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.TH PCAP_NEXT_EX 3PCAP "25 July 2018"

.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,

.ti +8

const u_char **pkt_data);

const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h);

.ft

.fi

.SH DESCRIPTION

.B 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

.I 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

.I 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

.B pcap_next()

reads the next packet (by calling

.B pcap_dispatch()

with a

.I cnt

of 1) and returns a

.I 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

.I 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

.B pcap_datalink(PCAP)

routine when handed the

.B pcap_t

value also passed to

.B pcap_loop()

or

.BR pcap_dispatch() .

.I https://www.tcpdump.org/linktypes.html

lists the values

.B 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

.B 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

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

.B 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,

.B PCAP_ERROR

if an error occurred while reading the packet, and

.B PCAP_ERROR_BREAK

if packets

are being read from a ``savefile'' and there are no more packets to read

from the savefile.  If

.B PCAP_ERROR

is returned,

.B pcap_geterr(3PCAP)

or

.B pcap_perror(3PCAP)

may be called with

.I p

as an argument to fetch or display the error text.

.PP

.B 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

pcap(3PCAP)