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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.