Technical Q&A NW23

PSetSelfSend in Classic AppleTalk vs. Open Transport AppleTalk

Q: What's the relationship between the classic AppleTalk PSetSelfSend call and Open Transport AppleTalk?

A: In version 1.1, Open Transport AppleTalk shares a "self send" variable with classic AppleTalk, i.e., if you set the variable using the classic PSetSelfSend, the effects are seen by native clients, and vice versa.

If you're using OT native interfaces, you can change the variable using an ioctl call, as shown in the snippet below.

enum {
    kATalkFullSelfSend        = MIOC_CMD(MIOC_ATALK, 47)
};
static OSStatus OTSetSelfSend(EndpointRef ep, Boolean enable_self_send)
{
  OSStatus result;

  result = OTIoctl(ep, kATalkFullSelfSend, (void *) enable_self_send);
  if (result > noErr) {
    result = noErr;
  }
  return result;
}

Note that, like the PSetSelfSend call, the ioctl returns the previous value of the self send variable as either 0 (it was previously disabled) or 1 (it was previously enabled). As in classic AppleTalk, it's rarely appropriate to restore the value of self send when you're done, so the code above maps both results to noErr.

The self send value is a Boolean, not a counter. For example, if the following sequence happens:

1. self send starts out false
2. client A sets self send to true, is returned false as the previous value
3. client B sets self send to true, is returned true as the previous value
4. client A quits, "restoring" self send to false

then client B is left with self send set to false.

For this reason, the standard practice is to set self send if you need it and not attempt to restore it. Because many clients follow this convention, it's important that your program work even if self send is true.

Future versions of OT will most probably have self send always on for OT native clients and loop-back packets will be filtered out only for classic clients if PSetSelfSend wasn't called.