cpp.info: Include Syntax

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Include Syntax

   Both user and system header files are included using the
preprocessing directive `#include'.  It has two variants:
`#include <FILE>'
     This variant is used for system header files.  It searches for a
     file named FILE in a standard list of system directories.  You can
     prepend directories to this list with the `-I' option (*note
     Invocation::).
`#include "FILE"'
     This variant is used for header files of your own program.  It
     searches for a file named FILE first in the directory containing
     the current file, then in the same directories used for `<FILE>'.
   The argument of `#include', whether delimited with quote marks or
angle brackets, behaves like a string constant in that comments are not
recognized, and macro names are not expanded.  Thus, `#include <x/*y>'
specifies inclusion of a system header file named `x/*y'.
   However, if backslashes occur within FILE, they are considered
ordinary text characters, not escape characters.  None of the character
escape sequences appropriate to string constants in C are processed.
Thus, `#include "x\n\\y"' specifies a filename containing three
backslashes.  (Some systems interpret `\' as a pathname separator.  All
of these also interpret `/' the same way.  It is most portable to use
only `/'.)
   It is an error if there is anything (other than comments) on the line
after the file name.