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(), 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 select() or poll() (for example, regu-
       lar network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so
       -1 is returned for those devices.

       Note  that  a  descriptor  on which a read can be done without blocking
       may, on some platforms, not have any packets to read if the read  time-
       out has expired.  A call to pcap_dispatch() 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()  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,  includ-
       ing  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(3), select(2), poll(2)



                                18 October 2014      PCAP_GET_SELECTABLE_FD(3)