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From: "Gisle Vanem (gisle DOT vanem AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Subject: Re: malloc() returns pointer to already allocated memory
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Date: Fri, 21 Jun 2019 09:44:34 +0200
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Rod Pemberton wrote:

> .. was caused by non-reentrant code being reentered.  That could be your
> issue as well, e.g., if using hardware or software interrupt routines.
...

> Of course, not saving/restoring C/C++ code's in-use registers does much
> the same thing.  Certain registers must be preserved and can't be
> clobbered.  These can also be referred to as callee-savee or
> caller-saved registers.  For DJGPP, my notes say that EBX, EDI, ESI,
> EBP, DS, ES, SS registers must preserved.  I.e., check your inlined
> assembly's clobber list.

And the GS, FS registers too. These are *not* preserved in a
real-mode callback (RMCB) like '_go32_dpmi_allocate_real_mode_callback_retf()'
Not sure about '_dpmi_allocate_real_mode_callback()'

Ref: <djgpp_root>/src/LIBC/GO32/gormcb.c

I figured this after similar bugs in the network-driver
call-back in Watt-32 tcp/ip stack. I'm not sure how/where
FS/GS are used; I suspect a modern gcc uses those in e.g.
built-in functions etc.


-- 
--gv