Both user and system header files are included using the preprocessing directive #include. It has two variants:
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).
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,
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,
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.