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PCAP_LOOP(3)               Library Functions Manual               PCAP_LOOP(3)



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(3), pcap_geterr(3), pcap_breakloop(3), pcap_datalink(3)



                                18 October 2014                   PCAP_LOOP(3)