Message-ID:	<38FB70A3.18D96638@bigfoot.com>
From:	"J.P. Morris" <doug-15 AT bigfoot DOT com>
Organization:	Aircraft Liberation Front
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Newsgroups:	comp.os.msdos.djgpp
Subject:	dead beef
Lines:	43
Date:	Mon, 17 Apr 2000 21:14:27 +0100
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp
Reply-To:	djgpp AT delorie DOT com

I have a complex game program which currently compiles in both DJGPP and Linux.
Allegro 3.9.x is used to manage most of the platform-specific code.

Under Linux, the program runs correctly for an indefinite period.
Under DJGPP, however, it crashes between 9:00 and 11:00 game time,
i.e. about 180-300 cycles of the game loop.

The reason it crashes is that some of my pointers appear to be
spontaneously changing, either to 0xA7 (which causes an RMCB fault)
or to 0x20320?. The memory these refer to is filled with 0xbeefdead.

I used the sbrk fill option to fill allocated memory with 0xdeadbeef,
but the memory I'm using isn't memory I've allocated, because I have
my own malloc() wrapper, which I used to try and find the rogue blocks.

Does sbrk fill memory with DEADBEEF only during allocation, or also
when memory is freed?

I have made utterly sure that memory is being locked in my interrupt
handlers.  In addition I have also used fortify, which detects memory
overruns.  It has not detected any.
Since each part of the program relies on something which has happened
previously and the problems only manifest when the program is fully
operational, I cannot effectively reduce the potential problem area.

I would guess that the problem must be one of these:

1. A memory overrun of statically-allocated data (is there a tool to detect
this?)
2. Memory overrun in a 3rd-party library
3. Memory is being used long after deallocation (NOT through M_get() or M_free()
wrappers)
4. Superstitious explanation

Does anyone recognise these symptoms, or another debugging technique?

-- 
JP Morris - aka DOUG the Eagle (Dragon) -=UDIC=-  DOUG-15 AT bigfoot DOT com
Fun things to do with the Ultima games            (http://ithe.cjb.net)
Developing a U6/U7 clone                          (http://fly.to/ire)
d+++ e+ N+ T++ Om U1234!56!7'!S'!8!9!KA u++ uC+++ uF+++ uG---- uLB----
uA--- nC+ nR---- nH+++ nP++ nI nPT nS nT wM- wC- y a(YEAR - 1976)

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