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)