pthread 静态编译版本在Windows下使用时的注意事项

作为通用的跨平台高性能线程库，在很多跨平台的项目中都可以看见pthread的身影。pthread本身的实现比较优雅，APIs使用起来也很方便。

但在Windows下使用静态编译的pthread时要特别注意一下，必须显式的调用如下四个函数，否则pthread用到的一些全局变量会没有被初始化，导致所有的pthread的APIs调用都crash.

BOOL pthread_win32_process_attach_np (void);

BOOL pthread_win32_process_detach_np (void);

BOOL pthread_win32_thread_attach_np (void);

BOOL pthread_win32_thread_detach_np (void);

pthread官方文档对此有如下的明确说明：

These functions contain the code normally run via dllMain

when the library is used as a dll but which need to be

called explicitly by an application when the library

is statically linked.

You will need to call pthread_win32_process_attach_np() before

you can call any pthread routines when statically linking.

You should call pthread_win32_process_detach_np() before

exiting your application to clean up.

pthread_win32_thread_attach_np() is currently a no-op, but

pthread_win32_thread_detach_np() is needed to clean up

the implicit pthread handle that is allocated to a Win32 thread if

it calls certain pthreads routines. Call this routine when the

Win32 thread exits.

These functions invariably return TRUE except for

pthread_win32_process_attach_np() which will return FALSE

if pthreads-win32 initialisation fails.

通过函数的名字我们不难猜测出如下调用顺序

在程序开始的时候要调用：

BOOL pthread_win32_process_attach_np (void);

BOOL pthread_win32_thread_attach_np (void);

在程序退出时要调用：

BOOL pthread_win32_thread_detach_np (void);

BOOL pthread_win32_process_detach_np (void);

比较通用的做法是在模块Load和UnLoad的时候做这个attach和detach操作，如下面所示：

/* Callback for our DLL so we can initialize pthread */

BOOL WINAPI DllMain( HANDLE hinstDLL, DWORD fdwReason, LPVOID lpvReserved )

{

#ifdef PTW32_STATIC_LIB

switch( fdwReason )

{

case DLL_PROCESS_ATTACH:

pthread_win32_process_attach_np();

case DLL_THREAD_ATTACH:

pthread_win32_thread_attach_np();

break;

case DLL_THREAD_DETACH:

pthread_win32_thread_detach_np();

break;

case DLL_PROCESS_DETACH:

pthread_win32_thread_detach_np();

pthread_win32_process_detach_np();

break;

}

#endif

return TRUE;

}

注意： PTW32_STATIC_LIB 宏为pthread静态编译的标志，这个可以通过pthread.h的配置或者CFLAGS传递进来。
下面是pthread的官方的dll.c的实现

BOOL WINAPI
DllMain (HINSTANCE hinstDll, DWORD fdwReason, LPVOID lpvReserved)
{
BOOL result = PTW32_TRUE;
switch (fdwReason)
{
case DLL_PROCESS_ATTACH:
result = pthread_win32_process_attach_np ();
break;
case DLL_THREAD_ATTACH:
/*
* A thread is being created
*/
result = pthread_win32_thread_attach_np ();
break;
case DLL_THREAD_DETACH:
/*
* A thread is exiting cleanly
*/
result = pthread_win32_thread_detach_np ();
break;
case DLL_PROCESS_DETACH:
(void) pthread_win32_thread_detach_np ();
result = pthread_win32_process_detach_np ();
break;
}
return (result);
} /* DllMain */

也就是说pthread官方代码在动态编译的版本中主动做了这个attach和detach操作。

而静态编译版本由于没有一个合适的地方来做这件事，就将attach和detach的的操作扔给用户来完成了。

上面的代码是针对调用方是Dll的情况做的初始化，如果调用方不是Dll呢？对此可以参照如下做法，虽然很暴力，但很简单，可以工作

1）定义如下函数

#ifdef PTW32_STATIC_LIB
static void detach_ptw32(void)
{
pthread_win32_thread_detach_np();
pthread_win32_process_detach_np();
}
#endif

2）在你的主程序的入口处，一般而言是main()中做如下调用即可

#ifdef PTW32_STATIC_LIB
pthread_win32_process_attach_np();
pthread_win32_thread_attach_np();
atexit(detach_ptw32);
#endif

也就是用atexit()将detach工作挂接到程序中去，使得程序在退出的时候可以对pthread进行detach.