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  22 .TH PCAP_LOOP 3PCAP "4 April 2008"
  23 .SH NAME
  24 pcap_loop, pcap_dispatch \- process packets from a live capture or savefile
  25 .SH SYNOPSIS
  26 .nf
  27 .ft B
  28 #include <pcap.h>
  29 .ft
  30 .LP
  31 .ft B
  32 typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h,
  33 .ti +8
  34                              const u_char *bytes);
  35 .ft
  36 .LP
  37 .ft B
  38 int pcap_loop(pcap_t *p, int cnt,
  39 .ti +8
  40 pcap_handler callback, u_char *user);
  41 int pcap_dispatch(pcap_t *p, int cnt,
  42 .ti +8
  43 pcap_handler callback, u_char *user);
  44 .ft
  45 .fi
  46 .SH DESCRIPTION
  47 .B pcap_loop()
  48 processes packets from a live capture or ``savefile'' until
  49 .I cnt
  50 packets are processed, the end of the ``savefile'' is
  51 reached when reading from a ``savefile'',
  52 .B pcap_breakloop()
  53 is called, or an error occurs.
  54 It does
  55 .B not
  56 return when live read timeouts occur.
  57 A value of \-1 or 0 for
  58 .I cnt
  59 is equivalent to infinity, so that packets are processed until another
  60 ending condition occurs.
  61 .PP
  62 .B pcap_dispatch()
  63 processes packets from a live capture or ``savefile'' until
  64 .I cnt
  65 packets are processed, the end of the current bufferful of packets is
  66 reached when doing a live capture, the end of the ``savefile'' is
  67 reached when reading from a ``savefile'',
  68 .B pcap_breakloop()
  69 is called, or an error occurs.
  70 Thus, when doing a live capture,
  71 .I cnt
  72 is the maximum number of packets to process before returning, but is not
  73 a minimum number; when reading a live capture, only one
  74 bufferful of packets is read at a time, so fewer than
  75 .I cnt
  76 packets may be processed. A value of \-1 or 0 for
  77 .I cnt
  78 causes all the packets received in one buffer to be processed when
  79 reading a live capture, and causes all the packets in the file to be
  80 processed when reading a ``savefile''.
  81 .PP
  82 .ft B
  83 (In older versions of libpcap, the behavior when
  84 \fIcnt\fP
  85 was 0 was undefined; different platforms and devices behaved
  86 differently, so code that must work with older versions of libpcap
  87 should use \-1, nor 0, as the value of
  88 \fIcnt\fP.)
  89 .ft R
  90 .PP
  91 .I callback
  92 specifies a routine to be called with three arguments:
  93 a
  94 .I u_char
  95 pointer which is passed in the
  96 .I user
  97 argument to
  98 .B pcap_loop()
  99 or
 100 .BR pcap_dispatch() ,
 101 a
 102 .I const struct pcap_pkthdr
 103 pointer pointing to the packet time stamp and lengths, and a
 104 .I const u_char
 105 pointer to the first
 106 .B caplen
 107 (as given in the
 108 .I struct pcap_pkthdr
 109 a pointer to which is passed to the callback routine)
 110 bytes of data from the packet.
 111 .PP
 112 .BR NOTE :
 113 when reading a live capture,
 114 .B pcap_dispatch()
 115 will not necessarily return when the read times out; on some platforms,
 116 the read timeout isn't supported, and, on other platforms, the timer
 117 doesn't start until at least one packet arrives.  This means that the
 118 read timeout should
 119 .B NOT
 120 be used in, for example, an interactive application, to allow the packet
 121 capture loop to ``poll'' for user input periodically, as there's no
 122 guarantee that
 123 .B pcap_dispatch()
 124 will return after the timeout expires.
 125 .SH RETURN VALUE
 126 .B pcap_loop()
 127 returns 0 if
 128 .I cnt
 129 is exhausted, \-1 if an error occurs, or \-2 if the loop terminated due
 130 to a call to
 131 .B pcap_breakloop()
 132 before any packets were processed.
 133 It does
 134 .B not
 135 return when live read timeouts occur; instead, it attempts to read more
 136 packets.
 137 .PP
 138 .B pcap_dispatch()
 139 returns the number of packets processed on success; this can be 0 if no
 140 packets were read from a live capture (if, for example, they were
 141 discarded because they didn't pass the packet filter, or if, on
 142 platforms that support a read timeout that starts before any packets
 143 arrive, the timeout expires before any packets arrive, or if the file
 144 descriptor for the capture device is in non-blocking mode and no packets
 145 were available to be read) or if no more packets are available in a
 146 ``savefile.''  It returns \-1 if an error occurs or \-2 if the loop
 147 terminated due to a call to
 148 .B pcap_breakloop()
 149 before any packets were processed.
 150 .ft B
 151 If your application uses pcap_breakloop(),
 152 make sure that you explicitly check for \-1 and \-2, rather than just
 153 checking for a return value < 0.
 154 .ft R
 155 .PP
 156 If \-1 is returned,
 157 .B pcap_geterr()
 158 or
 159 .B pcap_perror()
 160 may be called with
 161 .I p
 162 as an argument to fetch or display the error text.
 163 .SH SEE ALSO
 164 pcap_geterr(3PCAP), pcap_breakloop(3PCAP)