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From: Bart Oldeman <bartoldeman AT NOSPAMihug DOT co DOT nz>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: _CRT0_FLAG_NULLOK vs. DOSEMU
Date: Wed, 04 Aug 2004 12:00:46 +1200
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Egon Eckert wrote:
> 
> is null pointer dereference protection expected to work under DOSEMU/Linux?
> I have no success with that: reading/writing from/to *NULL does not trap. :(

It works in DOSEMU 1.3.1 (development version), not in the stable 
version 1.2.2 -- NULL pointer protection relies on uncommitted memory
support in the DPMI server; and supporting this in DOSEMU required some
moderately intrusive reorganization of its memory mapping mechanism, and 
this happened too late for 1.2.x.

This is what I see with DOSEMU 1.3.1 now:

C:\>type crash.c
int main(void)
{
     *(char*)0=0;
}

C:\>gcc crash.c

C:\>a
Exiting due to signal SIGSEGV
Page fault at eip=000015c3, error=0006
eax=00000000 ebx=0000b5cb ecx=00000000 edx=0000033f esi=00000054 
edi=0000cf4c
ebp=0008cf28 esp=0008cf28 program=C:\A.EXE
cs: sel=00c7  base=b5cb1000  limit=0009ffff
ds: sel=00cf  base=b5cb1000  limit=0009ffff
es: sel=00cf  base=b5cb1000  limit=0009ffff
fs: sel=00af  base=000028f0  limit=0000ffff
gs: sel=00df  base=00000000  limit=0010ffff
ss: sel=00cf  base=b5cb1000  limit=0009ffff
App stack: [0008cf4c..0000cf4c]  Exceptn stack: [0000ceac..0000af6c]

Call frame traceback EIPs:
   0x000015c3
   0x00002948

whereas dosemu-1.2.2 simply executes a.exe without any message.

Bart