bionic (3) pcap_get_selectable_fd.3pcap.gz

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,  regular  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 timeout 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, 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()
       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(3PCAP), select(2), poll(2)

                                                 18 October 2014                   PCAP_GET_SELECTABLE_FD(3PCAP)