#define is_string_literal(X)
...
...

is_string_literal("hello") == true;
const char * p = "hello";
is_string_literal(p) == false;

bool is_string_literal(const char * s);

is_string_literal("hello") == true;
const char * p = "hello";
is_string_literal(p) == false;

#define is_literal_(x) is_literal_f(#x, sizeof(#x) - 1)
#define is_literal(x) is_literal_(x)

bool is_literal_f(const char *s, size_t l)
{
    const char *e = s + l;
    if(s[0] == 'L') s++;
    if(s[0] != '"') return false;
    for(; s != e; s = strchr(s + 1, '"'))
      {
        if(s == NULL) return false;
        s++;
        while(isspace(*s)) s++;
        if(*s != '"') return false;
      }
    return true;
}

const char *p = "string";
// should is_literal(p) be true or false?

enum string_type { LITERAL, ARRAY, POINTER };

void string_func(/*const? */char *c, enum string_type t);

#define string_func(s)
    (string_func)(s, is_literal(s)  ? LITERAL :
        sizeof(s) == sizeof(char *) ? POINTER : ARRAY)

const char x[] = "Hello, World!";

#define is_string_literal(X) _is_string_literal("" X)
bool _is_string_literal (const char *str) { return true; } // practically not needed

#define is_string_literal(s)
  (memcmp(#s, """, 1) == 0)

template<size_t SIZE>  // for "abcd" or const char a[] = "abcd"
bool is_string_literal (const char (&arr)[SIZE])  { return true; }

template<size_t SIZE>  // for char a[] = "abcd"
bool is_string_literal (char (&arr)[SIZE])  { return false; }

template<typename CHAR_TYPE>  // for *p (char* or const char*)
bool is_string_literal(CHAR_TYPE *&p) { return false; }

#define is_string_literal(X) _is_string_literal("" X)
bool _is_string_literal (const char *str) { return true; }