Undefined output with prefix operator in C

Question

#include<stdio.h>
int main()
{
 int i=2;
 printf("%d %d\n",++i,++i);
 return 0;
}

Output is 4 4 on gcc. Please explain this output

You yourself wrote that it is `undefined` in the title. What more is there to explain than that? — Paul, Jul 19 '13 at 04:00
so everytime answers to such questions would be unpredictable? — user2492165, Jul 19 '13 at 04:03
You'd better not write code like this. It depends on compilers. You also don't need to struggle with this kind of problems. — Annie Kim, Jul 19 '13 at 04:41

phoxis · Answer 1 · 2013-07-19T04:16:35.260

This is undefined behaviour. The order of evaluation of the function parameters is not defined by the C standard.

Relevant sections: C99 Section 6.5.2.2 Paragraph 10

The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.

also in C99 Section 6.5.2.2 Paragraph 10

There is a sequence point after the evaluations of the function designator and the actual arguments but before the actual call. Every evaluation in the calling function (including other function calls) that is not otherwise specifically sequenced before or after the execution of the body of the called function is indeterminately sequenced with respect to the execution of the called function.94)

Undefined output with prefix operator in C

1 Answers1