what is the use of "#@" in c language?

Question

The # symbol is used at the start of preprocessor directives (#ifdef, #define etc). # is also used as the stringification operator.

## is the token pasting operator.

Then in an online quiz I saw this:

#define MAKECHAR(operand) #@operand

What operator is #@ and what is it used for?

Answer 1

It is an analogy to the stringify marker # but for characters, but it is not standardized. For example, clang/llvm does not support it.

To show the analogy:

#define MESSAGE(x) printf("%s: %d\n", #x, x)

int main(){
    int i = 5;
    MESSAGE(i); // expands to printf("%s: %d\n", "i", x)
    return 0;
}

Output is:

i: 5

With a compiler supporting #@, you could write:

#define MESSAGE(x) printf("%c: %d\n", #@x, x)

int main(){
    int i = 5;
    MESSAGE(i);  // expands to printf("%c: %d\n", 'i', x)
    return 0;
}