Re: [Libevent-users] Q: Exception callbacks and events that should run immediately

[Alexander Kolbasov <akolb@xxxxxxxxxxx>]

On Dec 5, 2012, at 2:15 PM, Mark Ellzey wrote:

> On Wed, Dec 05, 2012 at 07:28:25PM +0000, Alexander Kolbasov wrote:
>> On Dec 5, 2012, at 7:32 AM, Mark Ellzey wrote:
>> 
>>> On Wed, Dec 05, 2012 at 11:10:04AM +0200, Nir Soffer wrote:
>>>> On Wed, Dec 5, 2012 at 10:54 AM, Sashan Govender <sashang@xxxxxxxxx> wrote:
>>>> 
>>>>> Hi!
>>>>> 
>>>>> 
>>>>>> 1) I don't see any notion of event notification on FD exception - e.g.
>>>>>> socket close.
>>>>>>   There are some words for buffer events, but nothing for regular file
>>>>>>   descriptor based callbacks. So what is a good way to get a callback
>>>>>> when specific
>>>>>>   FD is closed/disconnected?
>>>>>> 
>>>>> 
>>>>> If the fd is a socket and the peer closes then you'll get a read callback.
>>>>> When you read from the fd 'read' will return 0 indicating that the peer
>>>>> closed.
>>>>> 
>>>> 
>>>> But if the peer die without closing the socket, you will never get a read
>>>> event. In this case you will fail when writing to the socket.
>>>> 
>>>> You can simulate this by disconnecting the peer from the network.
>>> 
>>> Writing to the socket, in most cases, not something you want to do just
>>> to determine if the socket is alive.
>>> 
>>> If a peer is disconnected, e.g., it shuts down, the OS *should* 
>>> eventually time-out the session which will result in your eventcb being
>>> executed. 
>>> 
>>> Another option is to set your own read timeout on the socket.
>> 
>> Does it mean that peer close is not triggering EV_READ callback and the user should
>> periodically timeout and attempt reads and check for read errors?
>> 
> 
> Depends on what you mean by peer-closed. I think in this thread we were
> speaking about the situation where a client never sends a RST/FIN, i.e.,
> the client host is shut down during transmission.
> 
> If you set a read timeout via libevent or the OS times the connection
> out, the event callback, not the read callback is executed.

Ok, let me clarify.

- If the peer does regular close and sends RST/FIN, we will get EV_READ event and actual read() will return -1
- If the peer just terminates and we only specify EV_READ, we would get nothing.
- If the peer just terminates and we specify EV_READ | EV_TIMEOUT and specify a timeout, we will get a callback with EV_TIMEOUT set.

Is this correct?

- Alex***********************************************************************
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