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