libusb源码学习：几个函数加载的宏(windows)

首先，我们复习一下C语言中的函数指针：

//C语言中的函数指针
#include <iostream>

// 下面typeFunc就一个函数指针，指向的函数类型 f 为
// void f(void)
typedef void (*typeFunc)(void);

void realFunc(void) {
	std::cout << "test" << std::endl;
}

int main() {
	typeFunc myf = realFunc;
	myf(); // 相当于调用realFunc();
	return 0;
}

然后我们看一下libusb中定义的那几个宏

/*
 * Macros for handling functions within a DLL
 */
#define DLL_FUNC_NAME(name) __dll_##name##_func_t

#define DLL_DECLARE_FUNC_PREFIXNAME(api, ret, prefixname, name, args)	\
	typedef ret (api * DLL_FUNC_NAME(name))args;			\
	static DLL_FUNC_NAME(name) prefixname = NULL

#define DLL_DECLARE_FUNC(api, ret, name, args)				\
	DLL_DECLARE_FUNC_PREFIXNAME(api, ret, name, name, args)
#define DLL_DECLARE_FUNC_PREFIXED(api, ret, prefix, name, args)		\
	DLL_DECLARE_FUNC_PREFIXNAME(api, ret, prefix##name, name, args)

#define DLL_LOAD_FUNC_PREFIXNAME(dll, prefixname, name, ret_on_failure)	\
	do {								\
		HMODULE h = DLL_HANDLE_NAME(dll);			\
		prefixname = (DLL_FUNC_NAME(name))GetProcAddress(h,	\
				DLL_STRINGIFY(name));			\
		if (prefixname)						\
			break;						\
		prefixname = (DLL_FUNC_NAME(name))GetProcAddress(h,	\
				DLL_STRINGIFY(name) DLL_STRINGIFY(A));	\
		if (prefixname)						\
			break;						\
		prefixname = (DLL_FUNC_NAME(name))GetProcAddress(h,	\
				DLL_STRINGIFY(name) DLL_STRINGIFY(W));	\
		if (prefixname)						\
			break;						\
		if (ret_on_failure)					\
			return FALSE;					\
	} while (0)

#define DLL_LOAD_FUNC(dll, name, ret_on_failure)			\
	DLL_LOAD_FUNC_PREFIXNAME(dll, name, name, ret_on_failure)
#define DLL_LOAD_FUNC_PREFIXED(dll, prefix, name, ret_on_failure)	\
	DLL_LOAD_FUNC_PREFIXNAME(dll, prefix##name, name, ret_on_failure)

我们发现，本来调用的函数名称如：SetupDiGetClassDevsA，

现在统统都加上了指针前缀，变成：pSetupDiGetClassDevsA

这些函数是在windows_usb.h中定义的，展开其中的一个定义，过程如下，

DLL_DECLARE_FUNC_PREFIXED(WINAPI, HDEVINFO, p, SetupDiGetClassDevsA, (LPCGUID, PCSTR, HWND, DWORD));
<==等价于==>
DLL_DECLARE_FUNC_PREFIXNAME(WINAPI, HDEVINFO, pSetupDiGetClassDevsA, SetupDiGetClassDevsA, (LPCGUID, PCSTR, HWND, DWORD))
<==等价于==>
typedef HDEVINFO (WINAPI * __dll_SetupDiGetClassDevsA_func_t)(LPCGUID, PCSTR, HWND, DWORD);
static __dll_SetupDiGetClassDevsA_func_t pSetupDiGetClassDevsA = NULL

就是说，
pSetupDiGetClassDevsA相当于定义了一个函数指针，类型为__dll_SetupDiGetClassDevsA_func_t的变量：

问题是，__dll_SetupDiGetClassDevsA_func_t这个函数指针类型是在哪里定义的呢？

static BOOL init_dlls(void)
{
	DLL_GET_HANDLE(Cfgmgr32);
	DLL_LOAD_FUNC(Cfgmgr32, CM_Get_Parent, TRUE);
	DLL_LOAD_FUNC(Cfgmgr32, CM_Get_Child, TRUE);

	// Prefixed to avoid conflict with header files
	DLL_GET_HANDLE(AdvAPI32);
	DLL_LOAD_FUNC_PREFIXED(AdvAPI32, p, RegQueryValueExW, TRUE);
	DLL_LOAD_FUNC_PREFIXED(AdvAPI32, p, RegCloseKey, TRUE);

	DLL_GET_HANDLE(OLE32);
	DLL_LOAD_FUNC_PREFIXED(OLE32, p, IIDFromString, TRUE);

	DLL_GET_HANDLE(SetupAPI);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiGetClassDevsA, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiEnumDeviceInfo, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiEnumDeviceInterfaces, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiGetDeviceInstanceIdA, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiGetDeviceInterfaceDetailA, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiGetDeviceRegistryPropertyA, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiDestroyDeviceInfoList, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiOpenDevRegKey, TRUE);
	DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiOpenDeviceInterfaceRegKey, TRUE);

	return TRUE;
}

其中，DLL_GET_HANLE相当于LoadLibraryA(),

#define DLL_GET_HANDLE(name)					\
	do {							\
		DLL_HANDLE_NAME(name) = DLL_LOAD_LIBRARY(name);	\
		if (!DLL_HANDLE_NAME(name))			\
			return FALSE;				\
	} while (0)


//API macros - leveraged from libusb-win32 1.x
#ifndef _WIN32_WCE
#define DLL_STRINGIFY(s) #s  //把传入的名称字符串化：名称变成字符串)
#define DLL_LOAD_LIBRARY(name) LoadLibraryA(DLL_STRINGIFY(name))
#else
#define DLL_STRINGIFY(s) L#s
#define DLL_LOAD_LIBRARY(name) LoadLibrary(DLL_STRINGIFY(name))
#endif

核心是这个DLL_LOAD_FUNC_PREFIXED的定义；在这里定义了类型为__dll_SetupDiGetClassDevsA_func_t的变量pSetupDiGetClassDevsA，并且其函数地址为GetProcAddress(h, "SetupDiGetClassDevsA");

其相应的宏展开过程如下所示，

DLL_LOAD_FUNC_PREFIXED(SetupAPI, p, SetupDiGetClassDevsA, TRUE);
<==eqivalent to ==>
DLL_LOAD_FUNC_PREFIXNAME(SetupAPI, pSetupDiGetClassDevsA, SetupDiGetClassDevsA, TRUE)	\
	do {
		HMODULE h = DLL_HANDLE_NAME(SetupAPI);
		pSetupDiGetClassDevsA = (DLL_FUNC_NAME(SetupDiGetClassDevsA))GetProcAddress(h,
				DLL_STRINGIFY(SetupDiGetClassDevsA));
		if (pSetupDiGetClassDevsA)
			break;
		pSetupDiGetClassDevsA = (DLL_FUNC_NAME(SetupDiGetClassDevsA))GetProcAddress(h,
				DLL_STRINGIFY(SetupDiGetClassDevsA) DLL_STRINGIFY(A));
		if (pSetupDiGetClassDevsA)
			break;
		pSetupDiGetClassDevsA = (DLL_FUNC_NAME(SetupDiGetClassDevsA))GetProcAddress(h,
				DLL_STRINGIFY(SetupDiGetClassDevsA) DLL_STRINGIFY(W));
		if (pSetupDiGetClassDevsA)
			break;
		if (TRUE)
			return FALSE;
	} while (0)