Why is my program evaluating arguments right-to-left?

Question

I am learning C so I tried the below code and am getting an output of 7,6 instead of 6,7. Why?

#include <stdio.h>
int f1(int);
void main()
{
    int b = 5;
    printf("%d,%d", f1(b), f1(b));
}
int f1(int b)
{
    static int n = 5;
    n++;
    return n;
}

Answer 1

The order of the evaluation of the function arguments is unspecified in C. (Note there's no undefined behaviour here; the arguments are not allowed to be evaluated concurrently for example.)

Typically the evaluation of the arguments is either from right to left, or from left to right.

As a rule of thumb don't call the same function twice in a function parameter list if that function has side-effects (as it does in your case), or if you pass the same parameter twice which allows something in the calling site to be modified (e.g. passing a pointer).