PCAP_NEXT_EX(3PCAP) PCAP_NEXT_EX(3PCAP)

NAME

pcap_next_ex, pcap_next - read the next packet from a pcap_t

SYNOPSIS

#include <pcap/pcap.h>

int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
const u_char **pkt_data);
const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);

DESCRIPTION

pcap_next_ex() reads the next packet and returns a success/failure
indication. If the packet was read without problems, the pointer
pointed to by the pkt_header argument is set to point to the
pcap_pkthdr struct for the packet, and the pointer pointed to by the
pkt_data argument is set to point to the data in the packet. The
struct pcap_pkthdr and the packet data are not to be freed by the
caller, and are not guaranteed to be valid after the next call to
pcap_next_ex(), pcap_next(), pcap_loop(3PCAP), or
pcap_dispatch(3PCAP); if the code needs them to remain valid, it must
make a copy of them.

pcap_next() reads the next packet (by calling pcap_dispatch() with a
cnt of 1) and returns a u_char pointer to the data in that packet.
The packet data is not to be freed by the caller, and is not
guaranteed to be valid after the next call to pcap_next_ex(),
pcap_next(), pcap_loop(), or pcap_dispatch(); if the code needs it to
remain valid, it must make a copy of it. The pcap_pkthdr structure
pointed to by h is filled in with the appropriate values for the
packet.

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(3PCAP) 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(3PCAP) 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 or DLT_LINUX_SLL2 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.

RETURN VALUE

pcap_next_ex() returns 1 if the packet was read without problems, 0
if packets are being read from a live capture and the packet buffer
timeout expired, PCAP_ERROR_BREAK if packets are being read from a
``savefile'' and there are no more packets to read from the savefile,
PCAP_ERROR_NOT_ACTIVATED if called on a capture handle that has been
created but not activated, or PCAP_ERROR if an error occurred while
reading the packet. If PCAP_ERROR is returned, pcap_geterr(3PCAP) or
pcap_perror(3PCAP) may be called with p as an argument to fetch or
display the error text.

pcap_next() returns a pointer to the packet data on success, and
returns NULL if an error occurred, or 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.'' Unfortunately, there is no way to determine whether an
error occurred or not.

SEE ALSO

pcap(3PCAP)

5 March 2022 PCAP_NEXT_EX(3PCAP)