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The body of the procedure say_hello is not defined in this program unit, but the program unit does contain a declaration of the interface to the procedure. The keyword extern declares that say_hello is declared in a module unit.1

•Module unit—This unit can begin with an optional module header, followed by a set of compilable declarations and definitions. Modules that call externally defined routines must have declarations for those routines.

Here is an example:

module module_unit; procedure say_hello; begin

writeln ('Hello, world.')

end;

Every program must have one and only one program unit; a program can have any number of module units. Any unit can call procedures declared in any other unit; each unit must have external declarations for every procedure it uses that is not defined in that unit.

A module unit can also be used as a library, that is, as a collection of useful routines that is shared among a number of programs.

Compiling with Units

Consider the units given in the previous section, “Using Program Units and Module Units.” You can compile and link these units on a single line by executing the following command, which then produces the executable, a.out.

hostname% pc program_unit.p module_unit.p

1.A statement that shows the interface of a routine is called a declaration, because it declares the name and parameters of the routine. The set of statements that shows the entire routine, including the body, is called the definition of the routine. There can be only one definition for a given routine, but every routine must be declared in every module or program unit that uses it.

Program Construction and Management

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