| Include Syntax

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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 (see 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.