Using external variables

 

Some libaries or code blocks have variables which they export. You can access these variables much in the same way as external functions. To access an external variable, you declare it as follows:

Var
  MyVar : MyType; external name 'varname';

The effect of this declaration is twofold:

1. No space is allocated for this variable.
2. The name of the variable used in the assebler code is varname. This is a case sensitive name, so you must be careful.

The variable will be accessible with it's declared name, i.e. MyVar in this case.

A second possibility is the declaration:

Var
  varname : MyType; cvar; external;

The effect of this declaration is twofold as in the previous case:

1. The external modifier ensures that no space is allocated for this variable.
2. The cvar modifier tells the compiler that the name of the variable used in the assebler code is exactly as specified in the declaration. This is a case sensitive name, so you must be careful.

In this case, you access the variable with it's C name, but case insensitive. The first possibility allows you to change the name of the external variable for internal use.

In order to be able to compile such statements, the compiler switch -Sv must be used.

As an example, let's look at the following C file (in extvar.c):

/* 
Declare a variable, allocate storage 
*/
int extvar = 12;

And the following program (in extdemo.pp):

Program ExtDemo;

{$L extvar.o}

Var { Case sensitive declaration !! }
    extvar : longint; cvar;external;
    I : longint; external name 'extvar';
begin
  { Extvar can be used case insensitive !! }
  Writeln ('Variable ''extvar'' has value : ',ExtVar);
  Writeln ('Variable ''I''      has value : ',i);
end.

Compiling the C file, and the pascal program:

gcc -c -o extvar.o extvar.c
ppc386 -Sv extdemo

Will produce a program extdemo which will print

Variable 'extvar' has value : 12
Variable 'I'      has value : 12

on your screen.