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(), poll(), 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() 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 a descriptor on which  a  read  can  be  done  without  blocking  may,  on  some
       platforms,  not  have  any  packets  to  read  if the read timeout has expired.  A call to
       pcap_dispatch() 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() 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 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(3PCAP), select(2), poll(2)

                                           22 July 2011             PCAP_GET_SELECTABLE_FD(3PCAP)