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.