HP SunSoft Pascal 4.0 Uživatelský manuál stáhnout pdf (Strana 93)

Program Construction and Management 69

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.

• 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:

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.

module module_unit;

procedure say_hello;

begin

writeln ('Hello, world.')

end;

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HP SunSoft Pascal 4.0 Uživatelský manuál Strana 93