Strange "invalid conversion from <type> to <type>" error

Question

Here is output:

g++ -DDEBUG -DUNITTEST -IC:/Users/Steven/Dropbox/Programming/entropy_p5_makefile/cpp/game/../include/ -O0 -g3 -Wall -c -fmessage-length=0 -o Input.o ..\Input.cpp
..\Input.cpp: In function 'void mousehookCustomRoutine(E_thread*, void*)':
..\Input.cpp:78:93: error: invalid conversion from 'LRESULT (*)(int, WPARAM, LPARAM)' to 'LRESULT (*)(int, WPARAM, LPARAM)'
..\Input.cpp:78:93: error:   initializing argument 2 of 'void* SetWindowsHookExA(int, LRESULT (*)(int, WPARAM, LPARAM), HINSTANCE__*, DWORD)'
Build error occurred, build is stopped

This is the code:

LRESULT CALLBACK MouseHookProc(int nCode, WPARAM wParam, LPARAM lParam) {
    //...
}

void mousehookCustomRoutine(E_thread *me, void *arg = (void *)&MouseHookProc) {
    // arg is the ptr to LL Mouse Routine

    me->sendMessage(0x14,me,(void*)GetCurrentThreadId());
    // send message to self in order for my parent to know how to identify me via threadID
    HHOOK mousehook = SetWindowsHookEx(WH_MOUSE_LL, (LRESULT (*)(int,WPARAM,LPARAM))arg,NULL, 0); // I am line 78
    if (mousehook == NULL) printf("Mousehook error %lu\n",GetLastError());
    //...
}

It makes no sense because I am casting to the exact type that it expects to receive, there aren't any qualifiers or anything that are different. What could possibly be going on here?

Answer 1

The compiler is omitting the calling convention from the error message -- SetWindowsHookEx wants a LRESULT (__stdcall *)(int,WPARAM,LPARAM), but you're passing it a LRESULT (__cdecl *)(int,WPARAM,LPARAM).

Answer 2

First, don't cast function pointers to void *. It is never safe to invoke a function of one type after casting it to another, and so there's rarely, if ever, a need to make them void *.

Second, probably you're seeing a difference in calling conventions. The type of the hook parameter is, properly stated, (LRESULT (CALLBACK *) (int, WPARAM, LPARAM)), or simply HOOKPROC. The prototype of the function should look like LRESULT CALLBACK MouseHookProc(int, WPARAM, LPARAM). Most likely gcc doesn't pretty-print the calling-convention specifier, but does check for it when checking type equivalence. Subtle problems like this are another reason not to cast function pointers - had you used a (HOOKPROC) cast, you would have had no compile-time error, but could have crashed at runtime .... but only on certain versions of windows.