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// Creates a temporary file which deletes itself automatically after closing
// the handle.
// Author:
//   Sebastian Jänicke
// Parameters:
//   [in]  Prefix:       Up to three characters (more are ignored) to be used
//                       as start of the filename.
//   [out] TempFileName: The filename of the created temporary file or an
//                       error description if the function fails.
// Return value:
//   Returns the file handle of the created temporary file.
//   If the function fails it returns INVALID_HANDLE_VALUE.
function CreateAutoTempFile(Prefix: String; out TempFileName: String): THandle;
// uses: Windows, SysUtils
const
  FILE_SHARE_NONE = 0;
  // There is no constant for this, but I assume this is better readable than 0.
var
  TempPath: PChar;
  TempPathBufferSize: Integer;
begin
  Result := INVALID_HANDLE_VALUE;
  // Fetch the system's temporary directory
  TempPathBufferSize := GetTempPath(0, nil);
  GetMem(TempPath, TempPathBufferSize);
  try
    if GetTempPath(TempPathBufferSize, TempPath) <> 0 then
    begin
      // Create a name for a temporary file in the temporary directory
      SetLength(TempFileName, MAX_PATH);
      if GetTempFileName(TempPath, PChar(Prefix), 0, PChar(TempFileName)) <> 0 then
      begin
        SetLength(TempFileName, StrLen(PChar(TempFileName)));
        // Create the temporary file
        Result := CreateFile(PChar(TempFileName), GENERIC_READ or GENERIC_WRITE,
          FILE_SHARE_NONE, nil, CREATE_ALWAYS,
          FILE_ATTRIBUTE_TEMPORARY or FILE_FLAG_DELETE_ON_CLOSE, 0);
        if Result = INVALID_HANDLE_VALUE then
          TempFileName := SysErrorMessage(GetLastError);
      end
      else
        TempFileName := SysErrorMessage(GetLastError);
    end
    else
      TempFileName := SysErrorMessage(GetLastError);
  finally
    FreeMem(TempPath, TempPathBufferSize);
  end;
end;					

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var
  TempFileName: string;
  TempFileHandle: THandle;
begin
  TempFileHandle := CreateAutoTempFile('aaa', TempFileName);
  if TempFileHandle <> INVALID_HANDLE_VALUE then
  begin
    ShowMessage('Dateiname: ' + TempFileName);
    CloseHandle(TempFileHandle);
  end
  else
    ShowMessage('Die Datei konnte nicht erstellt werden. Fehlermeldung:'#13#10
      + TempFileName);
end;