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,
#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.
#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.