libusb源码学习：几个函数加载的宏(windows)

太过爱你忘了你带给我的痛 2023-06-29 15:57 3阅读 0赞

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)