The macro X turns its argument into a string, but using it on its own means you can't turn other macro values into strings, only macro names.

    #define F 42
    #define X(s) #s
    X(5)  //  Expands to "5" (as a string literal)
    X(F)  //  Expands to "F" not "42"

If you add one level of recursive macro expansion, though, that expands the macro argument as well as the macro definition:

    #define F 42
    #define X(s) #s
    #define S(s) X(s)
    S(5) // "5" as expected
    S(F) // "42" because S(F) expands to X(42) expands to "42"