Delphi中GetLastError和FormatMessage的正确用法是什么？

function GetLastError: DWORD; stdcall;

function FormatMessage(dwFlags: DWORD; lpSource: LPCVOID; dwMessageId: DWORD; dwLanguageId: DWORD; lpBuffer: LPVOID; nSize: DWORD; Arguments: Pointer): DWORD; stdcall;

uses
  Windows, SysUtils;

function GetLastErrorMessage: string;
var
  dwError: DWORD;
  sMessage: array[0..255] of Char;
begin
  dwError := GetLastError;
  if FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM or FORMAT_MESSAGE_IGNORE_INSERTS or FORMAT_MESSAGE_MAX_WIDTH_MASK or FORMAT_MESSAGE_ARGUMENT_ARRAY, nil, dwError, 0, @sMessage, SizeOf(sMessage), nil) <> 0 then
    Result := sMessage
  else
    Result := 'Unknown error';
end;