pcap_loop(3PCAP) man page | TCPDUMP & LIBPCAP (original) (raw)

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This man page documents libpcap version 1.11.0-PRE-GIT (see also: 1.10.5, 1.10.4, 1.10.2, 1.10.1, 1.10.0, 1.9.1, 1.8.1, 1.7.4, 1.6.2, 1.5.3).

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NAME

pcap_loop, pcap_dispatch - process packets from a live capture or savefile

SYNOPSIS

#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);

DESCRIPTION

pcap_loop() processes packets from a live capture or ``savefile'' until_cnt_packets are processed, the end of the ``savefile'' is reached when reading from a ``savefile'',pcap_breakloop(3PCAP) is called, or an error occurs. It doesnotreturn when live packet buffer timeouts occur. A value of-1or0for_cnt_is equivalent to infinity, so that packets are processed until another ending condition occurs.

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-1or0for_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.

_callback_specifies apcap_handlerroutine to be called with three arguments: au_charpointer which is passed in the_user_argument topcap_loop() orpcap_dispatch(), aconst struct pcap_pkthdrpointer pointing to the packet time stamp and lengths, and aconst u_charpointer to the firstcaplen(as given in thestruct pcap_pkthdr, a pointer to which is passed to the callback routine) bytes of data from the packet. Thestruct pcap_pkthdrand 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 thepcap_datalink(3PCAP) routine when handed thepcap_tvalue also passed topcap_loop() orpcap_dispatch()._https://www.tcpdump.org/linktypes.html_lists the valuespcap_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 untilpcap_set_datalink(3PCAP) is called; after a successful call topcap_set_datalink(), all subsequent packets will have a link-layer header of the type specified by the link-layer header type value passed topcap_set_datalink().

DoNOTassume that the packets for a given capture or ``savefile`` will have any given link-layer header type, such asDLT_EN10MBfor Ethernet. For example, the "any" device on Linux will have a link-layer header type ofDLT_LINUX_SLLorDLT_LINUX_SLL2even if all devices on the system at the time the "any" device is opened have some other data link type, such asDLT_EN10MBfor Ethernet.

RETURN VALUE

pcap_loop() returns0if_cnt_is exhausted or if, when reading from a ``savefile'', no more packets are available. It returnsPCAP_ERROR_BREAKif the loop terminated due to a call topcap_breakloop() before any packets were processed,PCAP_ERROR_NOT_ACTIVATEDif called on a capture handle that has been created but not activated, orPCAP_ERRORif another error occurs. It doesnotreturn when live packet buffer timeouts occur; instead, it attempts to read more packets.

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 returnsPCAP_ERROR_BREAKif the loop terminated due to a call topcap_breakloop() before any packets were processed,PCAP_ERROR_NOT_ACTIVATEDif called on a capture handle that has been created but not activated, orPCAP_ERRORif another error occurs.If your application uses pcap_breakloop(), make sure that you explicitly check for PCAP_ERROR and PCAP_ERROR_BREAK, rather than just checking for a return value < 0.

IfPCAP_ERRORis returned,pcap_geterr(3PCAP) orpcap_perror(3PCAP) may be called with_p_as an argument to fetch or display the error text.

BACKWARD COMPATIBILITY

In libpcap versions before 1.5.0, the behavior when_cnt_was0was undefined; different platforms and devices behaved differently, so code that must work with these versions of libpcap should use-1, not0, as the value of_cnt_.

COLOPHON

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