207 lines
6.6 KiB
Plaintext
207 lines
6.6 KiB
Plaintext
.\" 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_LOOP 3PCAP "25 July 2018"
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.SH NAME
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pcap_loop, pcap_dispatch \- process packets from a live capture or savefile
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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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typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h,
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.ti +8
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const u_char *bytes);
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.ft
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.LP
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.ft B
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int pcap_loop(pcap_t *p, int cnt,
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.ti +8
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pcap_handler callback, u_char *user);
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int pcap_dispatch(pcap_t *p, int cnt,
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.ti +8
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pcap_handler callback, u_char *user);
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.ft
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.fi
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.SH DESCRIPTION
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.B pcap_loop()
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processes packets from a live capture or ``savefile'' until
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.I cnt
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packets are processed, the end of the ``savefile'' is
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reached when reading from a ``savefile'',
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.B pcap_breakloop(3PCAP)
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is called, or an error occurs.
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It does
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.B not
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return when live packet buffer timeouts occur.
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A value of \-1 or 0 for
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.I cnt
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is equivalent to infinity, so that packets are processed until another
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ending condition occurs.
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.PP
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.B pcap_dispatch()
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processes packets from a live capture or ``savefile'' until
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.I cnt
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packets are processed, the end of the current bufferful of packets is
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reached when doing a live capture, the end of the ``savefile'' is
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reached when reading from a ``savefile'',
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.B pcap_breakloop()
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is called, or an error occurs.
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Thus, when doing a live capture,
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.I cnt
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is the maximum number of packets to process before returning, but is not
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a minimum number; when reading a live capture, only one
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bufferful of packets is read at a time, so fewer than
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.I cnt
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packets may be processed. A value of \-1 or 0 for
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.I cnt
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causes all the packets received in one buffer to be processed when
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reading a live capture, and causes all the packets in the file to be
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processed when reading a ``savefile''.
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.PP
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Note that, when doing a live capture on some platforms, if the read
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timeout expires when there are no packets available,
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.B pcap_dispatch()
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will return 0, even when not in non-blocking mode, as there are no
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packets to process. Applications should be prepared for this to happen,
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but must not rely on it happening.
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.PP
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.ft B
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(In older versions of libpcap, the behavior when
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\fIcnt\fP
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was 0 was undefined; different platforms and devices behaved
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differently, so code that must work with older versions of libpcap
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should use \-1, not 0, as the value of
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\fIcnt\fP.)
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.ft R
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.PP
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.I callback
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specifies a
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.I pcap_handler
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routine to be called with three arguments:
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a
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.I u_char
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pointer which is passed in the
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.I user
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argument to
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.B pcap_loop()
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or
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.BR pcap_dispatch() ,
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a
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.I const struct pcap_pkthdr
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pointer pointing to the packet time stamp and lengths, and a
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.I const u_char
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pointer to the first
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.B caplen
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(as given in the
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.I struct pcap_pkthdr
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a pointer to which is passed to the callback routine)
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bytes of data from the packet. The
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.I struct pcap_pkthdr
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and the packet data are not to be freed by the callback routine, and are
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not guaranteed to be valid after the callback routine returns; if the
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code needs them to be valid after the callback, it must make a copy of
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them.
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.PP
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The bytes of data from the packet begin with a link-layer header. The
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format of the link-layer header is indicated by the return value of the
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.B pcap_datalink(3PCAP)
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routine when handed the
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.B pcap_t
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value also passed to
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.B pcap_loop()
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or
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.BR pcap_dispatch() .
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.I https://www.tcpdump.org/linktypes.html
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lists the values
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.B pcap_datalink()
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can return and describes the packet formats that
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correspond to those values. The value it returns will be valid for all
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packets received unless and until
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.B pcap_set_datalink(3PCAP)
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is called; after a successful call to
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.BR pcap_set_datalink() ,
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all subsequent packets will have a link-layer header of the type
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specified by the link-layer header type value passed to
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.BR pcap_set_datalink() .
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.PP
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Do
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.B NOT
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assume that the packets for a given capture or ``savefile`` will have
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any given link-layer header type, such as
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.B DLT_EN10MB
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for Ethernet. For example, the "any" device on Linux will have a
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link-layer header type of
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.B DLT_LINUX_SLL
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even if all devices on the system at the time the "any" device is opened
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have some other data link type, such as
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.B DLT_EN10MB
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for Ethernet.
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.SH RETURN VALUE
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.B pcap_loop()
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returns 0 if
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.I cnt
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is exhausted or if, when reading from a ``savefile'', no more packets
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are available. It returns
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.B PCAP_ERROR
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if an error occurs or
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.B PCAP_ERROR_BREAK
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if the loop terminated due to a call to
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.B pcap_breakloop()
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before any packets were processed.
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It does
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.B not
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return when live packet buffer timeouts occur; instead, it attempts to
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read more packets.
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.PP
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.B pcap_dispatch()
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returns the number of packets processed on success; this can be 0 if no
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packets were read from a live capture (if, for example, they were
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discarded because they didn't pass the packet filter, or if, on
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platforms that support a packet buffer timeout that starts before any
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packets arrive, the timeout expires before any packets arrive, or if the
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file descriptor for the capture device is in non-blocking mode and no
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packets were available to be read) or if no more packets are available
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in a ``savefile.'' It returns
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.B PCAP_ERROR
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if an error occurs or
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.B PCAP_ERROR_BREAK
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if the loop terminated due to a call to
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.B pcap_breakloop()
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before any packets were processed.
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.ft B
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If your application uses pcap_breakloop(),
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make sure that you explicitly check for PCAP_ERROR and PCAP_ERROR_BREAK,
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rather than just checking for a return value < 0.
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.ft R
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.PP
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If
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.B PCAP_ERROR
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is returned,
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.B pcap_geterr(3PCAP)
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or
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.B pcap_perror(3PCAP)
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may be called with
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.I p
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as an argument to fetch or display the error text.
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.SH SEE ALSO
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pcap(3PCAP)
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