What ! (char*) means in C

Question

I was looking at an example and I saw this:

char *str;

/* ... */

if (!str || !*str) {
    return str;
}

Does it mean it's empty or something?

Answer 1

str is a char pointer. ! negates it. Basically, !str will evaluate to true (1) when str == NULL.

The second part is saying, (if str points to something) evaluate to true (1) if the first character is a null char ('\0') - meaning it's an empty string.

Note:
*str dereferences the pointer and retrieves the first character. This is the same as doing str[0].

Answer 2

!str means that there is no memory allocated to str. !*str means that str points to an empty string.