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';
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The effect of this declaration is twofold:
- No space is allocated for this variable.
- The name of the variable used in the assembler 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;
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The effect of this declaration is twofold as in the previous case:
- The external modifier ensures that no space is allocated for this variable.
- The cvar modifier tells the compiler that the name of the variable used in the assembler
code is exactly as specified in the declaration. This is a case sensitive name, so you
must be careful.
The first possibility allows you to change the name of the external variable for internal
use.
As an example, let’s look at the following C file (in extvar.c):
/*
Declare a variable, allocate storage
*/
int extvar = 12;
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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.
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Compiling the C file, and the pascal program:
gcc -c -o extvar.o extvar.c
ppc386 -Sv extdemo
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Will produce a program extdemo which will print
Variable 'extvar' has value: 12
Variable 'I' has value: 12
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on your screen.