Question

I’m reading raw bytes from a disc and trying to print them in Hex. I’m getting an “Unhandled exception at 0x666CDF46 (msvcr110d.dll) in ConsoleApp.exe: 0xC0000005: Access violation writing location 0x002EC000.” inside the for loop. My guess is I’m running off the end of the TCHAR array str but I can’t figure out why. dwBytesRead and dwSize are 4096. The for loop stops for the exception when i is 4027 and I believe it should get to 4096. Can someone shed some light on this?

int _tmain(int argc, _TCHAR* argv[])
{
    HANDLE  hCD, hFile;
    DWORD   dwBytesRead;

    hFile = CreateFile (L"sector.dat",...);

    hCD = CreateFile (L"\\\\.\\E:", ...);

    if (hCD != INVALID_HANDLE_VALUE)
    {
        DISK_GEOMETRY         dgCDROM;

        ...

        LPBYTE lpSector;
        DWORD  dwSize = 2 * dgCDROM.BytesPerSector;  // 2 sectors

        lpSector = (LPBYTE) VirtualAlloc (NULL, dwSize,
            MEM_COMMIT|MEM_RESERVE,
            PAGE_READWRITE);

        ....

        if (ReadFile (hCD, lpSector, dwSize, &dwBytesRead, NULL)) {
            const int size = (int) dwBytesRead;
            TCHAR *str = new TCHAR[size*2+1];
            int i;

            for (i=0; i<size;i++) {
                _stprintf_s(str+2*i, (size_t) dwBytesRead, L"%02x", lpSector[i]);
            }
            str[2*i]=L'\0'; 
            OutputDebugString(str);
            ...
        }

        ...
    }
}

Answer 1

Your use of dwBytesRead inside the call to _stprintf_s is not valid. You are telling _stprintf_s that the size of the destination buffer, for every iteration through the loop, starts at a particular point in str and extends for dwBytesRead characters. This is not true, especially as your loop iterator i reaches the end of the buffer.

You can fix this with something like:

            _stprintf_s(str+2*i, (size*2+1) - (2*i), L"%02x", lpSector[i]);

It’s hard for me to say whether this will actually fix your problem, because it’s unclear what the root of the problem really is.