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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SEE ALSO | COLOPHON |
PCAP_GET_SELECTABLE_FD(3PCAP) PCAP_GET_SELECTABLE_FD(3PCAP)
pcap_get_selectable_fd - get a file descriptor on which a select()
can be done for a live capture
#include <pcap/pcap.h>
int pcap_get_selectable_fd(pcap_t *p);
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 packet
buffer 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 packet 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
packet buffer 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.
A selectable file descriptor is returned if one exists; otherwise, -1
is returned.
pcap(3PCAP), select(2), poll(2)
This page is part of the libpcap (packet capture library) project.
Information about the project can be found at
⟨http://www.tcpdump.org/⟩. If you have a bug report for this manual
page, see ⟨http://www.tcpdump.org/#patches⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2017-07-05. If
you discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for the
page, or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
20 January 2017 PCAP_GET_SELECTABLE_FD(3PCAP)