The built-in C operators that you have learned so far aren't very useful for handling strings.

For example, suppose you'd like to know the number of characters stored in a string variable.
Let's examine a piece of code where we use the function sizeof on a string stored in a char array.

Let's compile and run this program. 

You might be surprised that the size reported is 10, not 6 (the number of characters in 'Monday') or 7 (the number of characters in 'Monday' plus a null terminator character).

On an array, sizeof always gives the number of bytes occupied by the array. It is actually a compile-time operation; it doesn't look at the contents of the array at all to determine the size of that array. So, sizeof is not the way to determine the number of characters in a string.
Nor, by the way, is sizeof useful to determine the length of an array if that length isn't known at
compile time. A common mistake made by students is to try to use sizeof in a function that was passed
an array as a parameter.

Getting back to strings, here's one more example why built-in operators aren't useful on strings. 
Suppose that you have two strings s1 and s2 with values "Mon" and "day" respectively, and you want to concatenate them together into a larger string of "Monday".

Using the plus operator won't work. As you learned in the pointers and arrays videos, the name of an array evaluates to the address of its first element. Therefore, s1 + s2 simply adds two pointers together; it has nothing to do with string concatenation.

Fortunately, the C standard library contains many functions for working with strings. Among other things, C's string functions help us determine the length of a string or concatenate two strings together.

To use the C string functions, we must include the string header file in our program. This header file contains function prototypes of all of the C string functions.

One of the functions we can now use is strlen.

Here's the prototype for strlen.

strlen returns the number of characters in the string s, not including the null termination character. size_t is simply an unsigned integer type, and you can treat it as an integer. 
Let's compile and run this new program.

Now the program produces the desired output. strlen counts the number of characters before the '\0' character.