Behaviour of macros and increment operator in C

Question

When I compile the following code

#include<stdio.h>
#define CUBE(x) (x*x*x)

int main()
{
    int a, b=3;
    a = CUBE(b++);
    printf("%d, %d\n", a, b);
    return 0;
}

It gives 27 , 6

But shouldn't the expression a=b++*b++*b++; be calculated as a=3*4*5 and should give 60?

Undefined behavior aside, the macro should be `#define CUBE(x) ((x)*(x)*(x))` — Yu Hao, Sep 20 '13 at 15:46
Read [error with define macro definition](http://stackoverflow.com/questions/17038558/error-with-define-macro-definition/17038596#17038596) — Grijesh Chauhan, Sep 20 '13 at 16:22

score 4 · Answer 1 · answered Sep 20 '13 at 15:44

4

Your expression causes undefined behaviour, so you could get any answer. Trying to modify the same value more than twice between sequence points is bad news.

answered Sep 20 '13 at 15:44

Carl Norum

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Behaviour of macros and increment operator in C

1 Answers1