Include Guards syntax in C
Hello every one I want to ask a question about include guards in C programming.I know there purpose but in some codes I have seen "1" written after #define  like

    #ifndef MYFILE_H
    #define MYFILE_H 1

What is the purpose of this "1" ? Is it necessary ?
Thanks 
Include Guards syntax in C
===

Hello every one I want to ask a question about include guards in C programming.I know there purpose but in some codes I have seen "1" written after #define  like

    #ifndef MYFILE_H
    #define MYFILE_H 1

What is the purpose of this "1" ? Is it necessary ?
Thanks 