c++cwinapiformatmessage

Why do I need to cast the lpBuffer (LPTSTR) parameter in FormatMessage?

In Windows' FormatMessage() function, the parameter:

  _Out_     LPTSTR lpBuffer

Is doing my head in. Following along from Hart's Windows System Programming book, I'm declaring an LPTSTR pointer to be used as the lpBuffer (e.g. LPTSTR errortext;), and then calling the FormatMessage() function.

The correct way to pass in this parameter is: (LPTSTR)&errorText

This works fine. But I don't understand why I need to write (LPTSTR). I understand that's typecasting and I read about it but it doesn't make sense to me, because I'm not changing the variable type or anything, I declared it as an LPTSTR and I'm passing its memory address to the function, the function expects an LPTSTR and I passed it an LPTSTR, so why do I need to put (LPTSTR) as part of the lpBuffer parameter?

Solution

  • The parameter lpBuffer of FormatMessage() is documented as follows:

    A pointer to a buffer that receives the null-terminated string that specifies the formatted message. If dwFlags includes FORMAT_MESSAGE_ALLOCATE_BUFFER, the function allocates a buffer using the LocalAlloc function, and places the pointer to the buffer at the address specified in lpBuffer.

    So there are 2 different usages of FormatMessage(),

    1) Provide your own buffer

    const DWORD bufsize = ....;
TCHAR buf[bufsize];
FormatMessage(.... buf, bufsize, ....); // buf is passed as a TCHAR*

    2) FormatMessage allocates a buffer for you

    const DWORD bufsize = ....;
TCHAR* buf = 0;
FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | ....,
    .... (LPTSTR)&buf, bufsize, ....); // &buf is a TCHAR** so type-cast needed!
....
LocalFree(buf);

    In #1, you have to pass the address of the first TCHAR in your buffer, and the function simply fills it the buffer.

    In #2, the function needs to tell you where it allocates a new buffer, so you have to tell it where to place that address. You have to pass the address of a pointer variable that receives the address.

    In short:

    That is why the lpBuffer parameter has to be type-casted when using #2.