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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SEE ALSO | COLOPHON |
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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(2),
poll(2), epoll_wait(2), kevent(2), 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(3PCAP) and
pcap_activate(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 Endace 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
attempt 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 timeout 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 select(), 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 packets after that
timeout expires, regardless of whether the call 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, 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 select() can be used on the descriptor returned by
pcap_get_selectable_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 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), kqueue(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 2025-08-11.
(At that time, the date of the most recent commit that was found
in the repository was 2025-08-10.) 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
29 January 2020PCAP_GET_SELECTABLE_FD(3PCAP)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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