRunThread (void *data) { pThreadStartParms pThreadParms = (pThreadStartParms) data; pfnSDL_CurrentEndThread pfnEndThread = pThreadParms->pfnCurrentEndThread; void *args = pThreadParms->args; SDL_free(pThreadParms); SDL_RunThread(args); if (pfnEndThread != NULL ) pfnEndThread(0 ); return (0 ); } static DWORD WINAPIRunThreadViaCreateThread (LPVOID data) { return RunThread(data); } static unsigned __stdcallRunThreadViaBeginThreadEx (void *data) { return (unsigned ) RunThread(data); }
从代码可见 RunThread 调用 SDL_RunThread，而 SDL_RunThread 内部由 SDL_TLSCleanup() 来调用析构函数：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 void SDL_RunThread (void *data) { thread_args *args = (thread_args *) data; int (SDLCALL * userfunc) (void *) = args->func; void *userdata = args->data; SDL_Thread *thread = args->info; int *statusloc = &thread->status; SDL_SYS_SetupThread(thread->name); thread->threadid = SDL_ThreadID(); SDL_SemPost(args->wait); *statusloc = userfunc(userdata); SDL_TLSCleanup(); if (!SDL_AtomicCAS(&thread->state, SDL_THREAD_STATE_ALIVE, SDL_THREAD_STATE_ZOMBIE)) { if (SDL_AtomicCAS(&thread->state, SDL_THREAD_STATE_DETACHED, SDL_THREAD_STATE_CLEANED)) { if (thread->name) { SDL_free(thread->name); } SDL_free(thread); } } }
