Getting unexpected results when using macros with arguments

#include <stdio.h>
#define big(a, b) a > b ? a : b
#define swap(a, b) temp = a; a = b; b = temp;

int main() {
    int a = 3, b = 5, temp = 0;
    if ((3 + big(a, b)) > b)
        swap(a, b);
    printf("%d %d", a, b);
}

Above code was given in a multiple choice question. I expected the answer to be 5 3. On running the code, output comes out to be 5 0. I've tried to make sense of this but my efforts have gone in vain. What's the working of this code?

Just expand the macros in the code and you will get

int main() {
    int a = 3, b = 5, temp = 0;
    if ((3 + a > b ? a : b ) > b)
        temp = a;
    a = b;
    b = temp;    
    printf("%d %d", a, b);
}

As the first expression 3 + a in the conditional operator is equal to 6 then the result of the conditional operator is the value of a that is less than b.

So the condition of the if statement evaluates to logical false and the statement

temp = a;

is skipped.

As a result a will be equal to 5 and b will be equal to 0.

You need to use parentheses. For example

#define big(a, b) ( ( a ) > ( b ) ? ( a ) : ( b ) )
#define swap(a, b) do { temp = ( a ); ( a ) = ( b ); ( b ) = temp; } while ( 0 )