pcap_loop.3pcap

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.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,
    const u_char *bytes);
int pcap_loop(pcap_t *p, int cnt,
    pcap_handler callback, u_char *user);
int pcap_dispatch(pcap_t *p, int cnt,
    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
.B pcap_handler
routine to be called with three arguments:
a
.B u_char
pointer which is passed in the
.I user
argument to
.BR pcap_loop ()
or
.BR pcap_dispatch (),
a
.B const struct pcap_pkthdr
pointer pointing to the packet time stamp and lengths, and a
.B const u_char
pointer to the first
.B caplen
(as given in the
.BR "struct pcap_pkthdr" ,
a pointer to which is passed to the callback routine)
bytes of data from the packet.  The
.B 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)