How to vfscanf in Win32

[Emmanuel Mogenet]

 
For some reason, vfprintf is in the Windows API, but vfscanf isn't.

Quite annoying, especially if you consider that the call actually exists in msvcrt (it's called _input)

Unfortunately, it isn't exported.

However, there is a way, something along the lines of:

#ifdef WIN32
static uint32_t savedESP;
static uint32_t savedEAX;
static uint32_t savedRET;
static uint32_t savedThis;
static void     *truefscanf= (void*)fscanf;

class File
{
public:
                ...
                int     scanf(const char*,...);
                ...
private:
                void    *file;
};

/*
 * The things you gotta do on NT ...
 * No vfscanf, so got to trick the fscanf into
 * believing it's called directly.
 */
__declspec(naked) int File::scanf(
const char *,
...
)
{
    _asm
    {
        mov     dword ptr savedESP,esp          // Save esp in static

        mov     eax, dword ptr [esp]            // eax <- return address
        mov     dword ptr savedRET,eax          // Save return address in static

        mov     eax,dword ptr [esp+4]           // eax <- this
        mov     dword ptr savedThis,eax         // Save this in static

        add     esp, 8                          // esp+= 8 (crush retaddr,this)

        mov     eax,dword ptr [eax]             // eax <- this->filePtr
        push    eax                             // Push filePtr on argStack

        call    truefscanf                      // Call regular fscanf
        mov     dword ptr savedEAX,eax          // Save eax value in static

        mov     esp, dword ptr savedESP         // Restore esp

        mov     eax, dword ptr savedRET         // Restore return value
        mov     dword ptr [esp], eax

        mov     eax, dword ptr savedThis        // Restore this
        mov     dword ptr [esp+4], eax

        mov     eax, dword ptr savedEAX         // Restore eax

        ret
    }
}
#endif // WIN32