Error in macro expansion

Question

I have been trying to understand macro expansion and found out that the second printf gives out an error. I am expecting the second print statement to generate the same output as the first one. I know there are functions to do string concatenation. I am finding it difficult to understand why first print statement works and the second doesn't.

#define CAT(str1, str2) str1 str2

void main()
{
    char *string_1 = "s1", *string_2 = "s2";
    printf(CAT("s1", "s2"));
    printf(CAT(string_1, string_2));
}

Answer 1

Concatenating string literals, like "s1" "s2", is part of the language specification. Just placing two variables next to each other, like string_1 string_2 is not part of the language.

If you want to concatenate two string variables, consider using strcat instead, but remember to allocate enough space for the destination string.

Answer 2

Try to do the preprocessing "by hand":

CAT is supposed to take 2 input variables, and print them one after the other, with a space between. So... if we preprocess your code, it becomes:

void main()
{
    char *string_1 = "s1", *string_2 = "s2";
    printf("s1" "s2");
    printf(string_1 string_2);
}

While "s1" "s2" is automatically concatenated to "s1s2" by the compiler, string_1 string_2 is invalid syntax.