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From:	Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: segfault on context switch
Date:	Thu, 02 May 2002 22:25:20 CDT
Organization:	Rice University, Houston TX
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> 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.

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