Q.What does #inlcude directive specifies?

The C preprocessor is a program that runs before the compiler. It scans the source file for lines that begin with a hash symbol #, called preprocessor directives, and carries out the instructions they contain, producing an expanded source file that is then compiled.

The two most important directives are:

• #include - inserts the contents of another file into the program. It has two forms: #include <file name> searches the standard system directories (used for library headers such as stdio.h), while #include "file name" also searches the current directory (used for the programmer's own files). One included file may include others, but a file cannot include itself.
• #define - creates a macro or symbolic constant. Its general form is #define identifier string; wherever the identifier later appears, the preprocessor substitutes the string. For example #define PI 3.1415926 lets you write PI in place of the number throughout the program, so a value used in many places can be changed in a single line.

Directives are not ordinary statements: they are not terminated by a semicolon and are handled entirely by the preprocessor, not the compiler. Using them makes programs shorter, easier to read, and easier to modify - which is why the link and definition sections at the top of a C program are built almost entirely from #include and #define.