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NetBSD 6.1.5 - man page for pcap_get_selectable_fd (netbsd section 3)


       pcap_get_selectable_fd  - get a file descriptor on which a select() can be done for a live

       #include <pcap/pcap.h>

       int pcap_get_selectable_fd(pcap_t *p);

       pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a file  descriptor
       on  which  one  can  do a select() or poll() 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  on  most versions of most BSDs (including Mac OS X) 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), a simple select() or
       poll() will not return even after the read timeout  expires.   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.

       A selectable file descriptor is returned if one exists; otherwise, -1 is returned.

       pcap(3), select(2), poll(2)

					   5 April 2008 		PCAP_GET_SELECTABLE_FD(3)

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