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From: hdg@otter.hpl.hp.com (Hugh Duggan)
Newsgroups: comp.windows.ms.programmer
Subject: Re: Strange clobberings.
Message-ID: <72170013@otter.hpl.hp.com>
Date: 12 Jun 91 08:08:36 GMT
References: <1991Jun11.210451.14087@athena.mit.edu>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 34

> 
> I am trying to call MessageBox() from a Dialog Box.  If I call
> it with MB(hDlg, (LPSTR) "No filename specified", NULL, MB...);
> 
> the string is not displayed, and junk is in it's place, and memory
> is cold screwed up.
> 
> If I call it with a string used elsewhere in the program, defined at the
> top as:
> 	static char DefSpec[10] = "*.*";
> 
> It works.
> 
> But if I define my own:
> 	static char Err[100] = "Yo mama";
> 
> I get the same error.
> 
> Any clues??????
> ----------

    Your dialogue box function is being called by Windows, not directly
by any code you have written.  Consequently, you cannot guarantee to
have the correct data segment loaded (it is just chance that your
second example worked- in the other cases, Windows has loaded its own
data segment).
    Solution: make sure that the dialogue box function is declared
with the _loadds keyword, or compile your code with a switch which
forces DS to be reloaded on entry to all functions.  This problem will
be seen whenever you try to use any static or global data (ie
allocated from the heap) within a callback function.

    Hugh Duggan
    HP Labs, Bristol