Technical Q&A NW03

CTB & the AppleTalk ADSP Tool

Q: I have a question about the CTB & the AppleTalk ADSP Tool.

I have a connection between two machines using the ADSP Tool. The local machine writes a bunch of data to the remote machine, then wants to shut down the connection. I use a CMStatus call to verify that there isn't a write pending, and also check the buffer sizes returned by the CMStatus call and verify that there is nothing in the cmDataOut part of the sizes array. I then call CMClose, and the connection closes. The problem is the remote machine has not received all of the data before the connection is closed! How do I detect if the ADSP Tool has "really" written all the data? Or how do I invoke the CMClose so that it properly concludes the write (i.e., setting the abort parameter to 0 when doing the dspRemove to the ADSP Driver)?

I've tried two CMClose variations:

In one case, I make a synchronous call to CMClose as follows (in Pascal):

     theErr := CMClose(theConnection, FALSE, NIL, 0, FALSE);

This call exhibits the behavior I've described above.

I've also tried an asynchronous call, in the hope that the ADSP Tool would handle finishing off the connection properly:

  theErr := CMClose(theConnection, TRUE, nil, 60, FALSE);
  while (theErr = noErr) and
    (BAND(status, cmStatusOPEN + cmStatusClosing) <> 0) do
  begin
    CPI_Idle(theConnection);
    theErr := CPI_Status(theConnection, sizes, status);
   end;

What I've found in this case is that the ADSP Tool never completes the write. I've varied the timeout from 60 above to -1 and also 6000. In all of these cases, the cmStatusClosing bit is ALWAYS set, and never goes low. Any suggestions?

A: You're using the correct options on your call to CMClose, i.e., finishing all pending writes and then closing the connection.

Making the assumption that you're also using the ADSP Tool on the receiving end of the connection, it's possible that the data has been received and then the connection closed down. The receiver should be called with CMStatus to see if there is data to read, regardless of whether the cmStatusOpen bit is set; if cmStatusDataAvail is set, the connection can still be read for additional data, even if it is closed. The Tool may have buffered the incoming data and then completed the close connection request from the sender.

Check to see that you are testing the flags parameter of CMStatus on the receiver side against cmStatusDataAvail and basing your final read on that.