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Subj : Re: Challenge: Multithreading & Synchronization
To   : comp.programming.threads
From : Maciej Sobczak
Date : Sat May 21 2005 04:40 pm

Giancarlo Niccolai wrote:

>>Let's say there is no cout, but instead the "print" function:
>>
>>print("I'm doing something");
> 
> More. The terminal operation may be a LONG operation.

It is not the point here. The point is that synchronizing threads is not 
necessarily about protecting accesses to some data, but also about 
orchestrating the work of multiple threads so that the overall sequence 
of actions results in well-defined program behaviour.
In the case of output devices, the "well-defined" might just mean that 
the output is semantically meaningful when many "unit" operations are used.
This example was given to motivate Uenal Mutlu to think in terms of 
operations, not in terms of separate (and possibly independent) objects.

> If a terminal get
> disconnected while accessed

If you are concerned about this possibility, then that's fine - 
depending on your environment. Others might be concerned about 
spontaneously disconnecting hard drives or even CPUs (you may not 
believe, but I remember having disconnected hard drive whereas I do not 
remember any disconnected terminal).

It was not the focus of this example, however.

> Locking an
> operation (print) to protect an object (terminal) is wrong.

In this example, it is not the terminal (as a physical object) which was 
protected, but rather the "continuity" of the information that is printed.

> You must find another way [...] The way exists, and it's simple, just look a
> little further...

If you are talking about "transactions", then no. No matter what (and 
how much effort you want to put into it), sooner or later you will have 
to call this print function and if done from many threads, the problem 
is still there.
If you are talking about having a separate thread (or even process) that 
will be responsible for printing, then here also the problem is still 
there, just moved from one place to another.

Anyway - it was not the point of this example.

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/

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