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Man Pages


Manual Reference Pages  -  PCAP_LOOP (3)

NAME

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

CONTENTS

Synopsis
Description
Return Value
See Also

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() is called, or an error occurs. It does not return when live read timeouts occur. A value of -1 or 0 for 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 -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’’.

(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.

RETURN VALUE

pcap_loop() returns 0 if cnt is exhausted or if, when reading from a ‘‘savefile’’, no more packets are available. 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. It does not return when live read 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 read 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.

SEE ALSO

pcap(3), pcap_geterr(3), pcap_breakloop(3)
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--> PCAP_LOOP (3) 24 December 2008

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