gdb, stderr, totally baffing experience!

[Paul Johnson]

Hello everybody:

One of the Swarm users sent me a program that has a seg fault.  I ran it 
through gdb and found that the line causing the trouble was a call to a 
+createBegin method. I mean, literally, the top line in the backtrace 
was in the ModelSwarm

newAgent = [Agent createBegin: self];

That was the one the debugger pointed to in step 1. That is somewhat 
unusual in my experience because usually gdb points at something you can 
see would cause a segfault.  If the cause is inside +createBegin, for 
example, it would list a line in there.

So I rewrote the class to eliminate the need for it to override 
SwarmObject's +createBegin, so I could comment out +createBegin.  Then 
the backtrace started pointing at a call to Swarm's built in uniformIntRand.

So I was starting to think that something is so dramatically wrong that 
gdb has no idea what to do, and I started putting in debug prints like this:

fprintf(stderr,"update family num_family is %d\n",num_family);

And then, to my total surprise and astonishment, I started getting 
bactraces where the fprintf was the causing the seg fault.  Here's the 
bit from the backtrace

Program received signal SIGSEGV, Segmentation fault.
0x420503da in vfprintf () from /lib/i686/libc.so.6
(gdb) bt
#0  0x420503da in vfprintf () from /lib/i686/libc.so.6
#1  0x4205a297 in fprintf () from /lib/i686/libc.so.6
#2  0x08050ff4 in _i_Agent__updateFamily (self=0x8b6d2d8, 
_cmd=0x8058790) at Agent.m:438
#3  0x08050907 in _i_Agent__step (self=0x8b6d2d8, _cmd=0x8056e00) at 
Agent.m:256
#4  0x40017ddb in L10 () from /usr/lib/swarm/libswarm.so.0
#5  0x40205120 in objc_call (fa=0x8c36f10)

If I change all those fprintf statements to printf, then I still get the 
 seg fault pointing at the fprintf, but it is back pointing at 
uniformIntRand or such.

So there's obviously something about gdb or stderr I don't understand. 
Do you?

One bit of information about this program. It is rather like the 
ArtificialStockMarket when I started revising that code--liberal use of 
C (structs, C arrays) interspersed with Obj-C.  There could be 
significant array problems, like accesses of value[-1] or value[N+1] in 
an array that has valid index values from 0 to N.

Is it possible that, when some part of a program accesses memory so 
badly that it could damage memory in use by uniformIntRand in a way that 
totally baffles the gdb?


--
Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66045                FAX: (785) 864-5700


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