Re: [Boost-users] ASIO cancel question

Subject: Re: [Boost-users] ASIO cancel question
From: J.W.F. Thirion (derik_at_[hidden])
Date: 2009-08-06 15:22:43

Thank you so much. It makes sense from what I've observed. Three final
questions on this:

1) Let's say I've posted a request for a function, called handleClose,
onto the handler Queue from a function called close() which can be
called from a thread different from the IO service thread. The
handleClose function will call the cancel on the socket. Would it be
possible while I'm inside this handler, for ASIO to post more requests
onto the queue. E.g. let's say I've just entered my handleClose handler.
Before issuing the cancel, is it possible that ASIO could schedule a
handleRead for a successful read of a packet, or is the asynchronous
socket operations blocked by the fact that I'm inside a handler, meaning
that I can assume that if I'm inside my handleClose function, I won't
get any handlers posted by ASIO in the mean time, and thus the only
handleRead and handleWrite, etc. that I'll get will be ones with a
cancelled/aborted error (i.e. not success as error code when the
handleRead is called after my handleClose has executed) due to the
cancel called inside handleClose?

2) Does the above logic apply to timers also? In other words, I won't
get a handleTimer function posted with a success as error code for a
deadline timer while I'm inside the handleClose function? I'll need to
cancel the timer inside my handleClose function, but I'm afraid that
I'll get a handleTimer with a success instead of cancelled/aborted after
my handleClose function is done, if, while I'm inside my handleClose
function, just before the timer is cancelled, the timer expires, meaning
that even though I've cancelled the timer, I'll still get the
handleTimer called with success as error code indicating that the timer
has expired?

3) Are the events for the handlers placed on the queue always in
sequence. In other words, if I cancel a read operation and then post a
function handleDestroy, then I'll always get handleRead with
aborted/cancelled called before I get a handleDestroy called? In this
case the cancel interrupts the async read, which will post a handleRead
at the point of the cancel being called and not after the handleClose
function exits, which means that handleRead will occur before the
handleDestroy?

Many thanks for the advice!

Kind regards,
Derik

On Thu, 2009-08-06 at 18:16 +0300, Igor R wrote:
> > In ASIO, when I execute a cancel() on a tcp::socket, does it remove all
> > of the handlers associated with the socket from the handler queue?
> 
> No. Every async_XXX request must end with its handler invocation.
> 
> <...>
> >  Let's assume now that
> > the object destructs before the handleRead is called
> 
> This is very bad and will cause crash/segfault. Parts of your handler
> (incl. the bound object) must not cease before the async.operation is
> complete.
> 
> > Also, as reading and writing operations are done in the IO service thread, would it be safe to call the tcp::socket cancel from another
> thread (e.g. in the destructor of a class)?
> 
> No. You should post() the cancel operation to io_service thread.
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