how is this post increment working?

Question

#include <stdio.h>

int main ()
{

  int a=10;

  printf("%d %d %d",a,a++,a);

  return 0;
}

The output I am getting is "11 10 11". I thought the output would be "10 10 11".

why a is incrementing like this?

Answer 1

Because there is no guarantee about the order in which a C compiler evaluates the arguments. The only thing guaranteed (by the standard) is that they are all evaluated before doing the call. Therefore, you should never count on the order of evaluation of the arguments. Just consider it as random.

Hence, in general, avoid using auto-increment if the same variable exists more than once in an argument list.