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(cpp-295.info)Header Uses

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Uses of Header Files
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   Header files serve two kinds of 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 in C compilation as
copying the header file into each source file that needs it.  But 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.

   The usual convention is to give header files names that end with
`.h'.  Avoid unusual characters in header file names, as they reduce
portability.