NAME¶
pcap_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);
DESCRIPTION¶
pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a
file descriptor on which one can do a
select(),
poll(), or other
such call to wait for it to be possible to read packets without blocking, if
such a descriptor 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, regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG
devices), so -1 is returned for those devices.
Note that a descriptor on which a read can be done without blocking may, on some
platforms, not have any packets to read if the read timeout has expired. A
call to
pcap_dispatch() will return 0 in this case, but will not block.
Note that in:
- FreeBSD prior to FreeBSD 4.6;
- NetBSD prior to NetBSD 3.0;
- OpenBSD prior to OpenBSD 2.4;
- Mac OS X prior to Mac OS X 10.7;
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), but a simple
select() or
poll() will not indicate that the descriptor is
readable until a full buffer's worth of packets is received, even if the read
timeout expires before then. 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 workaround 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, including
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 ALSO¶
pcap(3PCAP),
select(2),
poll(2)