Weird multiplication result

Question

Here's come Arduino code.

#define M(x, y) x * y;
void setup() {
  Serial.begin(9600);
  Serial.println(M(3 + 2, 4 + 2));
}
void loop() {
}

Why is the result 13?

Answer 1

Because M(3 + 2, 4 + 2) gets replaced to 3 + 2 * 4 + 2 and then the normal priority of operations does the multiply before the addition.

This is one of the many reasons why you should prefer functions over macros. The inliner of the compiler will negate any call overhead.

If you truly want to use a macro then surround each use of the macro arguments in parenthesis, surround the macro itself in parenthesis as well.

#define M(x, y) ((x) * (y))