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)