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RegisterUserApiHook seems to be implemented in user32 to be used exclusively by uxtheme. Actually it abstracts the functionality needed to apply themes in uxtheme. The problem with this function is that it is changed through Windows releases so it is difficult to test it. RegisterUserApiHook works in the same way a global hook works. The only difference is that right after user32 loads the specified library into every application, it calls the (specified) callback to give the loaded library the ability to override some functionality that exists in user32.

This is the prototype in Windows XP:

BOOL WINAPI RegisterUserApiHook(HINSTANCE hInstance, PVOID CallbackFunc);

And this is the prototype in win2k3:

typedef struct _USERAPIHOOKINFO
DWORD m_size;
LPCWSTR m_dllname1;
LPCWSTR m_funname1;
LPCWSTR m_dllname2;
LPCWSTR m_funname2;


In both cases we give to RegisterUserApiHook the dll that is going to be loaded and the callback function. This is the prototype of the callback function:

typedef struct
DWORD size;
WNDPROC DefWindowProcA;
WNDPROC DefWindowProcW;
DWORD* DefWndProcArray;
DWORD DefWndProcArraySize;
FARPROC GetScrollInfo;
FARPROC SetScrollInfo;
FARPROC EnableScrollBar;
FARPROC AdjustWindowRectEx;
FARPROC SetWindowRng;
WNDPROC PostWndProc;
DWORD* WndProcArray;
DWORD WndProcArraySize;
WNDPROC PreDefDlgProc;
WNDPROC PostDefDlgProc;
DWORD* DlgProcArray;
DWORD DlgProcArraySize;
FARPROC GetSystemMetrics;
FARPROC SystemParametersInfoA;
FARPROC SystemParametersInfoW;
FARPROC ForceResetUserApiHook;
FARPROC DrawFrameControl;
FARPROC DrawCaption;


User32 gives to the callback function an APIHOOKINFO struct filled with the original implementation of the functions that the library can use from user32. Then the function replaces the functions from user32 with its own functions.

In Windows XP the callback function is internal in uxtheme.dll and isn’t exported. However in Windows 2003 this function must be exported because RegisterUserApiHook must be given its name and not a pointer to it. It is exported from uxtheme.dll with the name ThemeInitApiHook.

The weirdest things in the APIHOOKINFO struct are the DefWndProcArray, WndProcArray and DlgProcArray fields. These seem to control when the overridden DefWindowProc, Pre/PostWndProc and PreDefDlgProc will be called. If they are set to NULL, the overridden functions never get called.

By testing ThemeInitApiHook, I could dump the contents of these arrays. If I use these, the overridden functions are called correctly in both Windows XP and 2003 :

DWORD DefWindowProcMagic[25]= {0x1000, 0x0, 0x80, 0x28000000, 0x75, 0xc003, 0x0, 0x0,
DWORD PrePostWindowProcMagic[25]= {0x4000002,0x1800,0xc0,0x30000000,0x26,0x0,0x0,0x0,0x0,0x1,