pcap_loop(3PCAP) man page
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#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);
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''.
Note that, when doing a live capture on some platforms, if the read timeout expires when there are no packets available, pcap_dispatch() will return 0, even when not in non-blocking mode, as there are no packets to process. Applications should be prepared for this to happen, but must not rely on it happening.
(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(). https://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.
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 packet buffer 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.