Multi-file programs in C

           Unit IV-multi-file programs.

       File Inclusion

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.

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.