Question

Is it possible to create string variables using pointers? So that I don’t have to pass its size everytime, like char x[4] = “aaa”?

How can I get the size of such string?

And can I initialize an empty string with a pointer?

Answer 1

Remember that strings in C are ended by the null terminator character, written as \0. If you have a well-formed string stored in your pointer variable, you can therefore determine the length by searching for this character:

char *x = "hello"; // automatically appends a null terminator
int len = 0;
while (x[len] != '\0') {
    len++;
}

If your variables are uninitialized or otherwise not well-formed (e.g., by being NULL) you obviously cannot take this approach; however, most functions are generally written under the assumption that the strings are well-formed, because this leads to faster code.

If you wish to initialize a pointer, you have three options: NULL, a valid address (e.g., char *x = &someCharVar), or a string constant (e.g., char *x = "hello"). Note that if you use a string constant, it is illegal for you to write into that pointer unless you re-assign to it with the address of a non-constant string.

// Get enough space for 24 characters plus null terminator
char *myString = (char*) malloc(25 * sizeof(char));
strcpy(myString, "some text"); // fill the new memory
fgets(myString, 25, stdin); // fill with keyboard input

Note that sizeof(char) is unnecessary here, since a char is always defined to be exactly 1 byte. However, it’s a good habit to get into for when you’re using other data types, and it helps make your code self-documenting by making your intentions very clear.