How to distinguish between strings in heap or literals?

Question:

I have a use case where I can get pointers of strings allocated either in memory or literals. Now the latter can't be freed so that's a problem if I pass the wrong one. Is there a way to know which one is allocated and which not?

char *b = "dont free me!";
if(!IS_LITERAL(b)) {
free(b);
}

I imagine something like that.

My example:

Scenario 1: literal

char *b = "dont free me!";
scruct elem* my_element = mylib_create_element(b);
// do smth
int result = mylib_destroy_element(my_element); // free literal, very bad

Scenario 2: in heap

char *b = malloc(sizeof(char)*17); // example
strncpy(b, "you can free me!",17);
scruct elem* my_element = mylib_create_element(b);
// do smth
int result = mylib_destroy_element(my_element); // free heap, nice

How the user calls mylib_create_element(b); is not under my control. If he frees before mylib_destroy_element it can crash. So it has got to be mylib_destroy_element that cleans up.

Answer:

I've had a similar case recently. Here's what I did:

If you're making an API that accepts a string pointer and then uses it to create an object (mylib_create_element), a good idea would be to copy the string to a separate heap buffer and then free it at your discretion. This way, the user is repsonsible for freeing the string he used in the call to your API, which makes sense. It's his string, after all.

Note that this won't work if your API depends on the user changing the string after creating the object