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)