pcap_loop.3pcap

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.TH PCAP_LOOP 3PCAP "20 January 2017"
.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
.B 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'',
.B pcap_breakloop()
is called, or an error occurs.
It does
.B not
return when live packet buffer timeouts occur.
A value of \-1 or 0 for
.I cnt
is equivalent to infinity, so that packets are processed until another
ending condition occurs.
.PP
.B 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'',
.B 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 \-1 or 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,
.B 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
.ft B
(In older versions of libpcap, the behavior when
\fIcnt\fP
was 0 was undefined; different platforms and devices behaved
differently, so code that must work with older versions of libpcap
should use \-1, not 0, as the value of
\fIcnt\fP.)
.ft R
.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
.B 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
.B pcap_datalink()
routine when handed the
.B pcap_t
value also passed to
.B pcap_loop()
or
.BR pcap_dispatch() .
.I https://www.tcpdump.org/linktypes.html
lists the values
.B 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
.B pcap_set_datalink()
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
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
.B pcap_loop()
returns 0 if
.I cnt
is exhausted or if, when reading from a ``savefile'', no more packets
are available.  It returns \-1 if an error occurs or \-2 if the loop
terminated due to a call to
.B pcap_breakloop()
before any packets were processed.
It does
.B not
return when live packet buffer timeouts occur; instead, it attempts to
read more packets.
.PP
.B 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 \-1 if an error occurs or \-2 if the loop
terminated due to a call to
.B pcap_breakloop()
before any packets were processed.
.ft B
If your application uses pcap_breakloop(),
make sure that you explicitly check for \-1 and \-2, rather than just
checking for a return value < 0.
.ft R
.PP
If \-1 is returned,
.B pcap_geterr()
or
.B pcap_perror()
may be called with
.I p
as an argument to fetch or display the error text.
.SH SEE ALSO
pcap(3PCAP), pcap_geterr(3PCAP), pcap_breakloop(3PCAP),
pcap_datalink(3PCAP)