String concatenation behavior in C

Question

Why the following C code works?

char *str = "A""B"; /* str = "AB" */

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However I noticed that it is only in string statements, for example the following would not work:

char *a = "A";
char *ab = a "B"; /* Funcionaria se a fosse uma macro */

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I suspect it works because the compiler considers "A""B" as a single memory block and make the "concatenation" automatically.

In short I would like to know if my assumption is right or is for another reason.

Answer 1

Because the C compiler glues/concatenates literals of consecutive strings.

For example:

#include<stdio.h>
int main() {
    const char *hw = ""
        "Hello"
        " "
        "World" "!" "!" "!";

    printf("%s", hw);
}

Produces as output:

Hello World!!!

See here working on ideone.

However, this only applies to literal strings. It doesn’t work with variables or other things.