Subj : Re: I write a multithread program with pthread, and meet a program
To   : comp.programming.threads
From : David Butenhof
Date : Fri Apr 22 2005 11:45 am

Giancarlo Niccolai wrote:
> David Butenhof wrote:
> 
> 
>>That is, they may intend that you disable
>>cancelability around any area of code that might do this; and while you
>>can argue which makes most sense there's no objective authority to say
>>they're wrong.
> 
> Well, I never stated about correctness of one or the other possible
> solution. I just stated that as long as cout uses write(), and unless c++
> makes a future statement about the contrary, cout will be a cancellation
> point. Assuming that cout absolutely uses write() is arbitrary, and
> probably should not be taken for granted; what I meant is just that IF cout
> uses write(), and if C++ standard doesn't specify for the contrary, then
> cout will be a cancellation point.

That's only true if the C++ library in question doesn't disable 
cancellation around the write() call.

Yes, you're right that if someone naively implements cout with no 
consideration at all of threads, it'll likely be a cancellation point. 
(Though not a guaranteed cancellation point, even so, since not every 
cout method will call write()... most streams are buffered.)

MY point though was that you cannot assume that because cout (probably, 
sometimes/eventually) calls write() that is IS a cancellation point. It 
would be perfectly reasonable for a C++ library implementer to be aware 
of threads and to decide that cout operations should NOT be cancellation 
points, and to code accordingly. Neither POSIX nor ISO C++ says whether 
this is good or bad: it is entirely up to the judgement of the implementer.

So unless you're coding for a specific implementation and you KNOW which 
decision the implementer made, your best strategy is to avoid making any 
assumptions.

-- 
Dave Butenhof, David.Butenhof@hp.com
HP Utility Pricing software, POSIX thread consultant
Manageability Solutions Lab (MSL), Hewlett-Packard Company
110 Spit Brook Road, ZK2/3-Q18, Nashua, NH 03062

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