C order of passing Arguments . How is this code is working?

Question

CODE

#include <stdio.h>

int main()
{

    int a =1;
    printf("%d%d%d%d\n",a,++a,++a,a);

    a=1;
    printf("%d%d%d%d\n",a,a++,++a,a);

    a=1;
    printf("%d%d%d\n",a,++a,a++);

    return 0;
}

Output

3333
3233
331

Answer 1

This is "undefined behavior". You cannot rely on the order of evaluation of function arguments in C. When we say "undefined behavior" we mean that anything might happen: it might work on one compiler, it might not work on another compiler; it might work on one compiler with optimizations disabled, and not work on the same compiler with optimizations enabled; it might not work at all; it might work flawlessly; it might dump core; it might send sperm whales and bowls of petunias falling from the sky.

(See https://www.quora.com/What-is-the-passage-on-the-whale-and-the-bowl-of-petunias-about)