Создаем собственный UnRar, используя unrar.dll

// Объявления

function RAROpenArchive(ArchiveData : Pointer): Integer; stdcall;
external 'unrar.dll' name 'RAROpenArchive'; 
 
function RARCloseArchive(hArcData : Integer): Integer; stdcall;
external 'unrar.dll' name 'RARCloseArchive';
 
function RARReadHeader(hArcData : Integer; HeaderData : Pointer): Integer; stdcall;
external 'unrar.dll' name 'RARReadHeader';
 
function RARProcessFile(hArcData : Integer; Operation : Integer; DestPath : Pointer;
DestName : Pointer): Integer; stdcall;
external 'unrar.dll' name 'RARProcessFile';
 
 
const
  ERAR_END_ARCHIVE = 10;
  ERAR_NO_MEMORY = 11;
  ERAR_BAD_DATA = 12;
  ERAR_BAD_ARCHIVE = 13;
  ERAR_UNKNOWN_FORMAT = 14;
  ERAR_EOPEN = 15;
  ERAR_ECREATE = 16;
  ERAR_ECLOSE = 17;
  ERAR_EREAD = 18;
  ERAR_EWRITE = 19;
  ERAR_SMALL_BUF = 20;
 
  RAR_OM_LIST = 0;
  RAR_OM_EXTRACT = 1;
  RAR_SKIP = 0;
  RAR_TEST = 1;
  RAR_EXTRACT = 2;
  RAR_VOL_ASK = 0;
  RAR_VOL_NOTIFY = 1;
 
 
type
  Char260 = Array [1..260] of Char;
 
  TRAROpenArchiveData = record
   ArcName : PChar; // в C++ это будет выглядеть как: char *ArcName
   OpenMode : Cardinal;
   OpenResult : Cardinal;
   CmtBuf : PChar;
   CmtBufSize : Cardinal;
   CmtSize : Cardinal;
   CmtState : Cardinal;
  end;
 
  TRARHeaderData = record
   ArcName : Char260;
   FileName : Char260;
   Flags : Cardinal;
   PackSize : Cardinal;
   UnpSize : Cardinal;
   HostOS : Cardinal;
   FileCRC : Cardinal;
   FileTime : Cardinal;
   UnpVer : Cardinal;
   Method : Cardinal;
   FileAttr : Cardinal;
   CmtBuf : PChar;
   CmtBufSize : Cardinal;
   CmtSize : Cardinal;
   CmtState : Cardinal;
  end;
 
 
var
  RARExtract : boolean;
  RAROpenArchiveData : TRAROpenArchiveData;
  RARComment : array [1..256] of Char;
  RARHeaderData : TRARHeaderData;
 
...
 
procedure ExtractRARArchive;
var
  sDir : string;
  s : string;
  sTest : string;
  iTest : integer;
  bTestDone : boolean;
  RARhnd : Integer;
  RARrc : Integer;
  PDestPath : Char260;
 
begin
  RARExtract:=TRUE;
  lKBWritten:=0;
  ProgressBar2.Position := 0;
  ProgressBar2.Max := lTotalSize;
  RARStartTime:=Time;
 
  RAROpenArchiveData.OpenResult:=99;
  RAROpenArchiveData.OpenMode := RAR_OM_EXTRACT; // открываем для распаковки
  RAROpenArchiveData.ArcName:= @RARFileName;
  RAROpenArchiveData.CmtBuf := @RARComment; 
  RAROpenArchiveData.CmtBufSize := 255; 
 
// Открываем RAR архив и выделяем память
  RARhnd := RAROpenArchive (@RAROpenArchiveData);
  If RAROpenArchiveData.OpenResult <> 0 then
  begin
   case RAROpenArchiveData.OpenResult of
    ERAR_NO_MEMORY : s:='Not enough memory to initialize data structures';
    ERAR_BAD_DATA : s:='Archive header broken';
    ERAR_BAD_ARCHIVE : s:='File is not valid RAR archive';
    ERAR_EOPEN : s:='File open error';
   end;
   MessageDlg('Unable to open rar archive: ' + s + '!',mtError, [mbOK], 0);
  end;
 
  RARSetProcessDataProc(RARhnd,@Form.OnRarStatus);
  StrPCopy(@PDestPath, EInstallPath.Text);
 
  repeat
   RARrc := RARReadHeader (RARhnd, @RARHeaderData);// Читаем заголовок
   if RARrc <> ERAR_END_ARCHIVE then
   begin
    ProgressBar1.Position := 0;
    ProgressBar1.Max := RARHeaderData.UnpSize;
    s:=RARHeaderData.FileName;
    lblCurrentFile.Caption := s;
    lKBytesDone := 0;
   end;
 
   if RARrc = 0 then
   RARrc:=RARProcessFile (RARhnd, RAR_EXTRACT, @PDestPath, nil);
   if (RARrc <> 0) and (RARrc <> ERAR_END_ARCHIVE) then
   begin
    MessageDlg('An Error occured during extracting of ' + sTest+'!' + #13#10 +
    'RARProcessFile: ' + MakeItAString(RARrc),mtError, [mbOK], 0);
   end;
  until RARrc <> 0;
 
// закрываем RAR архив и освобождаем память
  If RARCloseArchive(RARhnd) <> 0 then
  begin
   MessageDlg('Unable to close rar archive!',mtError, [mbOK], 0);
  end;
end; // ExtractRARArchive

Взято из https://forum.sources.ru