Question

Two problems with the below code. To begin, I have been scouring this and various other forums for answers to my 1784 error code and everything I’ve tried has failed. Two of the threads I’ve checked on stackoverflow are WriteFile returning error 1784 and BlockWrite I/O Error 1784. I’ve checked some others on this forum but I’m not remembering exactly what the are right now.

I’m trying to save an array of structs to an empty binary file. The first problem is that I get an access violation if my size variable (nNumberOfBytesToWrite parameter) is anything less about 99000 bytes. That number jumps around. For awhile when I was testing it would have the access violation if it was 99,999 bytes but not 100,000 bytes. Of course, what I eventually want to do is set the size to the size of the entire array. The original code to handle that is now commented out so I can test with various sizes.

The second thing that happens (if I don’t get an access violation) is I get error code 1784 and WriteFile fails every time. As other threads on this topic have stated, this is defined on MSDN as ERROR_INVALID_USER_BUFFER and the description is “The supplied user buffer is not valid for the requested operation.” I’ve looked at MSDN’s own example for opening files like this (http://msdn.microsoft.com/en-us/library/windows/desktop/bb540534%28v=vs.85%29.aspx) and have tried some variations based on their code, but nothing seems to work.

This problem is probably massively noob and I’m sure I’m overlooking something ridiculously simple, but if anyone has suggestions they’d be greatly appreciated.

case IDM_SAVE1:
{
    HANDLE hFile = CreateFile("MineSave.mss", GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    int test_buffer[] = {1,2,3,4,5,6,7,8,9,10};

    if(hFile != INVALID_HANDLE_VALUE)
    {
        BOOL bSuccess;
        DWORD size = 100000; //DWORD size = (((sizeof(tile)) * tiles_total));
        LPDWORD bytes_written = 0;
        bSuccess = WriteFile(hFile, test_buffer, size, bytes_written, NULL);
        if(bSuccess)
        {
            MessageBox(hwnd, "File saved successfully.", "Great Job!", MB_OK);
        }
        else
        {
            DWORD error = GetLastError();
            MessageBox(hwnd, "Could not write to file.", "Error", MB_OK);
        }

        CloseHandle(hFile);
    }
    else
    {
        MessageBox(hwnd, "Could not create file.", "Error", MB_OK);
    }
}
break;

Answer 1

Your buffer is the size of 10 ints, which is 40 bytes on Windows. You are trying to write 100,000 bytes from that buffer. That is undefined behaviour, a buffer overrun. Hence the access violation.

You must not pass a value greater than sizeof(test_buffer), i.e. 40, to the nNumberOfBytesToWrite parameter of WriteFile.

You’ll need to write this file in a loop, writing 40 bytes at a time, until you have written as much as you need. Perhaps something like this:

BOOL bSuccess = TRUE;
DWORD bytesRemaining = 100000;
while (bSuccess && bytesRemaining>0)
{
    DWORD bytesToWrite = std::min(sizeof(test_buffer), bytesRemaining);
    DWORD bytesWritten;
    bSuccess = WriteFile(hFile, test_buffer, bytesToWrite, &bytesWritten, NULL);
    bytesRemaining -= bytesToWrite;
}
if (!bSuccess)
{
    //handle error;
}

Writing 40 bytes at a time is pretty slow. You’ll find it more efficient to write a few KB with each call to WriteFile.

Note that you aren’t allowed to pass NULL to the lpNumberOfBytesWritten parameter if you also pass NULL to lpOverlapped, as you do here. From the documentation:

lpNumberOfBytesWritten [out, optional]

……

This parameter can be NULL only when the lpOverlapped parameter is not NULL.