Date: Sat, 15 Mar 2008 16:16:17 -0500 From: Derek Ragona <derek@computinginnovations.com> To: Chuck Robey <chuckr@chuckr.org> Cc: freebsd-questions <freebsd-questions@freebsd.org> Subject: Re: C compiler issue perhaps? Message-ID: <6.0.0.22.2.20080315161039.0260cbe0@mail.computinginnovations.com> In-Reply-To: <47DC0136.5020707@chuckr.org> References: <A768FF06-4601-42C6-A491-634F5135DCCD@lafn.org> <6.0.0.22.2.20080314171533.023f7c88@mail.computinginnovations.com> <47DC0136.5020707@chuckr.org>
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At 12:02 PM 3/15/2008, Chuck Robey wrote:
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>Derek Ragona wrote:
> > At 05:10 PM 3/14/2008, Doug Hardie wrote:
> >> I have a program I was testing with gdb. I was trying to figure out
> >> why c.rmonths was always zero when it should have been 6. Stepped
> >> through using the gdb n command. Here is the output:
> >>
> >> (gdb)
> >> 215 c.rmonths = (edate - tdate) / toMONTHS;
> >> (gdb)
> >> 223 c.dial_in = u.dial_in[0];
> >> (gdb)
> >> 224 c.dsl = u.dsl[0];
> >> (gdb) p c.rmonths
> >> $1 = 0
> >> (gdb) p c
> >> $2 = {fa = 0, pwp = 0, disp_email = 0, imonths = 0, rmonths = 6,
> >> type = 73 'I', cd = 0 '\0', dial_in = 82 'R', dsl = 0 '\0',
> >> dsl_kit = 0 '\0', ip = 0 '\0', domain = 0 '\0', n_domain = 0 '\0',
> >> renewal = 89 'Y', program = "I\000\000"}
> >> (gdb) p c->rmonths
> >> $3 = 6
> >> (gdb) p c.rmonths
> >> $4 = 6
> >>
> >>
> >> Notice, the first time i print it its zero. The second time its 6.
> >> What gives here? I have seen this before but couldn't pin it down.
> >> The program is not compiled with any optimization. It is in a shared
> >> library though.
> >
> > It is hard to tell without the code you used. I would put some printf's
> > in the code and see what and when that variable gets set to in actual
> > running code.
>
>2points:
>
>(1) yes, you are right, without the source code, any guesses are at the
> same level as black magic, useless
>(2) if the user is learning to use gdb, then it is really bad manners to
> suggest that printfs should be used. While I have made massive use
> of printfs before I got used to gdb, gdb is incredibly more powerful,
> can do any and all that any prints might accomplish, and anyone who
> is willing to learn to use that debugger should be encouraged, not
> given bad habits that really should be a fallback only to environments
> where gdb won't work.
Chuck,
On your point 2 let me say that there are simply times when a developer
needs to check run-time code versus running the code in a
debugger. Debuggers are a great tool, but they do cause some side-effects
as was noted in the original post. If the debugger is not consistent in
the variable values, it is of little use if those values are causing a
problem. What I originally suggested was using conditionally compiled
fprintf's to check those variable values running the program by itself
instead of inside gdb. I know adding additional code, even just fprintf's,
can change the way a program is compiled and optimized. But this at least
gives another way to validate the variables.
-Derek
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