Provided by: libpcap0.8-dev_1.8.1-6ubuntu1.18.04.2_amd64 
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() is called, or an
error occurs. It does not return when live read 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.
(In older versions of libpcap, the behavior when cnt 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 cnt.)
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() routine when handed the pcap_t value also passed to
pcap_loop() or pcap_dispatch(). http://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() 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 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 -1 if an error occurs or -2 if the loop terminated due to a call to
pcap_breakloop() before any packets were processed. It does not return when live read 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 read 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 pcap_breakloop() before any packets
were processed. 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.
If -1 is returned, pcap_geterr() or pcap_perror() may be called with p as an argument to fetch or display
the error text.
SEE ALSO
pcap(3PCAP), pcap_geterr(3PCAP), pcap_breakloop(3PCAP), pcap_datalink(3PCAP) 18 October 2014 PCAP_LOOP(3PCAP)