[Dev-C++] Working with Modules

[André M. B. <and...@su...>]

Dear users,

  I will need to develop an larg app and
I´m interested to work with multiple modules.

  In these modules I want to define both
functions and variables.

  Because of this I wrote a dummy example to
check if it is all right.

  Please take a look at codes and answer me
one question: if the codes are ok, when I need
to use the extern qualifier?

Best Regards
André

-->main.c
#include <stdio.h>
#include <stdlib.h>

#include "sum.h"
#include "dif.h"

int main()
{
   int a, b, c;

   printf("Enter a-->");
   scanf("%d", &a);
   printf("Enter b-->");
   scanf("%d", &b);
   sr = sum(a, b);
   dr = dif(a, b);
   printf("The sum is %d\n", sr);
   printf("The difference is %d\n", dr);

   system("PAUSE");
   return 0;
}

-->sum.h
int sr;
int sum(int, int);

-->dif.h
int dr;
int dif(int, int);

-->sum.c
int sum(int x, int y)
{
    return(x + y);
}

-->dif.c
int dif(int x, int y)
{
    return(x - y);
}