The need for parentheses in macros in C [duplicate]

Answer 1

A macro is just a straight text substitution. After preprocessing, your code looks like:

int main()
{
    int a, b=3;
    a = b+5*b+5;
    printf("%d\n",a);
    return 0;
}

Multiplication has a higher operator precedence than addition, so it's done before the two additions when calculating the value for a. Adding parentheses to your macro definition fixes the problem by making it:

int main()
{
    int a, b=3;
    a = (b+5)*(b+5);
    printf("%d\n",a);
    return 0;
}

The parenthesized operations are evaluated before the multiplication, so the additions happen first now, and you get the a = 64 result that you expect.