(cpp.info.gz) Header Files

 
 2 Header Files
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 A header file is a file containing C declarations and macro definitions
 ( Macros) to be shared between several source files.  You request
 the use of a header file in your program by "including" it, with the C
 preprocessing directive `#include'.
 
    Header files serve two purposes.
 
    * System header files declare the interfaces to parts of the
      operating system.  You include them in your program to supply the
      definitions and declarations you need to invoke system calls and
      libraries.
 
    * Your own header files contain declarations for interfaces between
      the source files of your program.  Each time you have a group of
      related declarations and macro definitions all or most of which
      are needed in several different source files, it is a good idea to
      create a header file for them.
 
    Including a header file produces the same results as copying the
 header file into each source file that needs it.  Such copying would be
 time-consuming and error-prone.  With a header file, the related
 declarations appear in only one place.  If they need to be changed, they
 can be changed in one place, and programs that include the header file
 will automatically use the new version when next recompiled.  The header
 file eliminates the labor of finding and changing all the copies as well
 as the risk that a failure to find one copy will result in
 inconsistencies within a program.
 
    In C, the usual convention is to give header files names that end
 with `.h'.  It is most portable to use only letters, digits, dashes, and
 underscores in header file names, and at most one dot.
(cpp.info.gz) Header Files

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