PCAP_GET_SELECTABLE_FD(3PCAP) PCAP_GET_SELECTABLE_FD(3PCAP)
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(2), poll(2),
epoll_wait(2), kevent(2), or other such call to wait for it to be pos-
sible to read packets without blocking, if such a descriptor exists, or
-1, if no such descriptor exists.
Some network devices opened with pcap_create(3PCAP) and pcap_acti-
vate(3PCAP), or with pcap_open_live(3PCAP), do not support those calls
(for example, regular network devices on FreeBSD 4.3 and 4.4, and En-
dace DAG devices), so -1 is returned for those devices. In that case,
those calls must be given a timeout less than or equal to the timeout
returned by pcap_get_required_select_timeout(3PCAP) for the device for
which pcap_get_selectable_fd() returned -1, the device must be put in
non-blocking mode with a call to pcap_setnonblock(3PCAP), and an at-
tempt must always be made to read packets from the device when the call
returns. If pcap_get_required_select_timeout() returns NULL, it is not
possible to wait for packets to arrive on the device in an event loop.
Note that a device on which a read can be done without blocking may, on
some platforms, not have any packets to read if the packet buffer time-
out has expired. A call to pcap_dispatch(3PCAP) or pcap_next_ex(3PCAP)
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(), poll(), and kevent() 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 se-
lect(), poll(), or kevent() call will not indicate that the descriptor
is readable until a full buffer's worth of packets is received, even if
the packet timeout expires before then. To work around this, code that
uses those calls to wait for packets to arrive must put the pcap_t in
non-blocking mode, and must arrange that the call have a timeout less
than or equal to the packet buffer timeout, and must try to read pack-
ets after that timeout expires, regardless of whether the call indi-
cated 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, those calls work correctly on BPF devices, so
the workaround isn't necessary, although it does no harm.)
Note also that poll() and kevent() doesn't work on character special
files, including BPF devices, in Mac OS X 10.4 and 10.5, so, while se-
lect() can be used on the descriptor returned by pcap_get_se-
lectable_fd(), poll() and kevent() cannot be used on it those versions
of Mac OS X. poll(), but not kevent(), works on that descriptor in Mac
OS X releases prior to 10.4; poll() and kevent() work on that descrip-
tor 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), kqueue(2)
29 January 2020 PCAP_GET_SELECTABLE_FD(3PCAP)
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