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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | SEE ALSO | COLOPHON |
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PCAP_NEXT_EX(3PCAP) PCAP_NEXT_EX(3PCAP)
pcap_next_ex, pcap_next - read the next packet from a pcap_t
#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);
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.
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.
pcap(3PCAP)
This page is part of the libpcap (packet capture library) project.
Information about the project can be found at
⟨http://www.tcpdump.org/⟩. If you have a bug report for this
manual page, see ⟨http://www.tcpdump.org/#patches⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/the-tcpdump-group/libpcap.git⟩ on 2025-08-11.
(At that time, the date of the most recent commit that was found
in the repository was 2025-08-10.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
5 March 2022 PCAP_NEXT_EX(3PCAP)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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