True, We can prevent the same file from getting included again by using a preprocessor directive called #ifndef (short for "if not defined") to determine whether we've already defined a preprocessor 

symbol called XSTRING_H. If we have already defined this symbol, the compiler will ignore the rest 

of the file until it sees a #endif (which in this case is at the end of the file).

#ifndef XSTRING_H

#define XSTRING_H defines the same preprocessor symbol,

Finally, the last line of the file, #endif