C compiler issue perhaps?

Derek Ragona derek at computinginnovations.com
Sat Mar 15 13:00:32 UTC 2008

At 09:49 PM 3/14/2008, Doug Hardie wrote:

>On Mar 14, 2008, at 18:31, Derek Ragona wrote:
>>At 06:56 PM 3/14/2008, Doug Hardie wrote:
>>>There is no code running at that point.  Its just sitting there
>>>waiting for me to enter a gdb command.
>>>On Mar 14, 2008, at 15:16, 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
>>>>>why c.rmonths was always zero when it should have been 6.  Stepped
>>>>>through using the gdb n command.  Here is the output:
>>>>>215                             c.rmonths = (edate - tdate) /
>>>>>223                     c.dial_in = u.dial_in[0];
>>>>>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
>>>>>   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
>>>>>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.
>>>>         -Derek
>>I understand it is waiting at a breakpoint in gdb.  What I meant was
>>put printf's in your code and run the program and look at the
>>output.  You can use fprintf's to stderr if your prefer and just
>>look at the stderr output.
>>It is hard to diagnose what could be a compiler error, or a coding
>>error.  Remember in C you can do many things you really shouldn't.
>>It is also advisable to run lint over your source code too.
>All that lint shows is it doesn't like comments using // and lots of
>errors in /usr/include files.

This sounds more like a c++ program. c++ does a lot of variable initiation 
in code you usually won't see.

If this is a c++ program, put conditional printf's or cout's in to check 
the code at actual runtime rather than in the debugger.

You may want to use asserts.


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