1PCAP_LOOP(3PCAP) PCAP_LOOP(3PCAP)
2
6 pcap_loop, pcap_dispatch - process packets from a live capture or save‐
7 file
8
10 #include <pcap/pcap.h>
11 typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h,
13 const u_char *bytes);
14 int pcap_loop(pcap_t *p, int cnt,
16 pcap_handler callback, u_char *user);
17 int pcap_dispatch(pcap_t *p, int cnt,
18 pcap_handler callback, u_char *user);
19
21 pcap_loop() processes packets from a live capture or ``savefile'' until
22 cnt packets are processed, the end of the ``savefile'' is reached when
23 reading from a ``savefile'', pcap_breakloop(3PCAP) is called, or an
24 error occurs. It does not return when live packet buffer timeouts
25 occur. A value of -1 or 0 for cnt is equivalent to infinity, so that
26 packets are processed until another ending condition occurs.
27 pcap_dispatch() processes packets from a live capture or ``savefile''
29 until cnt packets are processed, the end of the current bufferful of
30 packets is reached when doing a live capture, the end of the ``save‐
31 file'' is reached when reading from a ``savefile'', pcap_breakloop() is
32 called, or an error occurs. Thus, when doing a live capture, cnt is
33 the maximum number of packets to process before returning, but is not a
34 minimum number; when reading a live capture, only one bufferful of
35 packets is read at a time, so fewer than cnt packets may be processed.
36 A value of -1 or 0 for cnt causes all the packets received in one buf‐
37 fer to be processed when reading a live capture, and causes all the
38 packets in the file to be processed when reading a ``savefile''.
39 Note that, when doing a live capture on some platforms, if the read
41 timeout expires when there are no packets available, pcap_dispatch()
42 will return 0, even when not in non-blocking mode, as there are no
43 packets to process. Applications should be prepared for this to hap‐
44 pen, but must not rely on it happening.
45 (In older versions of libpcap, the behavior when cnt was 0 was unde‐
47 fined; different platforms and devices behaved differently, so code
48 that must work with older versions of libpcap should use -1, not 0, as
49 the value of cnt.)
50 callback specifies a pcap_handler routine to be called with three argu‐
52 ments: a u_char pointer which is passed in the user argument to
53 pcap_loop() or pcap_dispatch(), a const struct pcap_pkthdr pointer
54 pointing to the packet time stamp and lengths, and a const u_char
55 pointer to the first caplen (as given in the struct pcap_pkthdr a
56 pointer to which is passed to the callback routine) bytes of data from
57 the packet. The struct pcap_pkthdr and the packet data are not to be
58 freed by the callback routine, and are not guaranteed to be valid after
59 the callback routine returns; if the code needs them to be valid after
60 the callback, it must make a copy of them.
61 The bytes of data from the packet begin with a link-layer header. The
63 format of the link-layer header is indicated by the return value of the
64 pcap_datalink(3PCAP) routine when handed the pcap_t value also passed
65 to pcap_loop() or pcap_dispatch(). https://www.tcpdump.org/link‐
66 types.html lists the values pcap_datalink() can return and describes
67 the packet formats that correspond to those values. The value it
68 returns will be valid for all packets received unless and until
69 pcap_set_datalink(3PCAP) is called; after a successful call to
70 pcap_set_datalink(), all subsequent packets will have a link-layer
71 header of the type specified by the link-layer header type value passed
72 to pcap_set_datalink().
73 Do NOT assume that the packets for a given capture or ``savefile`` will
75 have any given link-layer header type, such as DLT_EN10MB for Ethernet.
76 For example, the "any" device on Linux will have a link-layer header
77 type of DLT_LINUX_SLL even if all devices on the system at the time the
78 "any" device is opened have some other data link type, such as
79 DLT_EN10MB for Ethernet.
80
82 pcap_loop() returns 0 if cnt is exhausted or if, when reading from a
83 ``savefile'', no more packets are available. It returns PCAP_ERROR if
84 an error occurs or PCAP_ERROR_BREAK if the loop terminated due to a
85 call to pcap_breakloop() before any packets were processed. It does
86 not return when live packet buffer timeouts occur; instead, it attempts
87 to read more packets.
88 pcap_dispatch() returns the number of packets processed on success;
90 this can be 0 if no packets were read from a live capture (if, for
91 example, they were discarded because they didn't pass the packet fil‐
92 ter, or if, on platforms that support a packet buffer timeout that
93 starts before any packets arrive, the timeout expires before any pack‐
94 ets arrive, or if the file descriptor for the capture device is in non-
95 blocking mode and no packets were available to be read) or if no more
96 packets are available in a ``savefile.'' It returns PCAP_ERROR if an
97 error occurs or PCAP_ERROR_BREAK if the loop terminated due to a call
98 to pcap_breakloop() before any packets were processed. If your appli‐
99 cation uses pcap_breakloop(), make sure that you explicitly check for
100 PCAP_ERROR and PCAP_ERROR_BREAK, rather than just checking for a return
101 value < 0.
102 If PCAP_ERROR is returned, pcap_geterr(3PCAP) or pcap_perror(3PCAP) may
104 be called with p as an argument to fetch or display the error text.
105
107 pcap(3PCAP)
108 25 July 2018 PCAP_LOOP(3PCAP)