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Date: Wed, 02 Dec 2009 09:11:01 +0000
From: Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>
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Subject: Re: odd segfault with my c program using cygwin 1.7
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Andy Koppe wrote:
> 2009/12/2 Dave Korn:
>> Andy Koppe wrote:
>>> 2009/12/2 jeffunit:
>>>> My program ran fine under cygwin 1.5 as well as many versions of
>>>> linux including mandriva 2009, mandriva 2009.1 and mandriva 2010.0
>>>>
>>>> When compiled with
>>>> gcc cal.c -o cal
>>>> and run with
>>>> cal 2009
>>>> I get a segmentation fault.
>>>> When I uncomment line 62, the program runs successfully.
>>> It's the line after that that has the bug: it's writing to index 432
>>> of a 432-element array.
>>  Doesn't look like it does to me:
>>
>>>         for(j=0; j<6*72; j++) {
>>>           //printf("joe %d\n", j);
>>>           string[j] = '\0';
>>>       }
>>  That's less than 432, not less than or equal.
> 
> You're right, somehow I'd completely misread that, thinking that the
> string[j]='\0' is outside the loop.


  Still, it could be some other overflow somewhere else; as I'd guess you were
reasoning, that's the commonest reason for this sort of bug that crops up on
some platforms with some stack and memory layouts and not others.

  Jeff, recompile your code, adding the "-g" flag, then run it under gdb, and
when the segfault happens, use the "bt" command to get a backtrace and see
where you are.

    cheers,
      DaveK


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