Thread: MD5ing a file

I'm doing a file hasher, I'm starting with MD5, I'm following this example but I didn't want to use window's function, I'm using normal md5 functions.

Code:

    LPSTR lpOut[40];
    HANDLE hFile = NULL;
    BOOL bResult = FALSE;
    BYTE rgbFile[BUFSIZE];
    DWORD cbRead = 0;
    unsigned char* md5;

    hFile = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, NULL, 
                       OPEN_EXISTING, FILE_FLAG_SEQUENTIAL_SCAN, NULL);

    if (hFile == INVALID_HANDLE_VALUE)
    {
        wsprintf(lpOut, "Error opening file! Error: %d\0", GetLastError());
        MessageBox(hwnd, lpOut, "Error", MB_OK | MB_ICONERROR);
        break;
    }
                         
    MD5_CTX context;
    MD5Init(&context);

    while (bResult = ReadFile(hFile, rgbFile, BUFSIZE, &cbRead, NULL))
    {
        if (cbRead == 0)
        {
            break;
        }

        MD5Update(&context, rgbFile, BUFSIZE);
    }

    CloseHandle(hFile);
    MD5Final(md5, &context);
    SetDlgItemText(hwnd, HSH_MD5, md5);

It compiles fine but I get an error while running it. Aditionally I would like to know if is there a function that would take the number given by GetLastError() and convert it into the description of the error.

Code:

    unsigned char* md5;

    ....

    MD5Final(md5, &context);

Read this post. Assumably MD5Final is expecting an array of 16 unsigned chars where it will place the result. It is almost certainly not expecting a random pointer.

Aditionally I would like to know if is there a function that would take the number given by GetLastError() and convert it into the description of the error.

Yes, you can use the FormatMessage function:

Code:

TCHAR szErrorMsg[1024];

FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, 
              GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
              szErrorMsg, sizeof(szErrorMsg) / sizeof(szErrorMsg[0]), NULL);

Code:

MD5Update(&context, rgbFile, BUFSIZE);

This call is hashing BUFSIZE bytes on each call to MD5Update, even if a lesser number of bytes were read. You should use cbRead, which contains the number of bytes read, instead:

Code:

MD5Update(&context, rgbFile, cbRead);

Thanks a lot, again.