gcc/doc/cpp/header-files.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: header file
   7
   8 .. _header-files:
   9
  10 Header Files
  11 ------------
  12
  13 A header file is a file containing C declarations and macro definitions
  14 (see :ref:`macros`) to be shared between several source files.  You request
  15 the use of a header file in your program by :dfn:`including` it, with the
  16 C preprocessing directive :samp:`#include`.
  17
  18 Header files serve two purposes.
  19
  20 .. index:: system header files
  21
  22 * System header files declare the interfaces to parts of the operating
  23   system.  You include them in your program to supply the definitions and
  24   declarations you need to invoke system calls and libraries.
  25
  26 * Your own header files contain declarations for interfaces between the
  27   source files of your program.  Each time you have a group of related
  28   declarations and macro definitions all or most of which are needed in
  29   several different source files, it is a good idea to create a header
  30   file for them.
  31
  32 Including a header file produces the same results as copying the header
  33 file into each source file that needs it.  Such copying would be
  34 time-consuming and error-prone.  With a header file, the related
  35 declarations appear in only one place.  If they need to be changed, they
  36 can be changed in one place, and programs that include the header file
  37 will automatically use the new version when next recompiled.  The header
  38 file eliminates the labor of finding and changing all the copies as well
  39 as the risk that a failure to find one copy will result in
  40 inconsistencies within a program.
  41
  42 In C, the usual convention is to give header files names that end with
  43 :samp:`.h`.  It is most portable to use only letters, digits, dashes, and
  44 underscores in header file names, and at most one dot.
  45
  46 .. toctree::
  47   :maxdepth: 2
  48
  49   header-files/include-syntax
  50   header-files/include-operation
  51   header-files/search-path
  52   header-files/once-only-headers
  53   header-files/alternatives-to-wrapper-ifndef
  54   header-files/computed-includes
  55   header-files/wrapper-headers
  56   header-files/system-headers