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 if it would instead end the file name.

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.