Programming in C

Communication Between Modules
339
Static Versus Extern Variables and Functions
You now know that any variable defined outside of a function is not only a global variable,
but is also an external variable. Many situations arise in which you want to define a
variable to be global but not external. In other words, you want to define a global variable
to be local to a particular module (file). It makes sense to want to define a variable
this way if no functions other than those contained inside a particular file need access to
the particular variable.This can be accomplished in C by defining the variable to be
static.
The statement
static int moveNumber = 0;
if made outside of any function, makes the value of moveNumber accessible from any subsequent
point in the file in which the definition appears, but not from functions contained in
other files.
If you need to define a global variable whose value does not have to be accessed from
another file, declare the variable to be static.This is a cleaner approach to programming:The
static declaration more accurately reflects the variable’s usage and no conflicts
can be created by two modules that unknowingly both use different external global
variables of the same name.
As mentioned earlier in this chapter, you can directly call a function defined in
another file. Unlike variables, no special mechanisms are required; that is, to call a function
contained in another file, you don’t need an extern declaration for that function.
When a function is defined, it can be declared to be extern or static, the former
case being the default. A static function can be called only from within the same file as
the function appears. So, if you have a function called squareRoot, placing the keyword
static before the function header declaration for this function makes it callable only
from within the file in which it is defined:
static double squareRoot (double x)
{
...
}
The definition of the squareRoot function effectively becomes local to the file in which
it is defined. It cannot be called from outside the file.
The same motivations previously cited for using static variables also apply to the case
of static functions.
Figure 15.1 summarizes communication between different modules. Here two modules
are depicted, mod1.c and mod2.c.
mod1.c defines two functions: doSquare and main.The way things are set up here,
main calls doSquare, which in turn calls square.This last function is defined in the
module mod2.c.
Because doSquare is declared static, it can only be called from within mod1.c, and
by no other module.