pcap_loop(3pcap) — Linux manual page
PCAP_LOOP(3PCAP) PCAP_LOOP(3PCAP)
NAME
pcap_loop, pcap_dispatch - process packets from a live capture or
savefile
SYNOPSIS
#include <pcap/pcap.h>
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);
DESCRIPTION
pcap_loop() processes packets from a live capture or ``savefile''
until cnt packets are processed, the end of the ``savefile'' is
reached when reading from a ``savefile'', pcap_breakloop(3PCAP)
is called, or an error occurs. It does not return when live
packet buffer timeouts occur. A value of -1 or 0 for cnt is
equivalent to infinity, so that packets are processed until
another ending condition occurs.
pcap_dispatch() processes packets from a live capture or
``savefile'' until 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'', pcap_breakloop() is called, or an error occurs.
Thus, when doing a live capture, 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 cnt packets may be processed. A
value of -1 or 0 for 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''.
Note that, when doing a live capture on some platforms, if the
read timeout expires when there are no packets available,
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.
callback specifies a pcap_handler routine to be called with three
arguments: a u_char pointer which is passed in the user argument
to pcap_loop() or pcap_dispatch(), a const struct pcap_pkthdr
pointer pointing to the packet time stamp and lengths, and a
const u_char pointer to the first caplen (as given in the struct
pcap_pkthdr, a pointer to which is passed to the callback
routine) bytes of data from the packet. The 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.
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 pcap_datalink(3PCAP) routine when handed the pcap_t
value also passed to pcap_loop() or pcap_dispatch().
https://www.tcpdump.org/linktypes.html lists the values
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
pcap_set_datalink(3PCAP) is called; after a successful call to
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 pcap_set_datalink().
Do NOT assume that the packets for a given capture or
``savefile`` will have any given link-layer header type, such as
DLT_EN10MB for Ethernet. For example, the "any" device on Linux
will have a link-layer header type of DLT_LINUX_SLL or
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
DLT_EN10MB for Ethernet.
RETURN VALUE
pcap_loop() returns 0 if cnt is exhausted or if, when reading
from a ``savefile'', no more packets are available. It returns
PCAP_ERROR_BREAK if the loop terminated due to a call to
pcap_breakloop() before any packets were processed,
PCAP_ERROR_NOT_ACTIVATED if called on a capture handle that has
been created but not activated, or PCAP_ERROR if another error
occurs. It does not return when live packet buffer timeouts
occur; instead, it attempts to read more packets.
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 PCAP_ERROR_BREAK if the loop terminated
due to a call to pcap_breakloop() before any packets were
processed, PCAP_ERROR_NOT_ACTIVATED if called on a capture handle
that has been created but not activated, or PCAP_ERROR if another
error occurs. 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.
If PCAP_ERROR is returned, pcap_geterr(3PCAP) or
pcap_perror(3PCAP) may be called with p as an argument to fetch
or display the error text.
BACKWARD COMPATIBILITY
In libpcap versions before 1.5.0, the behavior when cnt was 0 was
undefined; different platforms and devices behaved differently,
so code that must work with these versions of libpcap should use
-1, not 0, as the value of cnt.
SEE ALSO
pcap(3PCAP)
COLOPHON
This page is part of the libpcap (packet capture library)
project. Information about the project can be found at
⟨http://www.tcpdump.org/⟩. If you have a bug report for this
manual page, see ⟨http://www.tcpdump.org/#patches⟩. This page
was obtained from the project's upstream Git repository
⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2024-06-14.
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