Re: [boost] Boost sprint: #3407 boost::call_once not re-entrant (atleast in win32)

Subject: Re: [boost] Boost sprint: #3407 boost::call_once not re-entrant (atleast in win32)
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2009-11-23 10:03:10

On Mon, Nov 23, 2009 at 9:45 AM, Francois Barel <frabar666_at_[hidden]> wrote:
> Anthony Williams wrote:
>> The current implementation handles exceptions "properly" in the sense
>> that if the once function exits with an exception then it is not marked
>> as called and another thread (possibly one of those waiting) will call
>> it.
>
> Does this mean the function will be called again? Isn't that
> disturbing for something named "call_once"?
>

I think the spec of the original POSIX pthread_once was that it was a
thread 'cancellation point', and they decided that the function would
be considered 'uncalled' on cancellation.
For C++ the analog would be exceptions.

But I agree, it doesn't match the name.  In our case, if the function
threw, it must have been *called*.  Maybe it didn't 'complete'
(depending on your definition of complete) but it was called.  At
times in the the past, I've toyed with having two functions -
call_once() and complete_once() (implemented by a common function
internally).  But then complete_once() would imply recursion also, so
I decided to separate the concepts.

IMO, the big advantage of re-calling on exception is that the same
exception would likely be thrown in each thread.  Which is hard to do
otherwise.

Tony