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(), or other such call to wait for it to be possible to
       read packets without blocking, if such a descriptor exists, or PCAP_ERROR, 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 PCAP_ERROR 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
       PCAP_ERROR, 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, PCAP_ERROR is returned.

SEE ALSO

       pcap(3PCAP), kqueue(2)

                                                  25 July 2018                     PCAP_GET_SELECTABLE_FD(3PCAP)