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153 lines
4.7 KiB
153 lines
4.7 KiB
4 months ago
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.\" Copyright (c) 1994, 1996, 1997
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that: (1) source code distributions
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.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" distributions including binary code include the above copyright notice and
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.\" this paragraph in its entirety in the documentation or other materials
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.\" provided with the distribution, and (3) all advertising materials mentioning
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.\" features or use of this software display the following acknowledgement:
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.\" ``This product includes software developed by the University of California,
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.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
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.\" the University nor the names of its contributors may be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.TH PCAP_GET_SELECTABLE_FD 3PCAP "29 January 2020"
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.SH NAME
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pcap_get_selectable_fd \- get a file descriptor on which a select() can
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be done for a live capture
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.SH SYNOPSIS
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.nf
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.ft B
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#include <pcap/pcap.h>
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.ft
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.LP
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.ft B
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int pcap_get_selectable_fd(pcap_t *p);
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.ft
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.fi
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.SH DESCRIPTION
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.BR pcap_get_selectable_fd ()
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returns, on UNIX, a file descriptor number for a file descriptor on
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which one can
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do a
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.BR select (2),
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.BR poll (2),
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.BR epoll_wait (2),
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.BR kevent (2),
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or other such call
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to wait for it to be possible to read packets without blocking, if such
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a descriptor exists, or
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.BR \-1 ,
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if no such descriptor exists.
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.PP
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Some network devices opened with
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.BR pcap_create (3PCAP)
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and
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.BR pcap_activate (3PCAP),
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or with
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.BR pcap_open_live (3PCAP),
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do not support those calls (for example, regular network devices on
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FreeBSD 4.3 and 4.4, and Endace DAG devices), so
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.B \-1
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is returned for
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those devices. In that case, those calls must be given a timeout less
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than or equal to the timeout returned by
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.BR pcap_get_required_select_timeout (3PCAP)
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for the device for which
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.BR pcap_get_selectable_fd ()
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returned
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.BR \-1 ,
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the device must be put in non-blocking mode with a call to
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.BR \%pcap_setnonblock (3PCAP),
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and an attempt must always be made to read packets from the device
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when the call returns. If
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.BR \%pcap_get_required_select_timeout ()
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returns
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.BR NULL ,
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it is not possible to wait for packets to arrive on the device in an
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event loop.
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.PP
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Note that a device on which a read can be done without blocking may,
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on some platforms, not have any packets to read if the packet buffer
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timeout has expired. A call to
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.BR pcap_dispatch (3PCAP)
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or
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.BR pcap_next_ex (3PCAP)
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will return 0 in this case, but will not block.
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.PP
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Note that in:
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.IP
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FreeBSD prior to FreeBSD 4.6;
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.IP
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NetBSD prior to NetBSD 3.0;
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.IP
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OpenBSD prior to OpenBSD 2.4;
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.IP
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Mac OS X prior to Mac OS X 10.7;
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.PP
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.BR select (),
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.BR poll (),
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and
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.BR kevent ()
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do not work correctly on BPF devices;
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.BR pcap_get_selectable_fd ()
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will return a file descriptor on most of those versions (the exceptions
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being FreeBSD 4.3 and 4.4), but a simple
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.BR select (),
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.BR poll (),
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or
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.BR kevent ()
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call will not indicate that the descriptor is readable until a full
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buffer's worth of packets is received, even if the packet timeout
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expires before then. To work around this, code that uses
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those calls to wait for packets to arrive must put the
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.B pcap_t
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in non-blocking mode, and must arrange that the call
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have a timeout less than or equal to the packet buffer timeout,
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and must try to read packets after that timeout expires, regardless of
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whether the call indicated that the file descriptor for the
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.B pcap_t
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is ready to be read or not. (That workaround will not work in FreeBSD
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4.3 and later; however, in FreeBSD 4.6 and later, those calls
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work correctly on BPF devices, so the workaround isn't necessary,
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although it does no harm.)
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.PP
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Note also that
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.BR poll ()
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and
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.BR kevent ()
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doesn't work on character special files, including BPF devices, in Mac
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OS X 10.4 and 10.5, so, while
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.BR select ()
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can be used on the descriptor returned by
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.BR pcap_get_selectable_fd (),
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.BR poll ()
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and
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.BR kevent ()
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cannot be used on it those versions of Mac OS X.
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.BR poll (),
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but not
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.BR kevent (),
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works on that descriptor in Mac OS X releases prior to
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10.4;
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.BR poll ()
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and
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.BR kevent ()
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work on that descriptor in Mac OS X 10.6 and later.
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.PP
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.BR pcap_get_selectable_fd ()
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is not available on Windows.
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.SH RETURN VALUE
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A selectable file descriptor is returned if one exists; otherwise,
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.B \-1
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is returned.
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.SH SEE ALSO
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.BR pcap (3PCAP),
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.BR kqueue (2)
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