NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SEE ALSO | COLOPHON

PCAP_LOOP(3PCAP)                                            PCAP_LOOP(3PCAP)

NAME         top

       pcap_loop,  pcap_dispatch  -  process  packets from a live capture or
       savefile

SYNOPSIS         top

       #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         top

       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 packet buffer
       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         top

       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 packet buffer 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 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 -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         top

       pcap(3PCAP), pcap_geterr(3PCAP), pcap_breakloop(3PCAP),
       pcap_datalink(3PCAP)

COLOPHON         top

       This page is part of the libpcap (packet capture library) project.
       Information about the project can be found at 
       ⟨http://www.tcpdump.org/⟩.  If you have a bug report for this manual
       page, see ⟨http://www.tcpdump.org/#patches⟩.  This page was obtained
       from the project's upstream Git repository 
       ⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2017-03-13.  If
       you discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for the
       page, or you have corrections or improvements to the information in
       this COLOPHON (which is not part of the original manual page), send a
       mail to man-pages@man7.org

                               20 January 2017              PCAP_LOOP(3PCAP)