From:	kagel AT quasar DOT bloomberg DOT com
Date:	Thu, 19 Dec 1996 09:44:21 -0500
Message-Id:	<9612191444.AA17414@quasar.bloomberg.com >
To:	Dik DOT Kettenis AT USERS DOT INFO DOT WAU DOT NL
Cc:	djgpp AT delorie DOT com
In-Reply-To:	<vines.DX49+RaIimA@vines2.wau.nl> (message from dik kettenis on Thu, 19 Dec 1996 14:56:45 +0100 (CET))
Subject:	re: Re: A problem for gurus, I guess
Reply-To:	kagel AT dg1 DOT bloomberg DOT com

   Errors-To: postmaster AT ns1
   Date: Thu, 19 Dec 1996 14:56:45 +0100 (CET)
   From: dik kettenis <Dik DOT Kettenis AT USERS DOT INFO DOT WAU DOT NL>
   X-Envelope-To: kagel AT dg1 DOT bloomberg DOT COM
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   On Wednesday, December 18, 1996 at 3:21:04 pm CET,
   kagel AT quasar DOT bloomberg DOT COM wrote:
   >   Errors-To: postmaster AT ns1
   >   Date: Wed, 18 Dec 1996 11:29:54 +0100 (MET)
   >   From: dik kettenis <Dik DOT Kettenis AT USERS DOT INFO DOT WAU DOT NL>
   >   X-Envelope-To: djgpp AT delorie DOT com
   >   Mime-Version: 1.0
   >   Content-Transfer-Encoding: 7BIT
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   >   Content-Length: 1163
   >
   >   Dear GPP-expert,
   >   A program compiled with the newest version of GNU C for MSDOS returned to
   >   MSDOS before the exit of the program. I started to debug this prgram with
   >   GDB. At this time I have found a trouble spot, but I do not know what is
   >   wrong. The following appears:
   >   1. Running the program under GDB without any breaks: computer hangs
   >   2. Stepping with s to the trouble spot: computer hangs
   >   3. Stepping to the trouble spot (a call to a user defined function) with
   >      s, continuing the execution with stepi executes the program to the
   >      normal exit.
   >   4. Running the program under GDB to a break at the assembler instruction
   >      following the assembler call to the function: 
   >      the program returns from the call, so the break will be reached.
   >      Two situations:
   >      a: stepping further with s: program executes to the normal exit.
   >      b: resuming execution by cont: machine hangs.
   >   I hope this sounds familiar to at least one of the experts. Hopefully
   >   someone can give me advise how to find the reason of my original
   >   problem. I can post the code but the code is including some other 
   >   functions rather lengthy.
   >   Please respond to my personal email address. Regards, Dik
   >
   >This effect is indicative of a memory overwrite, probably but not 
   necessarily
   >in the function you have identified (it might be in a previously called
   >routine).  The cause of the different runtime behaviors is that the GDB 
   loads
   >your program at a different location in memory than it would load normally 
   so
   >the memory overwrite may affect different code or data.  Also single 
   stepping
   >actually moves an instruction to a temp area inserting a break after the 
   next
   >statement or instruction.  This action can also change behavior.  
   Instruction
   >stepping would protect each instruction this way preventing the crash while
   >source level stepping only protects one out of a group of instructions
   >representing the corresponding "C" statement.
   >
   >That is the explanation.  The solution?  Check the function in question and 
   >the
   >last one or two functions called immediately before it (though I suspect the
   >same function which is crashing) for writes to uninitialized local pointers 
   >or
   >to arrays beyond their boundaries. (This last is the most likely cause as it
   >looks like the call stack is being trashed.  You could check this by having 
   >GDB
   >print the call stack periodically as you step through the function and see 
   >when
   >it is being trashed, if at all.)
   >
   >Hope that this is enough.
   >
   >-- 
   >Art S. Kagel, kagel AT quasar DOT bloomberg DOT com
   >
   >A proverb is no proverb to you 'till life has illustrated it.  -- John Keats
   >

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   Dear Art,
   Thanks for your reaction. In the meantime I have carefully checked the
   complete program for writes to uninitialized local pointers or arrays
   beyond their boundaries. No one found. But my experience is that such
   things might become clear only when you zoom in a bit. To be able to
   zoom in I did a 'bt' each time before and after a new call. No problem
   detected so far. I started to do it again with 'info f'. At a certain
   place I had a frame at 0x619c2. I entered a function and did:
   info f 0x619c2. I got some different information:
   eip was: 0x1f77 and is: 0x0
   ebx and esi were not mentioned the second time.
   Because I am very new to GNU C and GDB I wonder I have found the trouble
   spot? Can you give me advise where to look? Regards, Dik 

I am CCing this to the mailing list in case others are listening who know GDB
better than I.  Your info is sketchy (post both stack traces in their 
entirety) and also post output from the 'where' command at both points.  I am
not expert in GDB (I use another debugger day-to-day) but it does look like 
you have found the trouble spot.  Post the above output to the mailing list 
(or newsgroup) and let everyone pitch in.

-- 
Art S. Kagel, kagel AT quasar DOT bloomberg DOT com

A proverb is no proverb to you 'till life has illustrated it.  -- John Keats

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