pcap_get_selectable_fd(3pcap) — Linux manual page



NAME         top

       pcap_get_selectable_fd  -  get  a file descriptor on which a select()
       can be done for a live capture

SYNOPSIS         top

       #include <pcap/pcap.h>

       int pcap_get_selectable_fd(pcap_t *p);

DESCRIPTION         top

       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.

RETURN VALUE         top

       A selectable file descriptor is returned if one exists; otherwise, -1
       is returned.

SEE ALSO         top

       pcap(3PCAP), kqueue(2)

COLOPHON         top

       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 2020-06-09.
       (At that time, the date of the most recent commit that was found in
       the repository was 2020-06-07.)  If you discover any rendering prob‐
       lems in this HTML version of the page, or you believe there is a bet‐
       ter 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 2020 PCAP_GET_SELECTABLE_FD(3PCAP)