File Inclusion of the The C Preprocessor in C programming language

The second preprocessor directive we’ll explore in this chapter is file inclusion. This directive causes one file to be included in another. The preprocessor command for file inclusion looks like this:
#include "filename"
and it simply causes the entire contents of filename to be inserted into the source code at that point in the program. Of course this presumes that the file being included is existing. When and why this feature is used? It can be used in two cases:

(a) If we have a very large program, the code is best divided into several different files, each containing a set of related functions. It is a good programming practice to keep different sections of a large program separate. These files are #included at the beginning of main program file.

(b) There are some functions and some macro definitions that we need almost in all programs that we write. These commonly

needed functions and macro definitions can be stored in a file, and that file can be included in every program we write, which would add all the statements in this file to our program as if we have typed them in.
It is common for the files that are to be included to have a .h extension. This extension stands for ‘header file’, possibly because it contains statements which when included go to the head of your program. The prototypes of all the library functions are grouped into different categories and then stored in different header files. For example prototypes of all mathematics related functions are stored in the header file ‘math.h’, prototypes of console input/output functions are stored in the header file ‘conio.h’, and so on.
Actually there exist two ways to write #include statement. These are:
#include "filename"
#include <filename>
The meaning of each of these forms is given below:
#include "goto.c"
This command would look for the file goto.c in the current directory as well as the specified list of directories as mentioned in the include search path that might have been set up.
#include <goto.c>
This command would look for the file goto.c in the specified list of directories only.
The include search path is nothing but a list of directories that would be searched for the file being included. Different C compilers let you set the search path in different manners. If you are using Turbo C/C++ compiler then the search path can be set up by selecting ‘Directories’ from the ‘Options’ menu. On doing this needed functions and macro definitions can be stored in a file, and that file can be included in every program we write, which would add all the statements in this file to our program as if we have typed them in.
It is common for the files that are to be included to have a .h extension. This extension stands for ‘header file’, possibly because it contains statements which when included go to the head of your program. The prototypes of all the library functions are grouped into different categories and then stored in different header files. For example prototypes of all mathematics related functions are stored in the header file ‘math.h’, prototypes of console input/output functions are stored in the header file ‘conio.h’, and so on.
Actually there exist two ways to write #include statement. These are:
#include "filename"
#include <filename>
The meaning of each of these forms is given below:
#include "goto.c"
This command would look for the file goto.c in the current directory as well as the specified list of directories as mentioned in the include search path that might have been set up.
#include <goto.c>
This command would look for the file goto.c in the specified list of directories only.
The include search path is nothing but a list of directories that would be searched for the file being included. Different C compilers let you set the search path in different manners. If you are using Turbo C/C++ compiler then the search path can be set up by selecting ‘Directories’ from the ‘Options’ menu. On doing this