Re: low-level question
- From: Keith Thompson <kst-u@xxxxxxx>
- Date: Tue, 30 Aug 2005 20:15:37 GMT
"Robert Gamble" <rgamble99@xxxxxxxxx> writes:
> jesso wrote:
>> I couldn't get this on a midterm. Darn!
>>
>> Anyone want to help?
>>
>> 1. Why does the following program output a 0?
>> 2. Explain in detail.
>> 3. How could you prevent this outcome without changing the code?
>>
>>
>> #include <stdio.h>
>> #include <string.h>
>>
>> int
>> main( int argc, char *argv[] )
>> {
>> int ii = 1;
>> char buf[ 4 ];
>>
>> strcpy( buf, "AAAA" );
>>
>> printf( "%d\n", ii );
>>
>> return 0;
>> }
>
> It is undefined behavior in C because you are writing past the end of
> an arry ("AAAA" is 5 characters, buf is 4). Since it is undefined
> behavior, anything can happen. There might be certain behaviors that
> are more likely to occur than others due to nuances of your specific
> platform but I can't think of anything plausible that would account for
> the behavior suggested by the question.
If ii is allocated just after the end of buf, and if the platform uses
a little-endian representation for int, the '\0' of "AAAA" could
plausibly be written over the low-order byte of ii, causing it to be
set to 0.
--
Keith Thompson (The_Other_Keith) kst-u@xxxxxxx <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
.
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