PCAP_GET_SELECTABLE_FD(3)PCAP_GET_SELECTABLE_FD(3)NAMEpcap_get_selectable_fd - get a file descriptor on which a select() can
be done for a live capture
SYNOPSIS
#include <pcap/pcap.h>
int pcap_get_selectable_fd(pcap_t *p);
DESCRIPTIONpcap_get_selectable_fd() returns, on UNIX, a file descriptor number for
a file descriptor on which one can do a select() or poll() to wait for
it to be possible to read packets without blocking, if such a descrip‐
tor exists, or -1, if no such descriptor exists. Some network devices
opened with pcap_create() and pcap_activate(), or with
pcap_open_live(), do not support select() or poll() (for example, regu‐
lar network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so
-1 is returned for those devices.
Note that on most versions of most BSDs (including Mac OS X) select()
and poll() do not work correctly on BPF devices;
pcap_get_selectable_fd() will return a file descriptor on most of those
versions (the exceptions being FreeBSD 4.3 and 4.4), a simple select()
or poll() will not return even after the read timeout expires. To work
around this, an application that uses select() or poll() to wait for
packets to arrive must put the pcap_t in non-blocking mode, and must
arrange that the select() or poll() have a timeout less than or equal
to the read timeout, and must try to read packets after that timeout
expires, regardless of whether select() or poll() indicated that the
file descriptor for the pcap_t is ready to be read or not. (That work‐
around will not work in FreeBSD 4.3 and later; however, in FreeBSD 4.6
and later, select() and poll() work correctly on BPF devices, so the
workaround isn't necessary, although it does no harm.)
Note also that poll() doesn't work on character special files, includ‐
ing BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be
used on the descriptor returned by pcap_get_selectable_fd(), poll()
cannot be used on it those versions of Mac OS X. Kqueues also don't
work on that descriptor. poll(), but not kqueues, work on that
descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work
on that descriptor in Mac OS X 10.6 and later.
pcap_get_selectable_fd() is not available on Windows.
RETURN VALUE
A selectable file descriptor is returned if one exists; otherwise, -1
is returned.
SEE ALSOpcap(3), select(2), poll(2)
5 April 2008 PCAP_GET_SELECTABLE_FD(3)
