Header files and header inclusion

• Header files MUST be protected against multiple inclusion. The protection "key" MUST be the name of the file, entirely capitalized, which punctuation replaced with underscores, and an additional underscore appended. For example, if the file name is foo.h, the protection key SHALL be FOO_H_:

            #ifndef FOO_H_
            # define FOO_H_
            /*
            ** Contents of foo.h
            */
            #endif /* !FOO_H_ */
            

• When including headers, all inclusion directives (#include) SHOULD appear at the start of the file.
• Inclusion of system headers SHOULD precede inclusion of local headers.

  This is bad style: #ifndef FOO_H_ # define FOO_H_ int bar(); # include "bar.h" int foo(); # include <stdio.h> #endif /* !FOO_H_ */

  This is elegant:

            
  
            #ifndef FOO_H_
            # define FOO_H_
            
            # include <stdio.h>
            # include "bar.h"
            
            int bar();
            int foo();
            
            #endif /* !FOO_H_ */