Thread: Need Some explanation for this program output

Hi,

I am returning to C programs after a long time and i have confusion why 3rd output is 31! Remembering concepts of precedence and association i think i am either overlooking something or it may be undefined behavior. But i am 70% sure that it may not be come under criteria of undefined behavior and this is why posting it here to understand it.

Code:

If p = 10 then
(++p) + p + p = 33
p + (++p) + p = 33
p + p + (++p) = 31
p + p * (++p) = 132

I'm pretty sure all that nonsense results in undefined behaviour.

I'm pretty sure I agree with Salem

Edit: gcc even warns you about it :/

Code:

#include <stdio.h>

int main(void)
{    
    int p = 10;
    int a, b, c, d;
    
    a = (++p) + p + p;
    b = p + (++p) + p;
    c = p + p + (++p);
    d = p + p * (++p);
    
    return 0;
}

Code:

dumb.c: In function ‘main’:
dumb.c:8:10: warning: operation on ‘p’ may be undefined [-Wsequence-point]
    8 |     a = (++p) + p + p;
      |         ~^~~~
dumb.c:8:10: warning: operation on ‘p’ may be undefined [-Wsequence-point]
dumb.c:9:14: warning: operation on ‘p’ may be undefined [-Wsequence-point]
    9 |     b = p + (++p) + p;
      |             ~^~~~
dumb.c:9:14: warning: operation on ‘p’ may be undefined [-Wsequence-point]
dumb.c:10:18: warning: operation on ‘p’ may be undefined [-Wsequence-point]
   10 |     c = p + p + (++p);
      |                 ~^~~~
dumb.c:11:18: warning: operation on ‘p’ may be undefined [-Wsequence-point]
   11 |     d = p + p * (++p);
      |                 ~^~~~
dumb.c:11:18: warning: operation on ‘p’ may be undefined [-Wsequence-point]

Ok. Understood. Thanks a lot guys for clearing out my confusion.