Question

For example in Pascal, if I had a library that I’m compiling to DLL:

library Blah;

procedure AddToNum;
begin
  Num := Num + 1;
end;


procedure PrintNum;
begin
  WriteLN(Num);
end;

Exports AddToNum;
Exports PrintNum;


var
  Num: Integer;

begin
  Num := 0
end.

Ideally, the user could just call the AddToNum and PrintNum routines and it would do as such. However, you actually have to pass in arguments, and that means the user has to keep track of ALL of the variables I’d be using. What is the ideal method to do this? Pointers or something?

I’m looking for the variable to be the same between functions calls, much like some sort of “global”

Answer 1

Move your DLL code (the actual code that runs) into a separate unit (for instance, DLLCode.pas), declare the variable at the top of the implementation section, and have your .DPR file just use that unit. All of the actual code goes in DLLCode.pas, and visibility of the variable follows the normal Pascal scoping rules.

Here’s a sample DLL (DLLSample.dpr and DLLCode.pas), and a test console application that uses it. All of the code compiles and properly executes under Delphi 2007 and Windows 7 64-bit.

DllSample.dpr:

library DLLSample;

{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }

uses
  SysUtils,
  Classes,
  DLLCode in 'DLLCode.pas';

{$R *.res}

begin
end.

DllCode.pas:

unit DLLCode;

interface

procedure AddToNum; stdcall;
procedure PrintNum; stdcall;

exports
  AddToNum,
  PrintNum;

implementation

// Num  is only visible from here down, and starts with a value of zero when the
// DLL is first loaded. It keeps it's value until the DLL is unloaded, which is
// typically when your app is exited when using static linking, or when you 
// FreeLibrary() when dynamically linking.
var
  Num: Integer = 0;

procedure AddToNum;
begin
  Inc(Num);   // Same as Num := Num + 1;
end;

procedure PrintNum;
begin
  WriteLn(Num);
end;

end.

DllTestApp.dpr:

program DLLTestApp;

{$APPTYPE CONSOLE}

uses
  SysUtils;

// Links to the procedures in the DLL. Note that the DLL has to be
// in the same folder, or located somewhere on the Windows PATH. If 
// it can't be found, your app won't run.
procedure AddToNum; external 'DLLSample.dll';
procedure PrintNum; external 'DllSample.dll';


begin
  PrintNum;   // Print initial value
  AddToNum;   // Add to it twice
  AddToNum;
  PrintNum;   // Print new value
  ReadLn;     // Wait for Enter key
end.

This outputs:

0
2

in a console window and waits for you to hit Enter to close it.