Multifile Program in C

1.What is a Multifile program?

• If a program contains many functions then it is difficult to read and maintain because when the number of functions increases the size of the program/file will also increase. To avoid this some functions can be written in separate file 
• The individual files will be compiled separetely and then linked together to form one executable object program.
• This will help to editing and debugging because each file can be maintained at a manageable size.

2.How functions are defined in Multifile Program?

• Within a multi file program a function definition may be either external (or) static
• An external function will be recognized throughout the entire program(ie all the files)
• Static function will be recognized only with in the file in which it is defined.
• The default storage class for the function is external
• Syntax for Function definition

  Storage class datatype name(type1 arg1......typen,argn)
  {
  }

• Example:

  extern int calculate(int x)
  {
  }

3.How functions are declared in Multifile Program?

• When a function is defined in one file and accessed in another one file, the 2nd file must include a function declaration.
• This declaration identifies the function as an external function whose definition appears else where. 
• The function declarations are usually placed at the beginning of the file before any function definition.
• Syntax for external function declaration is same as function definition with additional one semicolon at the end

  Storage class datatype name(type1 arg1...typen,argn);

• Example:

  extern int calculate(int x);

• To execute a multifile program, each individual file must be compiled separetely then the main program file is compiled and executed.
  

4.A simple Multifile program in c

/****************File1*********/
#include<stdio.h>
//Include file test.c
#include "test.c"
//function declaration
extern void result(void);

int main()
{

    printf("In file1 main function\n");
    result();
    return;
}

/*****************File2*********/
extern void result(void)
{
    printf("Welcome to file2\n");
    return;
}

output:
./a.out
In file1 main function
Welcome to file2

5.Accessing external variables in Multifile function:

• Within a multifile program, external variables defined in one file and accessed in another file.
• An external variable definition can appear in only one file. 
• Its location within the file must be external to any function definition. Usually it will appear at the beginning of the file.
• External variable may include initial values.
• If no initial value is assigned then it will automatically initialized to zero.
• To access an external variable in another file,the variable must first be declared within that file .This declaration may appear anywhere within the file usually at the beginning of the file.
• The declaration must begin with the storage class specifier extern. Initial values cannot be included in external variable declaration
• Example:

   /*********file1**********/
  #include<stdio.h>
  //Include file test.c
  #include "test.c"
  //External variable definition
  int x=1,y=2,z=3;
  //function declaration
  extern void result(void);
  int main()
  {
      printf("In file1 main function\n");
      x=y+z;
      printf("x=%d y=%d z=%d\n",x,y,z);
      result();
      return;
  }

  /*****************File2*********/
  extern int x,y,z;/*external var declaration*/
  extern void result(void)
  {
      printf("Welcome to file2\n");
      printf("x=%d y=%d z=%d\n",x,y,z);
      return;
  }

  
  Output:./a.out
  In file1 main function
  x=5 y=2 z=3
  Welcome to file2
  x=5 y=2 z=3
• The value assigned to an external variable can be altered within any file in which the variable is recognized
• The changes will be recognized in all other files. Thus external variables provide a convenient means of transferring information between files.
• Example:

   /*********file1**********/
  #include<stdio.h>
  //Include file test.c
  #include "test.c"
  //External variable 
  definition
  int x=10,y=20,z=30;
  //function declaration
  extern void result(void);
  int main()
  {
      printf("In file1 main 
  function\n");
      x=y+z;
      printf("x=%d y=%d z=%d\n",x,y,z);
      result();
      return;
  }

  /*****************File2*********/
  extern int x,y,z;/*external 
  var declaration*/
  extern void result(void)
  {
      printf("Welcome to file2\n");
  
      y=y+10;
      z=z+10;
      printf("x=%d y=%d z=%d\n",x,y,z);
      return;
  }

  
  Output:./a.out
  In file1 main function
  x=50 y=20 z=30
  Welcome to file2
  x=50 y=30 z=40