Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!usc!ucsd!ucbvax!hplabs!hp-pcd!hpcvlx!nathanm
From: nathanm@hpcvlx.cv.hp.com (Nathan K. Meyers)
Newsgroups: comp.os.mach
Subject: Re: Wish explanation of some cthread calls
Message-ID: <113980003@hpcvlx.cv.hp.com>
Date: 27 Nov 89 20:55:37 GMT
References: <113980001@hpcvlx.cv.hp.com>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 31

And now that my question has been answered, I'm starting to get the "I'd
like to know also" messages.  So here's a cc of the clearest explanation
I got:

> Think of it as ways to store and access data from an arry
> indexed by the  thread ID. 
> 
> Something like 
> 
>    void* per_thread_data[MAX_THREAD_ID];
> 
>    per_thread_data[thread_id]=x;	
> 	/* equiv to 	cthread_set_data(thread_id,x);
>    x=per_thread_data[thread_id];
> 	/* equiv to 	x=cthread_data(thread_id);
> 
> would achieve the same thing if thread_ids were consecutive.
> It's useful in that it is a neat way to get a handle with which
> to create your own per-thread data structures.

Thanks to Arul A. Menezes for the clear explanation.

My big problem in understanding the explanation of these calls in the
1988 Cooper/Draves paper was trying to figure out how these simple
semantics allowed you to associate an arbitrary collection of data with
a thread.  The answer, of course, is that you can associate a single
pointer with the thread -- and hang whatever you want off of the
pointer.

Nathan Meyers
nathanm@cv.hp.com