Wireshark · Wireshark-dev: Re: [Wireshark-dev] D-Bus support

[frederic heem <frederic.heem@xxxxxxxxx>]

Alle 10:53, giovedì 26 ottobre 2006, Guy Harris ha scritto:
> frederic heem wrote:
> > Do you mean that on Solaris, when a read blocks and no packet arrives,
> > the read hangs forever ?
>
> Yes.
What about using select() + read() instead ?
>
> The purpose of the timer on Solaris is *NOT* to ensure that reads always
> complete within a fixed period of time.  The purpose of the timer is to
> ensure that if packets are arriving slowly, a packet won't remain queued
> up and unread indefinitely.

>
> The pcap man page explicitly avoids making any claim that the timer will
> ensure that a read will complete within the timeout period.
>
> >> I.e., any code that depends on the timer preventing a read on a pcap_t
> >> from taking more than the specified amount of time is depending on
> >> something that libpcap does not and will not guarantee (and the libpcap
> >> man page explicitly indicates this).  Does your code depend on that?
> >
> > Indeed, it depends on the fact that pcap_breakloop shall return in a time
> > which ha to be predictable.  See other bug report about this subject:
> > https://sourceforge.net/tracker/index.php?func=detail&aid=1575387&group_i
> >d=53067&atid=469577
>
> The problem there is that there's no way for a pcap_breakloop() call in
> one thread to "poke" the descriptor/handle associated with a pcap_t to
> force it to return (other than to close it, but pcap_breakloop() is
> intended to leave the pcap_t usable; a reopen of the device, followed by
> a replacement of the descriptor in the pcap_t with the new descriptor,
> followed by a close of the original device, might achieve that, although
> that'd have to be done carefully to ensure it's thread-safe).
At the moment, the model is one thread per capture, each thread blocks in 
capture_loop_start. The main program enters in the dbus loop waiting for some 
request to process. When a client requires to close a capture, the server 
calls pcap_breakloop(), hence  capture_loop_start which is running in its own 
thread returns, statistics can be computed and the capture can be really 
closed.
The dbus loop thread does not call pcap_close() because the client may require 
to get statistics, indeed, when a pcap handle is closed, it is too late to 
get statistics. The other reason is to avoid calling pcap_close from 
different threads.  
With this architecture, the only thread issue is that the pcap handle variable 
break_loop is not protected. One solution is to:
* add the private function _pcap_get_breakloop(pcap_t *) that returns 
breakloop variable
* use it instead of directly poking the member.
* add a mutex  in the pcap struture
* use it in pcap_breakloop() and  _pcap_get_breakloop

>
> If you call pcap_breakloop() within a signal handler, and the signal is
> delivered to the thread reading from the descriptor, and the signal is
> set to interrupt system calls, pcap_breakloop() will work.
The dbus loop does not use UNIX signal because it's not portable to other 
platform.
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