### 2.1 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 (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 quote directories and then the same directories used for `<`file`>`. You can prepend directories to the list of quote directories with the `-iquote` option.

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.