pcap_loop.3pcap - platform/external/libpcap - Git at Google

 .\" Copyright (c) 1994, 1996, 1997
 .\"	The Regents of the University of California.  All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that: (1) source code distributions
 .\" retain the above copyright notice and this paragraph in its entirety, (2)
 .\" distributions including binary code include the above copyright notice and
 .\" this paragraph in its entirety in the documentation or other materials
 .\" provided with the distribution, and (3) all advertising materials mentioning
 .\" features or use of this software display the following acknowledgement:
 .\" ``This product includes software developed by the University of California,
 .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 .\" the University nor the names of its contributors may be used to endorse
 .\" or promote products derived from this software without specific prior
 .\" written permission.
 .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 .\"
 .TH PCAP_LOOP 3PCAP "5 March 2022"
 .SH NAME
 pcap_loop, pcap_dispatch \- process packets from a live capture or savefile
 .SH SYNOPSIS
 .nf
 .ft B
 #include <pcap/pcap.h>
 .ft
 .LP
 .ft B
 typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h,
 .ti +8
 			     const u_char *bytes);
 .ft
 .LP
 .ft B
 int pcap_loop(pcap_t *p, int cnt,
 .ti +8
 pcap_handler callback, u_char *user);
 int pcap_dispatch(pcap_t *p, int cnt,
 .ti +8
 pcap_handler callback, u_char *user);
 .ft
 .fi
 .SH DESCRIPTION
 .BR pcap_loop ()
 processes packets from a live capture or ``savefile'' until
 .I cnt
 packets are processed, the end of the ``savefile'' is
 reached when reading from a ``savefile'',
 .BR pcap_breakloop (3PCAP)
 is called, or an error occurs.
 It does
 .B not
 return when live packet buffer timeouts occur.
 A value of
 .B \-1
 or
 .B 0
 for
 .I cnt
 is equivalent to infinity, so that packets are processed until another
 ending condition occurs.
 .PP
 .BR pcap_dispatch ()
 processes packets from a live capture or ``savefile'' until
 .I cnt
 packets are processed, the end of the current bufferful of packets is
 reached when doing a live capture, the end of the ``savefile'' is
 reached when reading from a ``savefile'',
 .BR pcap_breakloop ()
 is called, or an error occurs.
 Thus, when doing a live capture,
 .I cnt
 is the maximum number of packets to process before returning, but is not
 a minimum number; when reading a live capture, only one
 bufferful of packets is read at a time, so fewer than
 .I cnt
 packets may be processed. A value of
 .B \-1
 or
 .B 0
 for
 .I cnt
 causes all the packets received in one buffer to be processed when
 reading a live capture, and causes all the packets in the file to be
 processed when reading a ``savefile''.
 .PP
 Note that, when doing a live capture on some platforms, if the read
 timeout expires when there are no packets available,
 .BR pcap_dispatch ()
 will return 0, even when not in non-blocking mode, as there are no
 packets to process.  Applications should be prepared for this to happen,
 but must not rely on it happening.
 .PP
 .I callback
 specifies a
 .I pcap_handler
 routine to be called with three arguments:
 a
 .I u_char
 pointer which is passed in the
 .I user
 argument to
 .BR pcap_loop ()
 or
 .BR pcap_dispatch (),
 a
 .I const struct pcap_pkthdr
 pointer pointing to the packet time stamp and lengths, and a
 .I const u_char
 pointer to the first
 .B caplen
 (as given in the
 .I struct pcap_pkthdr
 a pointer to which is passed to the callback routine)
 bytes of data from the packet.  The
 .I struct pcap_pkthdr
 and the packet data are not to be freed by the callback routine, and are
 not guaranteed to be valid after the callback routine returns; if the
 code needs them to be valid after the callback, it must make a copy of
 them.
 .PP
 The bytes of data from the packet begin with a link-layer header.  The
 format of the link-layer header is indicated by the return value of the
 .BR pcap_datalink (3PCAP)
 routine when handed the
 .B pcap_t
 value also passed to
 .BR pcap_loop ()
 or
 .BR pcap_dispatch ().
 .I https://www.tcpdump.org/linktypes.html
 lists the values
 .BR pcap_datalink ()
 can return and describes the packet formats that
 correspond to those values.  The value it returns will be valid for all
 packets received unless and until
 .BR pcap_set_datalink (3PCAP)
 is called; after a successful call to
 .BR pcap_set_datalink (),
 all subsequent packets will have a link-layer header of the type
 specified by the link-layer header type value passed to
 .BR pcap_set_datalink ().
 .PP
 Do
 .B NOT
 assume that the packets for a given capture or ``savefile`` will have
 any given link-layer header type, such as
 .B DLT_EN10MB
 for Ethernet.  For example, the "any" device on Linux will have a
 link-layer header type of
 .B DLT_LINUX_SLL
 or
 .B DLT_LINUX_SLL2
 even if all devices on the system at the time the "any" device is opened
 have some other data link type, such as
 .B DLT_EN10MB
 for Ethernet.
 .SH RETURN VALUE
 .BR pcap_loop ()
 returns
 .B 0
 if
 .I cnt
 is exhausted or if, when reading from a ``savefile'', no more packets
 are available.  It returns
 .B PCAP_ERROR_BREAK
 if the loop terminated due to a call to
 .BR pcap_breakloop ()
 before any packets were processed,
 .B PCAP_ERROR_NOT_ACTIVATED
 if called on a capture handle that has been created but not activated,
 or
 .B PCAP_ERROR
 if another error occurs.
 It does
 .B not
 return when live packet buffer timeouts occur; instead, it attempts to
 read more packets.
 .PP
 .BR pcap_dispatch ()
 returns the number of packets processed on success; this can be 0 if no
 packets were read from a live capture (if, for example, they were
 discarded because they didn't pass the packet filter, or if, on
 platforms that support a packet buffer timeout that starts before any
 packets arrive, the timeout expires before any packets arrive, or if the
 file descriptor for the capture device is in non-blocking mode and no
 packets were available to be read) or if no more packets are available
 in a ``savefile.'' It returns
 .B PCAP_ERROR_BREAK
 if the loop terminated due to a call to
 .BR pcap_breakloop ()
 before any packets were processed,
 .B PCAP_ERROR_NOT_ACTIVATED
 if called on a capture handle that has been created but not activated,
 or
 .B PCAP_ERROR
 if another error occurs.
 .ft B
 If your application uses pcap_breakloop(),
 make sure that you explicitly check for PCAP_ERROR and PCAP_ERROR_BREAK,
 rather than just checking for a return value < 0.
 .ft R
 .PP
 If
 .B PCAP_ERROR
 is returned,
 .BR pcap_geterr (3PCAP)
 or
 .BR pcap_perror (3PCAP)
 may be called with
 .I p
 as an argument to fetch or display the error text.
 .SH BACKWARD COMPATIBILITY
 .PP
 In libpcap versions before 1.5.0, the behavior when
 .I cnt
 was
 .B 0
 was undefined; different platforms and devices behaved differently,
 so code that must work with these versions of libpcap should use
 .BR \-1 ,
 not
 .BR 0 ,
 as the value of
 .IR cnt .
 .SH SEE ALSO
 .BR pcap (3PCAP)
	.\" Copyright (c) 1994, 1996, 1997
	.\" The Regents of the University of California. All rights reserved.
	.\"
	.\" Redistribution and use in source and binary forms, with or without
	.\" modification, are permitted provided that: (1) source code distributions
	.\" retain the above copyright notice and this paragraph in its entirety, (2)
	.\" distributions including binary code include the above copyright notice and
	.\" this paragraph in its entirety in the documentation or other materials
	.\" provided with the distribution, and (3) all advertising materials mentioning
	.\" features or use of this software display the following acknowledgement:
	.\" ``This product includes software developed by the University of California,
	.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
	.\" the University nor the names of its contributors may be used to endorse
	.\" or promote products derived from this software without specific prior
	.\" written permission.
	.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
	.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
	.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
	.\"
	.TH PCAP_LOOP 3PCAP "5 March 2022"
	.SH NAME
	pcap_loop, pcap_dispatch \- process packets from a live capture or savefile
	.SH SYNOPSIS
	.nf
	.ft B
	#include <pcap/pcap.h>
	.ft
	.LP
	.ft B
	typedef void (pcap_handler)(u_char user, const struct pcap_pkthdr *h,
	.ti +8
	const u_char *bytes);
	.ft
	.LP
	.ft B
	int pcap_loop(pcap_t *p, int cnt,
	.ti +8
	pcap_handler callback, u_char *user);
	int pcap_dispatch(pcap_t *p, int cnt,
	.ti +8
	pcap_handler callback, u_char *user);
	.ft
	.fi
	.SH DESCRIPTION
	.BR pcap_loop ()
	processes packets from a live capture or ``savefile'' until
	.I cnt
	packets are processed, the end of the ``savefile'' is
	reached when reading from a ``savefile'',
	.BR pcap_breakloop (3PCAP)
	is called, or an error occurs.
	It does
	.B not
	return when live packet buffer timeouts occur.
	A value of
	.B \-1
	or
	.B 0
	for
	.I cnt
	is equivalent to infinity, so that packets are processed until another
	ending condition occurs.
	.PP
	.BR pcap_dispatch ()
	processes packets from a live capture or ``savefile'' until
	.I cnt
	packets are processed, the end of the current bufferful of packets is
	reached when doing a live capture, the end of the ``savefile'' is
	reached when reading from a ``savefile'',
	.BR pcap_breakloop ()
	is called, or an error occurs.
	Thus, when doing a live capture,
	.I cnt
	is the maximum number of packets to process before returning, but is not
	a minimum number; when reading a live capture, only one
	bufferful of packets is read at a time, so fewer than
	.I cnt
	packets may be processed. A value of
	.B \-1
	or
	.B 0
	for
	.I cnt
	causes all the packets received in one buffer to be processed when
	reading a live capture, and causes all the packets in the file to be
	processed when reading a ``savefile''.
	.PP
	Note that, when doing a live capture on some platforms, if the read
	timeout expires when there are no packets available,
	.BR pcap_dispatch ()
	will return 0, even when not in non-blocking mode, as there are no
	packets to process. Applications should be prepared for this to happen,
	but must not rely on it happening.
	.PP
	.I callback
	specifies a
	.I pcap_handler
	routine to be called with three arguments:
	a
	.I u_char
	pointer which is passed in the
	.I user
	argument to
	.BR pcap_loop ()
	or
	.BR pcap_dispatch (),
	a
	.I const struct pcap_pkthdr
	pointer pointing to the packet time stamp and lengths, and a
	.I const u_char
	pointer to the first
	.B caplen
	(as given in the
	.I struct pcap_pkthdr
	a pointer to which is passed to the callback routine)
	bytes of data from the packet. The
	.I struct pcap_pkthdr
	and the packet data are not to be freed by the callback routine, and are
	not guaranteed to be valid after the callback routine returns; if the
	code needs them to be valid after the callback, it must make a copy of
	them.
	.PP
	The bytes of data from the packet begin with a link-layer header. The
	format of the link-layer header is indicated by the return value of the
	.BR pcap_datalink (3PCAP)
	routine when handed the
	.B pcap_t
	value also passed to
	.BR pcap_loop ()
	or
	.BR pcap_dispatch ().
	.I https://www.tcpdump.org/linktypes.html
	lists the values
	.BR pcap_datalink ()
	can return and describes the packet formats that
	correspond to those values. The value it returns will be valid for all
	packets received unless and until
	.BR pcap_set_datalink (3PCAP)
	is called; after a successful call to
	.BR pcap_set_datalink (),
	all subsequent packets will have a link-layer header of the type
	specified by the link-layer header type value passed to
	.BR pcap_set_datalink ().
	.PP
	Do
	.B NOT
	assume that the packets for a given capture or ``savefile`` will have
	any given link-layer header type, such as
	.B DLT_EN10MB
	for Ethernet. For example, the "any" device on Linux will have a
	link-layer header type of
	.B DLT_LINUX_SLL
	or
	.B DLT_LINUX_SLL2
	even if all devices on the system at the time the "any" device is opened
	have some other data link type, such as
	.B DLT_EN10MB
	for Ethernet.
	.SH RETURN VALUE
	.BR pcap_loop ()
	returns
	.B 0
	if
	.I cnt
	is exhausted or if, when reading from a ``savefile'', no more packets
	are available. It returns
	.B PCAP_ERROR_BREAK
	if the loop terminated due to a call to
	.BR pcap_breakloop ()
	before any packets were processed,
	.B PCAP_ERROR_NOT_ACTIVATED
	if called on a capture handle that has been created but not activated,
	or
	.B PCAP_ERROR
	if another error occurs.
	It does
	.B not
	return when live packet buffer timeouts occur; instead, it attempts to
	read more packets.
	.PP
	.BR pcap_dispatch ()
	returns the number of packets processed on success; this can be 0 if no
	packets were read from a live capture (if, for example, they were
	discarded because they didn't pass the packet filter, or if, on
	platforms that support a packet buffer timeout that starts before any
	packets arrive, the timeout expires before any packets arrive, or if the
	file descriptor for the capture device is in non-blocking mode and no
	packets were available to be read) or if no more packets are available
	in a ``savefile.'' It returns
	.B PCAP_ERROR_BREAK
	if the loop terminated due to a call to
	.BR pcap_breakloop ()
	before any packets were processed,
	.B PCAP_ERROR_NOT_ACTIVATED
	if called on a capture handle that has been created but not activated,
	or
	.B PCAP_ERROR
	if another error occurs.
	.ft B
	If your application uses pcap_breakloop(),
	make sure that you explicitly check for PCAP_ERROR and PCAP_ERROR_BREAK,
	rather than just checking for a return value < 0.
	.ft R
	.PP
	If
	.B PCAP_ERROR
	is returned,
	.BR pcap_geterr (3PCAP)
	or
	.BR pcap_perror (3PCAP)
	may be called with
	.I p
	as an argument to fetch or display the error text.
	.SH BACKWARD COMPATIBILITY
	.PP
	In libpcap versions before 1.5.0, the behavior when
	.I cnt
	was
	.B 0
	was undefined; different platforms and devices behaved differently,
	so code that must work with these versions of libpcap should use
	.BR \-1 ,
	not
	.BR 0 ,
	as the value of
	.IR cnt .
	.SH SEE ALSO
	.BR pcap (3PCAP)