puzzling.md

OK, I'm puzzled, but maybe this warnings is a clue...?

Why do I get 9 here, but a friend gets 10?

macro-puzzle.c :

#include <stdio.h>


#define my_macro(a) (a) + (a)

int main() {
  int x = 3;
  x = my_macro(++x);
  printf("x: %d\n", x);
  return 0;
}

$ gcc macro-puzzle.c -o macro-puzzle && ./macro-puzzle 
macro-puzzle.c:8:16: warning: multiple unsequenced modifications to 'x' [-Wunsequenced]
  x = my_macro(++x);
               ^~
macro-puzzle.c:4:22: note: expanded from macro 'my_macro'
#define my_macro(a) (a) + (a)
                     ^     ~
1 warning generated.
x: 9