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''.

       (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)

                                                24 December 2008                                PCAP_LOOP(3PCAP)