Can I determine if an argument is string literal?

Question

Is it possible to determine if an argument passed in macro or function is a string literal at compile time or run time?

For example,

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

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

or

bool is_string_literal(const char * s);

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

Thanks.

Answer 1

YES! (Thanks to James McNellis and GMan for corrections. Updated to correctly handle concatenated literals like "Hello, " "World!" which get stringized before concatenation.)

#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;
}

This will stringify the argument before passing it to the function, so if the argument was a string literal, the argument passed to our function will be surrounded with quote characters.

If you consider this a string literal:

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

I cannot help you. You might be able to use some implementation-defined (or *shudder* undefined) behavior to test whether or not a string is stored in read-only memory, but on some (probably older) systems p could be modified.

For those who question the use of such a function, consider:

enum string_type { LITERAL, ARRAY, POINTER };

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

Rather than explicitly specifying the second argument to string_function on every call, is_literal allows us to wrap it with a macro:

#define string_func(s) \
    (string_func)(s, is_literal(s)  ? LITERAL :
        (void *)s == (void *)&s ? ARRAY : POINTER)

I can't imagine why it would make a difference, except in plain C where literals aren't const and for some reason you don't want to/can't write the function as taking a const char * instead of a char. But there are all kinds of reasons to want to do something. Someday you, too may feel the need to resort to a horrible hack.