Simple Assembly Hooking

typedef struct
{
	BOOL hooked;
	INT_PTR origAddress;
	INT_PTR hookAddress;
	BYTE origAsm[6];
	BYTE hookedAsm[6];
} HOOK;

BOOL HookFunction( HOOK* hook )
{
	if( hook->hooked )
		return TRUE;

	DWORD oldProtection;
	DWORD numBytes;

	if( *(INT_PTR*)(hook->hookedAsm + 1) == 0 )
	{
		hook->hookedAsm[0] = 0xe9; // jmp
		hook->hookedAsm[5] = 0xc3; // retn

		INT_PTR relativeAddress = hook->hookAddress - hook->origAddress - 5;
		*(INT_PTR*)(hook->hookedAsm + 1) = relativeAddress;
	}

	VirtualProtect( (LPVOID)hook->origAddress, 6, PAGE_READWRITE, &oldProtection );

	if( !ReadProcessMemory( GetCurrentProcess(), (LPVOID)hook->origAddress, hook->origAsm, 6, &numBytes ) || numBytes != 6 )
		return FALSE;

	numBytes = 0;

	if( !WriteProcessMemory( GetCurrentProcess(), (LPVOID)hook->origAddress, hook->hookedAsm, 6, &numBytes ) || numBytes != 6 )
		return FALSE;

	VirtualProtect( (LPVOID)hook->origAddress, 6, oldProtection, NULL );

	hook->hooked = TRUE;

	return TRUE;
}

BOOL UnhookFunction( HOOK* hook )
{
	if( !hook->hooked )
		return TRUE;

	DWORD oldProtection;
	DWORD numBytes;

	VirtualProtect( (LPVOID)hook->origAddress, 6, PAGE_READWRITE, &oldProtection );

	if( !WriteProcessMemory( GetCurrentProcess(), (LPVOID)hook->origAddress, hook->origAsm, 6, &numBytes ) || numBytes != 6 )
		return FALSE;

	VirtualProtect( (LPVOID)hook->origAddress, 6, oldProtection, NULL );

	hook->hooked = FALSE;

	return TRUE;
}

static HOOK hkLoadLibraryA;

extern "C" HMODULE WINAPI LoadLibraryAHook( LPCSTR lpLibFileName )
{
	UnhookFunction( &hkLoadLibraryA );
	HMODULE returned = LoadLibraryA( lpLibFileName );
	HookFunction( &hkLoadLibraryA );

	// Own code goes here

	return returned;
}

void Hook()
{
	hkLoadLibraryA.origAddress = (INT_PTR)LoadLibraryA;
	hkLoadLibraryA.hookAddress = (INT_PTR)LoadLibraryAHook;
	HookFunction( &hkLoadLibraryA );
}