pcap_get_selectable_fd(3) - NetBSD Manual Pages

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PCAP_GET_SELECTABLE_FD(3)                            PCAP_GET_SELECTABLE_FD(3)




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(), poll(), epoll_wait(),
       kevent(), 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() and pcap_activate(),  or
       with pcap_open_live(), 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()    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(), 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 time-
       out has expired.  A call  to  pcap_dispatch()  or  pcap_next_ex()  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  descrip-
       tor  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(3),   pcap_get_required_select_timeout(3),    pcap_setnonblock(3),
       select(2), poll(2), epoll_wait(2), kqueue(2)



                                20 January 2018      PCAP_GET_SELECTABLE_FD(3)