Updated: 2022/Sep/29

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PCAP_GET_SELECTABLE_FD(3)  Library Functions Manual  PCAP_GET_SELECTABLE_FD(3)



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
       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(3) and pcap_activate(3),
       or with pcap_open_live(3), 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(3) 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(3), 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(3) or pcap_next_ex(3)
       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
       A selectable file descriptor is returned if one exists; otherwise, -1
       is returned.

SEE ALSO
       pcap(3), kqueue(2)



                                29 January 2020      PCAP_GET_SELECTABLE_FD(3)