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From: ech@cbnewsk.att.com (ned.horvath)
Newsgroups: comp.sys.mac.programmer
Subject: Re: asynchronous question
Message-ID: <1990Nov19.195110.2339@cbnewsk.att.com>
Date: 19 Nov 90 19:51:10 GMT
References: <1990Nov19.130139.35978@eagle.wesleyan.edu>
Organization: AT&T Bell Laboratories
Lines: 48

From article <1990Nov19.130139.35978@eagle.wesleyan.edu>, by rcook@eagle.wesleyan.edu:
- Suppose I am making two asynchronous AppleTalk (or Device Manager, for that
- matter) like this:
- 
- var
-   MyParmBlock: ParmBlkPtr;
-   async: boolean;
- 
- begin
-   ...
-   async := true; { make the calls asynchronously }
-   MyParmBlk := ParmBlkPtr(NewPtr(sizeof(ParmBlkPtr)));
- 
-   with MyParmBlock^ do
-     begin
-       { fill in data }
-     end;
-   err := SomeAppleTalkCall(MyParmBlock,async);
- 
-   with MyParmBlock^ do
-     begin
-       { fill in some more data }
-     end;
-   err := SomeOtherAppleTalkCall(MyParmBlock,async);
-   ...
- end;
- 
- Assuming the first AppleTalk call has not yet been completed by the time the
- second call is made, is it safe to reuse MyParmBlock in this way?  Ignore for
- now the fact that I can't go back and check if the first call was completed
- successfully.  In other words, when something is placed in an I/O queue, is a
- copy of the parameter block made or is a pointer to your data placed in the
- queue instead?  I assume it makes a copy, but I want to hear this from someone
- who knows for sure.  Thanks.

Your intuition is wrong: your parameter block -- not a copy -- is linked
into the I/O queue, and becomes the property of the OS until the request
completes.

A common technique is to set ioResult to 1 before the call (some drivers
do this for you) since all valid completions are <= 0.  You can then poll
the ioResult field from your event loop if you don't feel up to doing an
ioCompletion routine.

Notice that the call "completes" -- i.e. never really gets started --
if the call returns anything but noErr (0).  ALWAYS check the err return!

=Ned Horvath=