Do you have "export" your function inside your Library??
Something like :
library TestLibrary;
uses SysUtils, Classes;
function MixedStr(AStr:PChar):PChar; stdcall; export;
var
ATemp:String;
begin
ATemp:=AStr;
ATemp2:=ATemp+' . Now, you get it from DLL';
Result:= PChar(ATemp2);
end;
exports
MixedStr;
begin
end.
Then you call it something like (Inside your EXE. It's Static Loaded, not the Dynamic one):
.....
var
Form1: TForm1;
function MixedStr(AStr:PChar):PChar; stdcall; external 'TestLibrary.dll'
implementation
{$R *.lfm}
procedure TForm1.Button1Click(Sender: TObject) ;
var
ATempStr, ATempStr2:String;
begin
ATempStr2:=MixedStr(PChar(ATempStr));
Caption:=ATempStr2;
end;
....