PCAP_GET_SELECTABLE_FD(3)                     FreeBSD Library Functions Manual

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)