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  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)

                                                  5 April 2008                     PCAP_GET_SELECTABLE_FD(3PCAP)