有关dll的一个问题？

[zhangkeus]

我用dll文件调用一个可视窗体，但是运行后老是报错：
access violation at address... in module "password.dll" write of
address...
请问是怎么回事？
各个模块的代码如下：
library mydll;
uses
SysUtils,
frm_call_dll,
Classes;

{$R *.res}

exports
area,volume;
begin
end.

unit frm_call_dll;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;

function area(R: integer): integer; stdcall; export;
function volume(R: integer): integer; stdcall; export;

type
Tfrm_call = class(TForm)
private
{ Private declarations }
public
{ Public declarations }
end;
//const pi = 3;
var
frm_call: Tfrm_call;

implementation

function area(R: integer): integer; stdcall; export;
begin
result := R*R;
end;

function volume(R: integer): integer; stdcall; export;
begin
result := R*R*R;
end;



{$R *.dfm}

end.

unit main;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

function area(R: integer): integer;
function volume(R: integer): integer;
type
Tfrm_main = class(TForm)
Edit1: TEdit;
Label1: TLabel;
Button1: TButton;
Label2: TLabel;
Label3: TLabel;
Label4: TLabel;
Label5: TLabel;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
frm_main: Tfrm_main;

implementation
function area(R: integer): integer; external 'mydll.dll' name 'area';
function volume(R: integer): integer; external 'mydll.dll' name 'volume';


{$R *.dfm}


procedure Tfrm_main.Button1Click(Sender: TObject);
begin
showmessage('开始计算');
label4.Caption := inttostr(area(strtoint(edit1.Text)));
label5.Caption := inttostr(volume(strtoint(edit1.Text)));
end;

end.

[sixgj]

write of
address...
错误。

[Eastunfail]

implementation
function area(R: integer): integer; external 'mydll.dll' name 'area';
function volume(R: integer): integer; external 'mydll.dll' name 'volume';

没有指定stdcall。

[Eastunfail]

function area(R: integer): integer;stdcall; external 'mydll.dll' name 'area';
function volume(R: integer): integer;stdcall; external 'mydll.dll' name 'volume';