Programming in C

340 Chapter 15 Working with Larger Programs
double x;
static double result;
static void doSquare (void)
{
double square (void);
}
x = 2.0;
result = square ();
int main (void)
{
doSquare ();
printf ("%g\n", result);
}
return 0;
extern double x;
double square(void)
{
}
return x * x;
mod1.c
mod2.c
Figure 15.1
Communication between modules.
mod1.c defines two global variables: x and result,both of type double. x can be
accessed by any module that is linked together with mod1.c. On the other hand, the
keyword static in front of the definition of result means that it can only be accessed
by functions defined inside mod1.c (namely main and doSquare).
When execution begins, the main routine calls doSquare.This function assigns the
value 2.0 to the global variable x and then calls the function square. Because square is
defined in another source file (inside mod2.c), and because it doesn’t return an int,
doSquare properly includes an appropriate declaration at the beginning of the function.
The square function returns as its value the square of the value of the global variable
x. Because square wants to access the value of this variable, which is defined in another
source file (in mod1.c), an appropriate extern declaration appears in mod2.c (and, in this
case, it makes no difference whether the declaration occurs inside or outside the square
function).
The value that is returned by square is assigned to the global variable result inside
doSquare, which then returns back to main.Inside main, the value of the global variable
result is displayed.This example, when run, produces a result of 4.0 at the terminal
(because that’s obviously the square of 2.0).
Study the example until you feel comfortable with it.This small—albeit impractical—
example illustrates very important concepts about communicating between modules, and
it’s necessary that you understand these concepts to work effectively with larger
programs.