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| 22 | .TH PCAP_LOOP 3PCAP "24 December 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/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, not 0, as the value of |
| 88 | \fIcnt\fP.) |
| 89 | .ft R |
| 90 | .PP |
| 91 | .I callback |
| 92 | specifies a |
| 93 | .I pcap_handler |
| 94 | routine to be called with three arguments: |
| 95 | a |
| 96 | .I u_char |
| 97 | pointer which is passed in the |
| 98 | .I user |
| 99 | argument to |
| 100 | .B pcap_loop() |
| 101 | or |
| 102 | .BR pcap_dispatch() , |
| 103 | a |
| 104 | .I const struct pcap_pkthdr |
| 105 | pointer pointing to the packet time stamp and lengths, and a |
| 106 | .I const u_char |
| 107 | pointer to the first |
| 108 | .B caplen |
| 109 | (as given in the |
| 110 | .I struct pcap_pkthdr |
| 111 | a pointer to which is passed to the callback routine) |
| 112 | bytes of data from the packet. The |
| 113 | .I struct pcap_pkthdr |
| 114 | and the packet data are not to be freed by the callback routine, and are |
| 115 | not guaranteed to be valid after the callback routine returns; if the |
| 116 | code needs them to be valid after the callback, it must make a copy of |
| 117 | them. |
| 118 | .PP |
| 119 | The bytes of data from the packet begin with a link-layer header. The |
| 120 | format of the link-layer header is indicated by the return value of the |
| 121 | .B pcap_datalink() |
| 122 | routine when handed the |
| 123 | .B pcap_t |
| 124 | value also passed to |
| 125 | .B pcap_loop() |
| 126 | or |
| 127 | .BR pcap_dispatch() . |
| 128 | .I http://www.tcpdump.org/linktypes.html |
| 129 | lists the values |
| 130 | .B pcap_datalink() |
| 131 | can return and describes the packet formats that |
| 132 | correspond to those values. The value it returns will be valid for all |
| 133 | packets received unless and until |
| 134 | .B pcap_set_datalink() |
| 135 | is called; after a successful call to |
| 136 | .BR pcap_set_datalink() , |
| 137 | all subsequent packets will have a link-layer header of the type |
| 138 | specified by the link-layer header type value passed to |
| 139 | .BR pcap_set_datalink() . |
| 140 | .PP |
| 141 | Do |
| 142 | .B NOT |
| 143 | assume that the packets for a given capture or ``savefile`` will have |
| 144 | any given link-layer header type, such as |
| 145 | .B DLT_EN10MB |
| 146 | for Ethernet. For example, the "any" device on Linux will have a |
| 147 | link-layer header type of |
| 148 | .B DLT_LINUX_SLL |
| 149 | even if all devices on the system at the time the "any" device is opened |
| 150 | have some other data link type, such as |
| 151 | .B DLT_EN10MB |
| 152 | for Ethernet. |
| 153 | .SH RETURN VALUE |
| 154 | .B pcap_loop() |
| 155 | returns 0 if |
| 156 | .I cnt |
| 157 | is exhausted or if, when reading from a ``savefile'', no more packets |
| 158 | are available. It returns \-1 if an error occurs or \-2 if the loop |
| 159 | terminated due to a call to |
| 160 | .B pcap_breakloop() |
| 161 | before any packets were processed. |
| 162 | It does |
| 163 | .B not |
| 164 | return when live read timeouts occur; instead, it attempts to read more |
| 165 | packets. |
| 166 | .PP |
| 167 | .B pcap_dispatch() |
| 168 | returns the number of packets processed on success; this can be 0 if no |
| 169 | packets were read from a live capture (if, for example, they were |
| 170 | discarded because they didn't pass the packet filter, or if, on |
| 171 | platforms that support a read timeout that starts before any packets |
| 172 | arrive, the timeout expires before any packets arrive, or if the file |
| 173 | descriptor for the capture device is in non-blocking mode and no packets |
| 174 | were available to be read) or if no more packets are available in a |
| 175 | ``savefile.'' It returns \-1 if an error occurs or \-2 if the loop |
| 176 | terminated due to a call to |
| 177 | .B pcap_breakloop() |
| 178 | before any packets were processed. |
| 179 | .ft B |
| 180 | If your application uses pcap_breakloop(), |
| 181 | make sure that you explicitly check for \-1 and \-2, rather than just |
| 182 | checking for a return value < 0. |
| 183 | .ft R |
| 184 | .PP |
| 185 | If \-1 is returned, |
| 186 | .B pcap_geterr() |
| 187 | or |
| 188 | .B pcap_perror() |
| 189 | may be called with |
| 190 | .I p |
| 191 | as an argument to fetch or display the error text. |
| 192 | .SH SEE ALSO |
| 193 | pcap(3PCAP), pcap_geterr(3PCAP), pcap_breakloop(3PCAP), |
| 194 | pcap_datalink(3PCAP) |