Re: pcap_read_linux_mmap is always blocking

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

On Jul 6, 2009, at 9:39 AM, Dragos Ilie wrote:

> I traced the issue to pcap-linux.c:pcap_read_linux_mmap(), which is  
> the
> read handler selected when HAVE_PACKET_RING is defined. The handler
> calls poll() with a negative value, which means an infinite timeout.
> This occurs when the libpcap variable md.timeout is equal to zero
> (default value). If the caller specifies a positive md.timeout then
> pcap_setnonblock_mmap() re-computes the timeout as shown below:
>
> p->md.timeout = p->md.timeout*-1 - 1;
>
> When the user-specified timeout is negative the expression above  
> becomes:
>
> p->md.timeout = (p->md.timeout+1)*-1;

The user-specified timeout isn't supposed to be negative; the behavior  
when that is done has never been specified.  If your application is  
setting the timeout to a negative value, it should stop doing so.

Internally, libpcap uses a negative timeout value in the pcap_t data  
structure to indicate non-blocking mode; if it's negative, poll()  
isn't called.  If, when pcap_setnonblock() is called, the p- 
>md.timeout is:

        zero, p->md.timeout becomes -1;

        1 through INT_MAX, p->md.timeout becomes -2 through -INT_MAX-1;

and that should be negative in all cases, so poll() shouldn't be called.
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