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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