Parentheses matter...

modtest.c

#include <stdio.h>
#define VAL0  123
#define VAL1  1*123
#define VAL2 (1*123)

int main()
{
        int mod0 = 2 % VAL0;
        printf("mod0: %d\n", mod0);

        int mod1 = 2 % VAL1;
        printf("mod1: %d\n", mod1);

        int mod2 = 2 % VAL2;
        printf("mod2: %d\n", mod2);

        int mod3 = 2 % 123;
        printf("mod3: %d\n", mod3);

        return 0;
}

output.txt

$ ./modtest 
mod0: 2
mod1: 0
mod2: 2
mod3: 2