Simple while loop

[FirebarUK]

I've got a simple while loop in C;

int i=1
while (i>0)
{
i=i*1000
}

In theory this should never end, but it does (because i becomes too big). What causes that? I presume this is a computing basic (i.e; "cant handle a number over XXX), but I'm not entirely sure.

Thanks.

 

[Voo]

"It appears that this will go on forever, but in fact the value of i will eventually reach the maximum value storable in an unsigned int and adding 1 to that number will wrap-around to 0, breaking the loop. The actual limit of i depends on the details of the system and compiler used. With arbitrary-precision arithmetic, this loop would continue until the computer's memory could no longer contain i."

Quoted from wiki

http://en.wikipedia.org/wiki/Infinite_loop

 

[FirebarUK]

Bah after all that, my boycott of wikipedia bites me in the arse. Thanks

 

[Dj_Jestar]

SmallTalk would continue for ever.

 

[happytechie]

and .NET would throw a numeric overflow exception

no coder worth his salt would be writing code like that anyway.

It looks to me like a snippet of code designed to test your knowledge of the maximum values that you can store store in an int type. In C this is platform dependent and is defined in one of the header files on most other systems (.NET, Java etc) it's fixed and predefined.

HT

 

[FirebarUK]

It looks to me like a snippet of code designed to test your knowledge of the maximum values that you can store store in an int type.

Exactly that, we're looking at code vulnerabilities. Thanks for your input guys.

 

[MrWhippy]

i f that sort of thing is entertaining you, go read the C-FAQ.
it's got loads of similarly easy to fall foul of gotchas...

 

[Una]

I've got a simple while loop in C;

int i=1
while (i>0)
{
i=i*1000
}

In theory this should never end, but it does (because i becomes too big). What causes that? I presume this is a computing basic (i.e; "cant handle a number over XXX), but I'm not entirely sure.

Thanks.

Its pretty simple to be honest, int is signed so it overflows and become negative thus terminating the loop. (over 2^31 + 1 it will wrap round to -2^31 - (Assuming 32bit ints - sizeof(int)))

http://en.wikipedia.org/wiki/Integer_overflow

Its actually quite common, and in some situations you can exploit it to cause arbitary random code execution.

If you want more info search phrack 60 - Webby is currently down atm, but google cache has it.

 

[DaveF]

Its pretty simple to be honest, int is signed so it overflows and become negative thus terminating the loop. (over 2^31 + 1 it will wrap round to -2^31 - (Assuming 32bit ints - sizeof(int)))

To be pedantic, 2^31-1 is the largest value representable as a signed 32 bit int.

 

[Una]

To be pedantic, 2^31-1 is the largest value representable as a signed 32 bit int.

And as default int is signed it would say unsigned int if it wasn't

 

[DaveF]

And as default int is signed it would say unsigned int if it wasn't

Sure, my nitpick was the overflow is when you go over 2^31-1, not 2^31+1...

 

[yer_averagejoe]

_int128 ***