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.TH PCAP_NEXT_EX 3PCAP "5 March 2022"
.SH NAME
pcap_next_ex, pcap_next \- read the next packet from a pcap_t
.SH SYNOPSIS
.nf
.ft B
#include <pcap/pcap.h>
.ft
.LP
.ft B
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);
.ft
.fi
.SH DESCRIPTION
.BR 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
.I pkt_header
argument is set to point to the
.B pcap_pkthdr
struct for the packet, and the
pointer pointed to by the
.I pkt_data
argument is set to point to the data in the packet.  The
.B 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
.BR pcap_next_ex (),
.BR pcap_next (),
.BR pcap_loop (3PCAP),
or
.BR pcap_dispatch (3PCAP);
if the code needs them to remain valid, it must make a copy of them.
.PP
.BR pcap_next ()
reads the next packet (by calling
.BR pcap_dispatch ()
with a
.I cnt
of 1) and returns a
.B 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
.BR pcap_next_ex (),
.BR pcap_next (),
.BR pcap_loop (),
or
.BR pcap_dispatch ();
if the code needs it to remain valid, it must make a copy of it.
The
.B pcap_pkthdr
structure pointed to by
.I h
is filled in with the appropriate values for the packet.
.PP
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
.BR pcap_datalink (3PCAP)
routine when handed the
.B pcap_t
value also passed to
.BR pcap_loop ()
or
.BR pcap_dispatch ().
.I https://www.tcpdump.org/linktypes.html
lists the values
.BR 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
.BR pcap_set_datalink (3PCAP)
is called; after a successful call to
.BR 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
.BR pcap_set_datalink ().
.PP
Do
.B NOT
assume that the packets for a given capture or ``savefile`` will have
any given link-layer header type, such as
.B DLT_EN10MB
for Ethernet.  For example, the "any" device on Linux will have a
link-layer header type of
.B DLT_LINUX_SLL
or
.B 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
.B DLT_EN10MB
for Ethernet.
.SH RETURN VALUE
.BR pcap_next_ex ()
returns
.B 1
if the packet was read without problems,
.B 0
if packets are
being read from a live capture and the packet buffer timeout expired,
.B PCAP_ERROR_BREAK
if packets
are being read from a ``savefile'' and there are no more packets to read
from the savefile,
.B PCAP_ERROR_NOT_ACTIVATED
if called on a capture handle that has been created but not activated,
or
.B PCAP_ERROR
if an error occurred while reading the packet.  If
.B PCAP_ERROR
is returned,
.BR pcap_geterr (3PCAP)
or
.BR pcap_perror (3PCAP)
may be called with
.I p
as an argument to fetch or display the error text.
.PP
.BR pcap_next ()
returns a pointer to the packet data on success, and returns
.B 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.
.SH SEE ALSO
.BR pcap (3PCAP)