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 er‐
24 ror occurs. It does not return when live packet buffer timeouts occur.
25 A value of -1 or 0 for cnt is equivalent to infinity, so that packets
26 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 callback specifies a pcap_handler routine to be called with three argu‐
47 ments: a u_char pointer which is passed in the user argument to
48 pcap_loop() or pcap_dispatch(), a const struct pcap_pkthdr pointer
49 pointing to the packet time stamp and lengths, and a const u_char
50 pointer to the first caplen (as given in the struct pcap_pkthdr a
51 pointer to which is passed to the callback routine) bytes of data from
52 the packet. The struct pcap_pkthdr and the packet data are not to be
53 freed by the callback routine, and are not guaranteed to be valid after
54 the callback routine returns; if the code needs them to be valid after
55 the callback, it must make a copy of them.
56 The bytes of data from the packet begin with a link-layer header. The
58 format of the link-layer header is indicated by the return value of the
59 pcap_datalink(3PCAP) routine when handed the pcap_t value also passed
60 to pcap_loop() or pcap_dispatch(). https://www.tcpdump.org/link‐
61 types.html lists the values pcap_datalink() can return and describes
62 the packet formats that correspond to those values. The value it re‐
63 turns will be valid for all packets received unless and until
64 pcap_set_datalink(3PCAP) is called; after a successful call to
65 pcap_set_datalink(), all subsequent packets will have a link-layer
66 header of the type specified by the link-layer header type value passed
67 to pcap_set_datalink().
68 Do NOT assume that the packets for a given capture or ``savefile`` will
70 have any given link-layer header type, such as DLT_EN10MB for Ethernet.
71 For example, the "any" device on Linux will have a link-layer header
72 type of DLT_LINUX_SLL or DLT_LINUX_SLL2 even if all devices on the sys‐
73 tem at the time the "any" device is opened have some other data link
74 type, such as DLT_EN10MB for Ethernet.
75
77 pcap_loop() returns 0 if cnt is exhausted or if, when reading from a
78 ``savefile'', no more packets are available. It returns PCAP_ER‐
79 ROR_BREAK if the loop terminated due to a call to pcap_breakloop() be‐
80 fore any packets were processed, PCAP_ERROR_NOT_ACTIVATED if called on
81 a capture handle that has been created but not activated, or PCAP_ERROR
82 if another error occurs. It does not return when live packet buffer
83 timeouts occur; instead, it attempts to read more packets.
84 pcap_dispatch() returns the number of packets processed on success;
86 this can be 0 if no packets were read from a live capture (if, for ex‐
87 ample, they were discarded because they didn't pass the packet filter,
88 or if, on platforms that support a packet buffer timeout that starts
89 before any packets arrive, the timeout expires before any packets ar‐
90 rive, or if the file descriptor for the capture device is in non-block‐
91 ing mode and no packets were available to be read) or if no more pack‐
92 ets are available in a ``savefile.'' It returns PCAP_ERROR_BREAK if the
93 loop terminated due to a call to pcap_breakloop() before any packets
94 were processed, PCAP_ERROR_NOT_ACTIVATED if called on a capture handle
95 that has been created but not activated, or PCAP_ERROR if another error
96 occurs. If your application uses pcap_breakloop(), make sure that you
97 explicitly check for PCAP_ERROR and PCAP_ERROR_BREAK, rather than just
98 checking for a return value < 0.
99 If PCAP_ERROR is returned, pcap_geterr(3PCAP) or pcap_perror(3PCAP) may
101 be called with p as an argument to fetch or display the error text.
102
104 In libpcap versions before 1.5.0, the behavior when cnt was 0 was unde‐
105 fined; different platforms and devices behaved differently, so code
106 that must work with these versions of libpcap should use -1, not 0, as
107 the value of cnt.
108
110 pcap(3PCAP)
111 5 March 2022 PCAP_LOOP(3PCAP)