Current User and Domain Names

A program may sometimes need to display the user name and domain name for the current thread. This article demonstrates how to do this on Windows NT by using security functions within the Win32 Application Programming Interface (API). Prior to Windows NT, it could be assumed that a thread was running under the account of the interactively logged on user. Windows NT, however, allows threads to run under multiple security contexts, potentially representing multiple users. For instance, in a client/server application, a thread in the server might impersonate a client through the ImpersonateNamedPipeClient function. In this case, it runs under the user context of the client. Another example of a thread running in a different security context is a service thread, which has a domain name of NT AUTHORITY and a user name of SYSTEM, assuming that the service is running in the local system account.
If you need to obtain both the user name and the domain name for the current thread, you must first extract the user's security identifier (SID) from the thread's access token by using the GetTokenInformation function. Then, a call to the LookupAccountSid function can be used to retrieve the account name and domain name associated with the SID.
The following sample code demonstrates how to programmatically retrieve the user name and domain name on Windows NT:

//**********************************************************************
// 
//  FUNCTION:     GetCurrentUserAndDomain - This function looks up
//                the user name and domain name for the user account
//                associated with the calling thread.
//
//  PARAMETERS:   szUser - a buffer that receives the user name
//                pcchUser - the size, in characters, of szUser
//                szDomain - a buffer that receives the domain name
//                pcchDomain - the size, in characters, of szDomain
// 
//  RETURN VALUE: TRUE if the function succeeds. Otherwise, FALSE and
//                GetLastError() will return the failure reason.
// 
//                If either of the supplied buffers are too small, 
//                GetLastError() will return ERROR_INSUFFICIENT_BUFFER
//                and pcchUser and pcchDomain will be adjusted to
//                reflect the required buffer sizes.
//
//**********************************************************************

function GetCurrentUserAndDomain(szUser : PChar;   pcchUser : DWORD;
       szDomain : PChar;  pcchDomain: DWORD) : boolean;

var
   fSuccess : boolean;
   hToken   : THandle;
   ptiUser  : PSIDAndAttributes;
   cbti     : DWORD;
   snu      : SID_NAME_USE;

begin
   ptiUser := nil;
   Result := false;

   try
      // Get the calling thread's access token.
      if (not OpenThreadToken(GetCurrentThread(), TOKEN_QUERY, TRUE,
            hToken)) then

       begin
         if (GetLastError() <> ERROR_NO_TOKEN) then
            exit;

         // Retry against process token if no thread token exists.
         if (not OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY,
               hToken)) then
            exit;
       end;

      // Obtain the size of the user information in the token.
      if (GetTokenInformation(hToken, TokenUser, nil, 0, cbti)) then

         // Call should have failed due to zero-length buffer.
         Exit

       else

         // Call should have failed due to zero-length buffer.
         if (GetLastError() <> ERROR_INSUFFICIENT_BUFFER) then
            Exit;


      // Allocate buffer for user information in the token.
      ptiUser :=  HeapAlloc(GetProcessHeap(), 0, cbti);
      if (ptiUser= nil) then
         Exit;

      // Retrieve the user information from the token.
      if ( not GetTokenInformation(hToken, TokenUser, ptiUser, cbti, cbti)) then
         Exit;

      // Retrieve user name and domain name based on user's SID.
      if ( not LookupAccountSid(nil, ptiUser.Sid, szUser, pcchUser,
            szDomain, pcchDomain, snu)) then
         Exit;

      fSuccess := TRUE;

   finally

      // Free resources.
      if (hToken > 0) then
         CloseHandle(hToken);

      if (ptiUser <> nil) then
         HeapFree(GetProcessHeap(), 0, ptiUser);
   end;

   Result :=  fSuccess;
end;