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PCAP_GET_SELECTABLE_FD(3PCAP)                                                  PCAP_GET_SELECTABLE_FD(3PCAP)



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() 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 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() 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), but a simple
       select() or poll() will not indicate that the descriptor is readable until a full buffer's  worth  of
       packets  is received, even if the read timeout expires before then.  To work around this, an applica-tion application
       tion 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 work-around workaround
       around 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(3PCAP), select(2), poll(2)



                                                5 April 2008                   PCAP_GET_SELECTABLE_FD(3PCAP)