Re: select() on BPF devices and Mac OS X

[Guy Harris <guy () alum mit edu>]

Eloy A. Paris wrote:

> So, according to a comment from Guy in the CVS log for pcap-bpf.c,
> select() should work on BPF devices on Tiger.

It works as well as it does on older versions of other BSDs.

This means that you can do a select() on it, and, if the BPF buffer 
fills up, the select() will return.

However, if the timer expires, the select() won't return.

The comment in the CVS log refers to something else - poll() in Panther 
is just a wrapper around select(), and works on character special files 
such as BPF devices, but poll() in Tiger is implemented using the same 
kernel infrastructure as kqueues, and neither kqueues nor poll() work on 
character special files.  I'd originally had the impression that 
select() was broken on character special files as well, but it turned 
out it wasn't.

The workaround for the select() problem on OS X - and on older versions 
of other BSDs (I *think* it might be fixed on the current versions of 
{Free,Net,Open,DragonFly}BSD), is to:

        1) put the BPF device into non-blocking mode;

	2) use the timeout passed to pcap_open_live() as the timeout for the 
select();

	3) try to read from the BPF device if select() says it's readable *or* 
if the timeout has expired.

(There are bugs filed against OS X for select() not returning when the 
BPF timout expires and for poll() and kqueues not working on character 
special files.)
-
This is the tcpdump-workers list.
Visit https://lists.sandelman.ca/ to unsubscribe.