Multiple inclusion guard tactic for C explained

includeGuard.h

/** To prevent your code having multiply included header files, 
  * and hence possibly multiply declared functions and variables,
  * the following tactic is used in the file to be included which will
  * prevent the code from being compiled more than once.
  */

// The "ifndef" combined with the matching "endif" directives serve as a pre-processor
// "if not defined" statement, where the code between these two directives will only
// Be compiled if, in this case, the symbol "NAME_OF_HEADER_FILE_H" has not been defined.

#ifndef NAME_OF_HEADER_FILE_H
#define NAME_OF_HEADER_FILE_H // This defines the symbol, which means that the "ifndef" directive in all
                              // other multiply included files will evaluate to false, and the code within
                              // it and the "endif" will not be compiled! Problem solved.

// == Header contents ==

#endif // This is the matching directive that encloses the code to be either compiled or ignored

// NOTE: that this is not the only use of the #define directive. There are a huge amount of other use cases!