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(2), poll(2), epoll_wait(2), kevent(2), 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(3PCAP) and pcap_activate(3PCAP), or with
       pcap_open_live(3PCAP), do not support those calls (for example, regular network devices on
       FreeBSD  4.3  and  4.4,  and Endace DAG devices), so -1 is returned for those devices.  In
       that case, those calls must be given a timeout less than or equal to the timeout  returned
       by     pcap_get_required_select_timeout(3PCAP)     for     the     device     for    which
       pcap_get_selectable_fd() returned -1, the device must be put in non-blocking mode  with  a
       call  to  pcap_setnonblock(3PCAP), and an attempt must always be made to read packets from
       the device when the call returns.  If pcap_get_required_select_timeout() returns NULL,  it
       is not possible to wait for packets to arrive on the device in an event loop.

       Note  that  a  device on which a read can be done without blocking may, on some platforms,
       not have any packets to read if  the  packet  buffer  timeout  has  expired.   A  call  to
       pcap_dispatch(3PCAP)  or  pcap_next_ex(3PCAP)  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(),   poll(),   and   kevent()   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(),  poll(),  or  kevent()  call
       will  not  indicate that the descriptor is readable until a full buffer's worth of packets
       is received, even if the packet timeout expires before then.  To work  around  this,  code
       that  uses  those  calls to wait for packets to arrive must put the pcap_t in non-blocking
       mode, and must arrange that the call have a timeout less  than  or  equal  to  the  packet
       buffer  timeout,  and  must  try to read packets after that timeout expires, regardless of
       whether the call 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, those calls work correctly on BPF  devices,  so  the  workaround  isn't  necessary,
       although it does no harm.)

       Note  also that poll() and kevent() 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()  and  kevent()  cannot be used on it those
       versions of Mac OS X.  poll(), but not kevent(), works on that  descriptor  in  Mac  OS  X
       releases  prior  to 10.4; poll() and kevent() 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), kqueue(2)

                                         29 January 2020            PCAP_GET_SELECTABLE_FD(3PCAP)