Why is this not an infinite loop in C/C++

Question

I cant figure out how the following loop can ever end:

for(int i;i<i+1;i++)

when i reach the max_int it will overflow and become the negative number, then the loop stop — simpletron, Sep 11 '14 at 14:04

score 7 · Answer 1 · answered Sep 11 '14 at 14:02

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Overflowing a signed integer is undefined behaviour. Which manifests itself either in loop terminating or not.

Also, you do not initialize i but access its value which is undefined behaviour as well.

answered Sep 11 '14 at 14:02

Maxim Egorushkin

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Never mind that; accessing an object with unspecified value is UB. – Lightness Races in Orbit Sep 11 '14 at 14:03

score 2 · Answer 2 · answered Sep 11 '14 at 14:07

2

signed integers are implemented using twos complement arithmetic, this has negative values when the top bit is set . When you do a the maximum positive value +1 you get the maximum negative value at which point the condition becomes false. This isn't defined by the C standard, but is very likely to be the case unless you are on odd hardware.

answered Sep 11 '14 at 14:07

James Snook

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... or you have a good compiler that optimizes this code by throwing away the condition check at all (or even the whole code :) due to undefined behavior which the code exposes. – user396672 Sep 11 '14 at 15:55

Why is this not an infinite loop in C/C++

2 Answers2