pcap_loop(3PCAP) man page
This man page documents libpcap version 1.11.0-PRE-GIT (see also: 1.10.4, 1.10.2, 1.10.1, 1.10.0, 1.9.1, 1.8.1, 1.7.4, 1.6.2, 1.5.3).Your system may have a different version installed, possibly with some local modifications. To achieve the best results, please make sure this version of this man page suits your needs. If necessary, try to look for a different version on this web site or in the man pages available in your installation.
#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);
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.
routine to be called with three arguments:
pointer which is passed in the
const struct pcap_pkthdr
pointer pointing to the packet time stamp and lengths, and a
pointer to the first
(as given in the
a pointer to which is passed to the callback routine)
bytes of data from the packet. The
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
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
routine when handed the
value also passed to
lists the values
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
is called; after a successful call to
all subsequent packets will have a link-layer header of the type
specified by the link-layer header type value passed to
assume that the packets for a given capture or ``savefile`` will have
any given link-layer header type, such as
for Ethernet. For example, the "any" device on Linux will have a
link-layer header type of
even if all devices on the system at the time the "any" device is opened
have some other data link type, such as
PCAP_ERROR_BREAKif the loop terminated due to a call to pcap_breakloop() before any packets were processed,
PCAP_ERROR_NOT_ACTIVATEDif called on a capture handle that has been created but not activated, or
PCAP_ERRORif another error occurs. It does not return when live packet buffer timeouts occur; instead, it attempts to read more packets.
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
if the loop terminated due to a call to
before any packets were processed,
if called on a capture handle that has been created but not activated,
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.
may be called with
as an argument to fetch or display the error text.
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.