PCAP_LOOP
Section: Misc. Reference Manual Pages (3PCAP)
Updated: 18 October 2014
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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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- SEE ALSO
-
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