PCAP_LOOP(3)           FreeBSD 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(3) 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.

     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(3) routine when handed the pcap_t value also passed to
     pcap_loop() or pcap_dispatch().  https://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(3) 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 or DLT_LINUX_SLL2 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 PCAP_ERROR_BREAK
     if the loop terminated due to a call to pcap_breakloop() before any
     packets were processed, PCAP_ERROR_NOT_ACTIVATED if called on a capture
     handle that has been created but not activated, or PCAP_ERROR if another
     error occurs.  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 PCAP_ERROR_BREAK if the loop terminated due to
     a call to pcap_breakloop() before any packets were processed,
     PCAP_ERROR_NOT_ACTIVATED if called on a capture handle that has been
     created but not activated, or PCAP_ERROR if another error occurs.  If
     your application uses pcap_breakloop(), make sure that you explicitly
     check for PCAP_ERROR and PCAP_ERROR_BREAK, rather than just checking for
     a return value < 0.

     If PCAP_ERROR is returned, pcap_geterr(3) or pcap_perror(3) may be called
     with p as an argument to fetch or display the error text.

BACKWARD COMPATIBILITY

     In libpcap versions before 1.5.0, the behavior when cnt was 0 was
     undefined; different platforms and devices behaved differently, so code
     that must work with these versions of libpcap should use -1, not 0, as
     the value of cnt.

SEE ALSO

     pcap(3)

                                 5 March 2022                     PCAP_LOOP(3)