what will be the correct output of the given program in c with explaination

Question

#include<stdio.h>
#define sqr(n) (n*n);
int main()
{
  int x =3,b;
  b=sqr(x+1);
  printf("%d\n", b);
  return 0;
}

//the output should be 16 but my on compiling in GCC compiler the output is coming 7.can anyone explain me the reason

Answer 1

This line is problematic:

#define sqr(n) (n*n);

Because there's no parentheses around n, n+1 evaluates to (n + 1 * n + 1). The output is seven because sqr(3+1) evaluates to (3 + 1 * 3 + 1) = 7. To fix, add parentheses:

#define sqr(n) ((n)*(n));

Then sqr(3+1) evaluates to ((3 + 1) * (3 + 1)) = 16.