为什么需要在FormatMessage中强制转换lpBuffer（LPTSTR）参数？

Question

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 (eg 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?

Answer 1

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:

• #1 needs a TCHAR* to an existing buffer
• #2 needs a TCHAR** that receives a new buffer

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