Question

I’m having a weird problem with avr-gcc. If I do this:

int i = 0;
i = ++i;

It results in the compiler warning:

warning: operation on ‘i’ may be undefined

What is wrong here?

If it is rewritten to

i = i + 1;

It does not result in the warning.

avr-gcc is version 4.3.4 and I’m running this on Ubuntu 10.04.

Answer 1

If you intended to simply increment i, then use either

i = i + 1;

or

++i;

(or i++), but not both. The rules of C don’t permit you to modify a variable twice before a single sequence point. Both the preincrement (++i) and the assignment (i =) modify the value of i.