How to concatenate two strings in a C macro?

concat.c

#include <stdio.h>

#define concat0(str1, str2) (str1 " " str2)
#define concat1(str1, str2) (#str1 " " #str2)

int main()
{
    printf("%s\n%s\n",
           concat0 ("a","b"),
           concat1 ("a","b"));

    return 0;
}

thank you, it works.