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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 <&lt;pcap/pcap.h>&gt;

       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() or poll() 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 on most versions of most BSDs (including Mac OS X) 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), a simple select()
       or poll() will not return even after the read timeout expires.  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 read 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.

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

SEE ALSO
       pcap(3), select(2), poll(2)



                                 5 April 2008        PCAP_GET_SELECTABLE_FD(3)