#### 26.4.1 `#include` Syntax ¶

You can specify inclusion of user and system header files with 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 Invoking GCC in Using the GNU Compiler Collection).

`#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, 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: character escape sequences such as used in string constants in C are not meaningful here. Thus, `#include "x\n\\y"` specifies a filename containing three backslashes. By the same token, there is no way to escape ‘`"`’ or ‘`>`’ to include it in the header file name rather than end the file name. We recommend not using those two characters in the names of files to be included.

Some systems interpret ‘`\`’ as a file name component separator. All these systems also interpret ‘`/`’ the same way. It is most portable to use only ‘`/`’.

It is an error to put anything other than comments on the `#include` line after the file name.