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From: Anton Helm <ahelm AT gmx DOT net>
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Newsgroups: comp.os.msdos.djgpp
Subject: Re: segfault on context switch
References: <31542 DOT 1020351804 AT www41 DOT gmx DOT net> <3cd20320 DOT sandmann AT clio DOT rice DOT edu>
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Date: Tue, 14 May 2002 08:24:44 +0100
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Charles, 

Thanks for your kind advice. I downloaded new binaries today,
recompiled, relinked and the problem disappeared.

Happy context-switching ... ;-)

Tony


Charles Sandmann wrote:
> 
> > The program is reading/writing data from/to a com port using
> > the svasync library. No problems in plain DOS, but when I'm
> > running WinNT4.0 and do a context switch (Alt-TAB) while the
> > program is running I get a segfault in malloc(). No problems
> > without context switch though ...
> 
> > Does that ring a bell anywhere? Is this a known WinNT "feature"?
> 
> I suspect you are turfing up a known problem with Windows NT - it
> does not allow address wrap on selector limits.  If a malloc asks
> for a memory block and it is returned "below" the starting address,
> in Win9x we just wrap the address space around (4Gb) to access it.
> 
> NT won't allow this, silently fails the selector limit set, then
> any access to that memory (malloc) fails.  Alt-Tab causes things
> outside the VDM to happen which causes a memory block at a low
> address to free up.
> 
> > I'm using DJGPP v2.03 with GCC v2.95.3, binaries about 6 months old.
> 
> Check your lib\crt0.o - if it is dated earlier than December 2001 try
> upgrading to the djdev203.zip on Simtel (refreshed version).  This is
> one of the fixes I put in the refresh.  Windows 2000 caused this problem
> at a much higher frequency than Windows NT so I fixed it.
> 
> If you have the the updated crt0.o - it's possible that I botched the
> patch - a full register list/traceback might help identify the problem.