Delphi 10.3 - Weird DLL behaviour - delphi

Question

I have met a really weird behaviour using Delphi and DLL.
I want to learn how to create and use DLL's. To do so, I created two files: one containing the DLL, and the other one calling it.
The DLL retrieves informations from a *.ini file (the GetInfos function). When calling it in a form, I get the following error: "Access violation at address XXXXXXXX in "dlltest.dll" module. Read of address 00000000.
I wanted to try some other things, so I created a simple procedure, that just display a message.
The interesting thing is that when I call this procedure (which only purpose is to display a message), I no longer get the Access violation error.
I really do not understand how it can make my code work. I'd really like if someone could give me some explanation. I am using Delphi 10.3 Rio Version 26.0.36039.7899
Here is the code I produced :
dlltest:
library dlltest;
uses
System.SysUtils, System.Classes, System.IniFiles, Winapi.Windows, vcl.dialogs;
var
// Ini file var
iniConfig: TInifile;
procedure SayHello;
var
hello: PChar;
begin
hello := 'Hello world!';
ShowMessage(hello);
end;
// -----------------------------------------------------------------------------
// Retrieving .ini file
function GetIni: TInifile;
begin
result := TInifile.Create('.\monitoring.ini');
end;
// -----------------------------------------------------------------------------
function GetInfos: ShortString;
begin
iniConfig := GetIni;
try
result := iniConfig.ReadString('filename', 'start', 'start');
finally
iniConfig.Free;
end; { try }
end; { function GetInfos }
exports
SayHello, GetInfos;
begin
end.
Using dll:
unit testUtilisationDll;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils,
System.Variants, System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms,
Vcl.Dialogs, Vcl.StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Déclarations privées }
public
{ Déclarations publiques }
end;
var
Form1: TForm1;
procedure SayHello; StdCall; external 'dlltest.dll';
function GetInfos: ShortString; StdCall;
external 'dlltest.dll';
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
SayHello;
ShowMessage(GetInfos(self));
end;
end.

Answer 1

When you export the functions, they are using the default calling convention (Register), whereas when you import them, you are telling the import that they're using StdCall calling convention.
The normal case for DLL exported functions is StdCall convention, so you should declare your functions to use this calling convention in your .DLL source:
function GetInfos: ShortString; stdcall;
and
procedure SayHello; stdcall;
Another problem is that when you call GetInfos:
ShowMessage(GetInfos(self));
you are passing it a parameter (self), but your declaration of the import:
function GetInfos: ShortString; StdCall; external 'dlltest.dll';
doesn't list any parameters. You won't be able to compile the program as it is shown in your question...