Errors in Delphi while trying to load procedures from dll - delphi

Question

I have a problem while loading procedures from a dll, either when loading it dynamically or statically. When I put procedures from dll to my unit, everything works fine. When I try to do it with dll it gives me
First chance exception at $00526399. Exception class $C0000005 with message 'access violation at 0x00526399: read of address 0x00000390'. Process Project1.exe (21988)
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ComCtrls,Unit2;
type
TForm1 = class(TForm)
ListView1: TListView;
Button1: TButton;
Button2: TButton;
Edit1: TEdit;
Edit2: TEdit;
Edit3: TEdit;
Edit4: TEdit;
Edit5: TEdit;
procedure FormCreate(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
procedure Refresh;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
type
plist = ^element;
element = record
artist,title,genre: string[20];
year,grade: integer;
wsk: plist;
end;
database = file of element;
var
base: database;
first: plist;
handler: HModule;
{$R *.dfm}
procedure TForm1.Refresh();
var
current: plist;
begin
ListView1.Clear;
current:= first;
while current<>nil do
begin
with ListView1.Items.Add do
begin
Caption:=current^.artist;
SubItems.Add(current^.title);
SubItems.Add(current^.genre);
SubItems.Add(IntToStr(current^.year));
SubItems.Add(IntToStr(current^.grade));
end;
current:=current^.wsk;
end;
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
var Save: procedure;
begin
handler:=LoadLibrary('lib.dll');
try
#Save:=GetProcAddress(handler, PChar(2));
if #Save = nil then raise Exception.Create('Load nie dziala');
Save();
finally
FreeLibrary(handler);
end;
end;
procedure TForm1.FormCreate(Sender: TObject);
var
Load: procedure;
begin
handler:=LoadLibrary('lib.dll');
try
#Load:=GetProcAddress(handler, PChar(1));
if #Load = nil then raise Exception.Create('Load nie dziala');
Load();
finally
FreeLibrary(handler);
end;
Refresh();
end;
procedure TForm1.Button1Click(Sender: TObject);
var
el: element;
Add: procedure(el:element);
begin
el.artist:=Edit1.Text;
el.title:=Edit2.Text;
el.genre:=Edit3.Text;
el.year:=StrToInt(Edit4.Text);
el.grade:=StrToInt(Edit5.Text);
handler:=LoadLibrary('lib.dll');
try
#Add:=GetProcAddress(handler, PChar(3));
if #Add = nil then raise Exception.Create('Load nie dziala');
Add(el);
finally
FreeLibrary(handler);
Refresh();
{Form2:=TForm2.Create(Form1);
Form2.ShowModal;
Form2.Free;}
end;
end;
end.
The dll file looks like this:
library lib;
{ 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
System.SysUtils,
System.Classes;
{$R *.res}
type plist = ^element;
element = record
artist,title,genre:string[20];
year,grade:integer;
wsk: plist;
end;
database = file of element;
var
first: plist;
base: database;
procedure add(el: element); stdcall;
var current,tmp: plist;
begin
New(current);
current^ := el;
current^.wsk := nil;
if first = nil then
begin
first:=current;
end else
begin
tmp:=first;
while tmp^.wsk<>nil do
begin
tmp:=tmp^.wsk;
end;
tmp^.wsk:=current;
end;
end;
procedure load();stdcall;
var
el: element;
i: integer;
begin
AssignFile(base, 'baza.dat');
if not FileExists('baza.dat') then
begin
Rewrite(base);
end else
begin
Reset(base);
for i := 0 to FileSize(base)-1 do
begin
read(base, el);
add(el);
end;
end;
CloseFile(base);
end;
procedure save();stdcall;
var
current: plist;
el: element;
begin
AssignFile(base, 'baza.dat');
Rewrite(base);
current:=first;
while current<>nil do
begin
el:=current^;
el.wsk:=nil;
write(base, el);
current:= current^.wsk;
end;
end;
exports
add index 1,
load index 2,
save index 3;
begin
end.
It also shows me an error:
Expected ';' but received and identifier 'index' at line 91
But exports are done like I red on web.

Answer 1

The obvious errors are:
You don't perform much error checking. You assume that the calls to LoadLibrary always succeed.
The calling conventions don't match. You use stdcall in the DLL and register in the executable.
The ordinals don't match. In the DLL it is add (1), load (2) and save (3). In the executable you have add (3), load (1) and save (2).
You load and unload the DLL every time you call functions from the DLL. That means that the global variables in the DLL that hold your state are lost each time the DLL is unloaded.
Frankly this code is a real mess. I suggest that you do the following:
Switch to load time linking using the function names rather than ordinals. This means to use the external keyword in the executable. This will greatly simplify your code by removing all those calls to LoadLibrary, GetProcAddress etc. If runtime linking is needed, you can add it later using the delayed keyword.
Stop using global state in the DLL and instead pass information back and forth between modules. Remove all global variables. But make sure you don't pass Delphi objects back and forth.
Use PChar rather than short strings across the module boundary.
Stop using linked lists and dynamic allocation. That's hard to get right. Use TList<T> in the DLL to store the list of elements.