pcap(3)
- NetBSD Manual Pages
PCAP(3) NetBSD Library Functions Manual PCAP(3)
NAME
pcap -- packet capture library
LIBRARY
Packet Capture Library (libpcap, -lpcap)
SYNOPSIS
#include <pcap.h>
char errbuf[PCAP_ERRBUF_SIZE];
pcap_t *
pcap_open_live(const char *device, int snaplen, int promisc, int to_ms,
char *errbuf);
pcap_t *
pcap_open_dead(int linktype, int snaplen);
pcap_t *
pcap_open_offline(const char *fname, char *errbuf);
pcap_dumper_t *
pcap_dump_open(pcap_t *p, const char *fname);
int
pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int
pcap_getnonblock(pcap_t *p, char *errbuf);
int
pcap_findalldevs(pcap_if_t **alldevsp, char *ebuf);
void
pcap_freealldevs(pcap_if_t *alldevs);
char *
pcap_lookupdev(char *errbuf);
int
pcap_lookupnet(const char *device, bpf_u_int32 *netp, bpf_u_int32 *maskp,
char *errbuf);
int
pcap_dispatch(pcap_t *p, int cnt, pcap_handler callback, u_char *user);
int
pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user);
void
pcap_dump(u_char *user, struct pcap_pkthdr *h, u_char *sp);
int
pcap_compile(pcap_t *p, struct bpf_program *fp, char *str, int optimize,
bpf_u_int32 netmask);
int
pcap_compile_nopcap(int snaplen, int linktype, struct bpf_program *fp,
char *str, int optimize, bpf_uint32 netmask, char *errbuf);
int
pcap_setfilter(pcap_t *p, struct bpf_program *fp);
void
pcap_freecode(struct bpf_program *);
const u_char *
pcap_next(pcap_t *p, struct pcap_pkthdr *h);
int
pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
const u_char **pkt_data);
void
pcap_breakloop(pcap_t *);
int
pcap_datalink(pcap_t *p);
int
pcap_list_datalinks(pcap_t *p, int **dlt_buf);
int
pcap_set_datalink(pcap_t *p, int dlt);
int
pcap_datalink_name_to_val(const char *name);
const char *
pcap_datalink_val_to_name(int dlt);
const char *
pcap_datalink_val_to_description(int dlt);
int
pcap_snapshot(pcap_t *p);
int
pcap_is_swapped(pcap_t *p);
int
pcap_major_version(pcap_t *p);
int
pcap_minor_version(pcap_t *p);
int
pcap_stats(pcap_t *p, struct pcap_stat *ps);
FILE *
pcap_file(pcap_t *p);
int
pcap_fileno(pcap_t *p);
int
pcap_get_selectable_fd(pcap_t *p);
void
pcap_perror(pcap_t *p, char *prefix);
char *
pcap_geterr(pcap_t *p);
char *
pcap_strerror(int error);
const char *
pcap_lib_version(void);
void
pcap_close(pcap_t *p);
int
pcap_dump_flush(pcap_dumper_t *p);
FILE *
pcap_dump_file(pcap_dumper_t *p);
void
pcap_dump_close(pcap_dumper_t *p);
DESCRIPTION
The pcap library provides a high level interface to packet capture sys-
tems. All packets on the network, even those destined for other hosts,
are accessible through this mechanism.
ROUTINES
NOTE: errbuf in pcap_open_live(), pcap_open_dead(), pcap_open_offline(),
pcap_setnonblock(), pcap_getnonblock(), pcap_findalldevs(),
pcap_lookupdev(), and pcap_lookupnet() is assumed to be able to hold at
least PCAP_ERRBUF_SIZE chars.
pcap_open_live() is used to obtain a packet capture descriptor to look at
packets on the network. device is a string that specifies the network
device to open; on Linux systems with 2.2 or later kernels, a device
argument of any or NULL can be used to capture packets from all inter-
faces. snaplen specifies the maximum number of bytes to capture. If
this value is less than the size of a packet that is captured, only the
first snaplen bytes of that packet will be captured and provided as
packet data. A value of 65535 should be sufficient, on most if not all
networks, to capture all the data available from the packet. promisc
specifies if the interface is to be put into promiscuous mode. (Note
that even if this parameter is false, the interface could well be in
promiscuous mode for some other reason.) For now, this doesn't work on
the any device; if an argument of any or NULL is supplied, the promisc
flag is ignored. to_ms specifies the read timeout in milliseconds. The
read timeout is used to arrange that the read not necessarily return
immediately when a packet is seen, but that it wait for some amount of
time to allow more packets to arrive and to read multiple packets from
the OS kernel in one operation. Not all platforms support a read time-
out; on platforms that don't, the read timeout is ignored. A zero value
for to_ms, on platforms that support a read timeout, will cause a read to
wait forever to allow enough packets to arrive, with no timeout. errbuf
is used to return error or warning text. It will be set to error text
when pcap_open_live() fails and returns NULL. errbuf may also be set to
warning text when pcap_open_live() succeeds; to detect this case the
caller should store a zero-length string in errbuf before calling
pcap_open_live() and display the warning to the user if errbuf is no
longer a zero-length string.
pcap_open_dead() is used for creating a pcap_t structure to use when
calling the other functions in libpcap. It is typically used when just
using libpcap for compiling BPF code.
pcap_open_offline() is called to open a ``savefile'' for reading. fname
specifies the name of the file to open. The file has the same format as
those used by tcpdump(8). The name ``-'' is a synonym for stdin. errbuf
is used to return error text and is only set when pcap_open_offline()
fails and returns NULL.
pcap_dump_open() is called to open a ``savefile'' for writing. The name
``-'' is a synonym for stdout. NULL is returned on failure. p is a pcap
struct as returned by pcap_open_offline() or pcap_open_live(). fname
specifies the name of the file to open. If NULL is returned,
pcap_geterr() can be used to get the error text.
pcap_setnonblock() puts a capture descriptor, opened with
pcap_open_live(), into ``non-blocking'' mode, or takes it out of
``non-blocking'' mode, depending on whether the nonblock argument is non-
zero or zero. It has no effect on ``savefiles''. If there is an error,
-1 is returned and errbuf is filled in with an appropriate error message;
otherwise, 0 is returned. In ``non-blocking'' mode, an attempt to read
from the capture descriptor with pcap_dispatch() will, if no packets are
currently available to be read, return 0 immediately rather than blocking
waiting for packets to arrive. pcap_loop() and pcap_next() will not work
in ``non-blocking'' mode.
pcap_getnonblock() returns the current ``non-blocking'' state of the cap-
ture descriptor; it always returns 0 on ``savefiles''. If there is an
error, -1 is returned and errbuf is filled in with an appropriate error
message.
pcap_findalldevs() constructs a list of network devices that can be
opened with pcap_open_live(). (Note that there may be network devices
that cannot be opened with pcap_open_live() by the process calling
pcap_findalldevs(), because, for example, that process might not have
sufficient privileges to open them for capturing; if so, those devices
will not appear on the list.) alldevsp is set to point to the first ele-
ment of the list; each element of the list is of type pcap_if_t, and has
the following members:
next if not NULL, a pointer to the next element in the
list; NULL for the last element of the list
name a pointer to a string giving a name for the device to
pass to pcap_open_live()
description if not NULL, a pointer to a string giving a human-
readable description of the device
addresses a pointer to the first element of a list of addresses
for the interface
flags interface flags:
PCAP_IF_LOOPBACK set if the interface is a
loopback interface
Each element of the list of addresses is of type pcap_addr_t, and has the
following members:
next if not NULL, a pointer to the next element in the list;
NULL for the last element of the list
addr a pointer to a struct sockaddr containing an address
netmask if not NULL, a pointer to a struct sockaddr that con-
tains the netmask corresponding to the address pointed
to by addr
broadaddr if not NULL, a pointer to a struct sockaddr that con-
tains the broadcast address corresponding to the address
pointed to by addr; may be NULL if the interface doesn't
support broadcasts
dstaddr if not NULL, a pointer to a struct sockaddr that con-
tains the destination address corresponding to the
address pointed to by addr; may be null if the interface
isn't a point-to-point interface
-1 is returned on failure, in which case errbuf is filled in with an
appropriate error message; 0 is returned on success.
pcap_freealldevs() is used to free a list allocated by
pcap_findalldevs().
pcap_lookupdev() returns a pointer to a network device suitable for use
with pcap_open_live() and pcap_lookupnet(). If there is an error, NULL
is returned and errbuf is filled in with an appropriate error message.
pcap_lookupnet() is used to determine the network number and mask associ-
ated with the network device device. Both netp and maskp are bpf_u_int32
pointers. A return of -1 indicates an error in which case errbuf is
filled in with an appropriate error message.
pcap_dispatch() is used to collect and process packets. cnt specifies
the maximum number of packets to process before returning. This is not a
minimum number; when reading a live capture, only one bufferful of pack-
ets is read at a time, so fewer than cnt packets may be processed. A cnt
of -1 processes all the packets received in one buffer when reading a
live capture, or all the packets in the file when reading a ``savefile''.
A cnt of 0 processes all packets until an error occurs (or EOF is
reached). callback specifies a routine to be called with three argu-
ments: a u_char pointer which is passed in from pcap_dispatch(), a const
struct pcap_pkthdr pointer to a structure (which precedes the actual net-
work headers and data) with the following members:
ts a struct timeval containing the time when the packet was
captured
caplen a bpf_u_int32 giving the number of bytes of the packet that
are available from the capture
len a bpf_u_int32 giving the length of the packet, in bytes
(which might be more than the number of bytes available
from the capture, if the length of the packet is larger
than the maximum number of bytes to capture)
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 (which won't necessarily be the entire packet; to
capture the entire packet, you will have to provide a value for snaplen
in your call to pcap_open_live() that is sufficiently large to get all of
the packet's data - a value of 65535 should be sufficient on most if not
all networks).
The number of packets read is returned. Zero is returned 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 sup-
port a read timeout that starts before any packets arrive, the timeout
expires before any packets arrive, or if the file descriptor for the cap-
ture device is in non-blocking mode and no packets were available to be
read) or if no more packets are available in a ``savefile''. A return of
-1 indicates an error in which case pcap_perror() or pcap_geterr() may be
used to display the error text. A return of -2 indicates that the loop
terminated due to a call to pcap_breakloop() before any packets were pro-
cessed. 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.
NOTE: when reading a live capture, pcap_dispatch() will not necessarily
return when the read times out; on some platforms, the read timeout isn't
supported, and, on other platforms, the timer doesn't start until at
least one packet arrives. This means that the read timeout should not be
used in, for example, an interactive application, to allow the packet
capture loop to ``poll for user input periodically, as there's no'' guar-
antee that pcap_dispatch() will return after the timeout expires.
pcap_loop() is similar to pcap_dispatch() except it keeps reading packets
until cnt packets are processed or an error occurs. It does not return
when live read timeouts occur. Rather, specifying a non-zero read time-
out to pcap_open_live() and then calling pcap_dispatch() allows the
reception and processing of any packets that arrive when the timeout
occurs. A negative cnt causes pcap_loop() to loop forever (or at least
until an error occurs). -1 is returned on an error; 0 is returned if cnt
is exhausted; -2 is returned 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.
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
pcap_pkthdr struct for that packet is not supplied.) NULL is returned if
an error occured, 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 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''. Unfortunately, there is no way to determine whether
an error occured or not.
pcap_next_ex() reads the next packet and returns a success/failure indi-
cation:
1 the packet was read without problems
0 packets are being read from a live capture, and the timeout
expired
-1 an error occurred while reading the packet
-2 packets are being read from a ``savefile'', and there are no
more packets to read from the savefile.
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.
pcap_breakloop() sets a flag that will force pcap_dispatch() or
pcap_loop() to return rather than looping; they will return the number of
packets that have been processed so far, or -2 if no packets have been
processed so far.
This routine is safe to use inside a signal handler on UNIX or a console
control handler on Windows, as it merely sets a flag that is checked
within the loop.
The flag is checked in loops reading packets from the OS - a signal by
itself will not necessarily terminate those loops - as well as in loops
processing a set of packets returned by the OS. Note that if you are
catching signals on UNIX systems that support restarting system calls
after a signal, and calling pcap_breakloop() in the signal handler, you
must specify, when catching those signals, that system calls should NOT
be restarted by that signal. Otherwise, if the signal interrupted a call
reading packets in a live capture, when your signal handler returns after
calling pcap_breakloop(), the call will be restarted, and the loop will
not terminate until more packets arrive and the call completes.
Note also that, in a multi-threaded application, if one thread is blocked
in pcap_dispatch(), pcap_loop(), pcap_next(), or pcap_next_ex(), a call
to pcap_breakloop() in a different thread will not unblock that thread;
you will need to use whatever mechanism the OS provides for breaking a
thread out of blocking calls in order to unblock the thread, such as
thread cancellation in systems that support POSIX threads.
Note that pcap_next() will, on some platforms, loop reading packets from
the OS; that loop will not necessarily be terminated by a signal, so
pcap_breakloop() should be used to terminate packet processing even if
pcap_next() is being used.
pcap_breakloop() does not guarantee that no further packets will be pro-
cessed by pcap_dispatch() or pcap_loop() after it is called; at most one
more packet might be processed.
If -2 is returned from pcap_dispatch() or pcap_loop(), the flag is
cleared, so a subsequent call will resume reading packets. If a positive
number is returned, the flag is not cleared, so a subsequent call will
return -2 and clear the flag.
pcap_dump() outputs a packet to the ``savefile'' opened with
pcap_dump_open(). Note that its calling arguments are suitable for use
with pcap_dispatch() or pcap_loop(). If called directly, the user param-
eter is of type pcap_dumper_t as returned by pcap_dump_open().
pcap_compile() is used to compile the string str into a filter program.
program is a pointer to a bpf_program struct and is filled in by
pcap_compile(). optimize controls whether optimization on the resulting
code is performed. netmask specifies the IPv4 netmask of the network on
which packets are being captured; it is used only when checking for IPv4
broadcast addresses in the filter program. If the netmask of the network
on which packets are being captured isn't known to the program, or if
packets are being captured on the Linux "any" pseudo-interface that can
capture on more than one network, a value of 0 can be supplied; tests for
IPv4 broadcast addreses won't be done correctly, but all other tests in
the filter program will be OK. A return of -1 indicates an error in
which case pcap_geterr() may be used to display the error text.
pcap_compile_nopcap() is similar to pcap_compile() except that instead of
passing a pcap structure, one passes the snaplen and linktype explicitly.
It is intended to be used for compiling filters for direct BPF usage,
without necessarily having called pcap_open(). A return of -1 indicates
an error; the error text is unavailable. (pcap_compile_nopcap() is a
wrapper around pcap_open_dead(), pcap_compile(), and pcap_close(); the
latter three routines can be used directly in order to get the error text
for a compilation error.)
pcap_setfilter() is used to specify a filter program. fp is a pointer to
an array of bpf_program struct, usually the result of a call to
pcap_compile(). -1 is returned on failure, in which case pcap_geterr()
may be used to display the error text; 0 is returned on success.
pcap_freecode() is used to free up allocated memory pointed to by a
bpf_program struct generated by pcap_compile() when that BPF program is
no longer needed, for example after it has been made the filter program
for a pcap structure by a call to pcap_setfilter().
pcap_datalink() returns the link layer type; link layer types it can
return include:
DLT_NULL BSD loopback encapsulation; the link layer header
is a 4-byte field, in host byte order, containing a
PF_ value from <sys/socket.h> for the network-layer
protocol of the packet.
Note that ``host byte order'' is the byte order of
the machine on which the packets are captured, and
the PF_ values are for the OS of the machine on
which the packets are captured; if a live capture
is being done, ``host byte order'' is the byte
order of the machine capturing the packets, and the
PF_ values are those of the OS of the machine cap-
turing the packets, but if a ``savefile'' is being
read, the byte order and PF_ values are not neces-
sarily those of the machine reading the capture
file.
DLT_EN10MB Ethernet (10Mb, 100Mb, 1000Mb, and up)
DLT_IEEE802 IEEE 802.5 Token Ring
DLT_ARCNET ARCNET
DLT_SLIP SLIP; the link layer header contains, in order:
a 1-byte flag, which is 0 for packets
received by the machine and 1 for packets
sent by the machine;
a 1-byte field, the upper 4 bits of which
indicate the type of packet, as per RFC 1144:
0x40 an unmodified IP datagram
(TYPE_IP);
0x70 an uncompressed-TCP IP datagram
(UNCOMPRESSED_TCP), with that
byte being the first byte of the
raw IP header on the wire, con-
taining the connection number in
the protocol field;
0x80 a compressed-TCP IP datagram
(COMPRESSED_TCP), with that byte
being the first byte of the com-
pressed TCP/IP datagram header;
for UNCOMPRESSED_TCP, the rest of the modi-
fied IP header, and for COMPRESSED_TCP, the
compressed TCP/IP datagram header;
for a total of 16 bytes; the uncompressed IP data-
gram follows the header.
DLT_PPP PPP; if the first 2 bytes are 0xff and 0x03, it's
PPP in HDLC-like framing, with the PPP header fol-
lowing those two bytes, otherwise it's PPP without
framing, and the packet begins with the PPP header.
DLT_FDDI FDDI
DLT_ATM_RFC1483
RFC 1483 LLC/SNAP-encapsulated ATM; the packet
begins with an IEEE 802.2 LLC header.
DLT_RAW raw IP; the packet begins with an IP header.
DLT_PPP_SERIAL PPP in HDLC-like framing, as per RFC 1662, or Cisco
PPP with HDLC framing, as per section 4.3.1 of RFC
1547; the first byte will be 0xFF for PPP in HDLC-
like framing, and will be 0x0F or 0x8F for Cisco
PPP with HDLC framing.
DLT_PPP_ETHER PPPoE; the packet begins with a PPPoE header, as
per RFC 2516.
DLT_C_HDLC Cisco PPP with HDLC framing, as per section 4.3.1
of RFC 1547.
DLT_IEEE802_11 IEEE 802.11 wireless LAN
DLT_FRELAY Frame Relay
DLT_LOOP OpenBSD loopback encapsulation; the link layer
header is a 4-byte field, in network byte order,
containing a PF_ value from OpenBSD's
<sys/socket.h> for the network-layer protocol of
the packet.
Note that, if a ``savefile'' is being read, those
PF_ values are not necessarily those of the machine
reading the capture file.
DLT_LINUX_SLL Linux "cooked" capture encapsulation; the link
layer header contains, in order:
a 2-byte "packet type", in network byte
order, which is one of:
0 packet was sent to us by somebody
else
1 packet was broadcast by somebody
else
2 packet was multicast, but not
broadcast, by somebody else
3 packet was sent by somebody else to
somebody else
4 packet was sent by us
a 2-byte field, in network byte order, con-
taining a Linux ARPHRD_ value for the link
layer device type;
a 2-byte field, in network byte order, con-
taining the length of the link layer address
of the sender of the packet (which could be
0);
an 8-byte field containing that number of
bytes of the link layer header (if there are
more than 8 bytes, only the first 8 are
present);
a 2-byte field containing an Ethernet proto-
col type, in network byte order, or contain-
ing 1 for Novell 802.3 frames without an
802.2 LLC header or 4 for frames beginning
with an 802.2 LLC header.
DLT_LTALK Apple LocalTalk; the packet begins with an
AppleTalk LLAP header.
DLT_PFLOG OpenBSD pflog; the link layer header contains, in
order:
a 1-byte header length, in host byte order;
a 4-byte PF_ value, in host byte order;
a 2-byte action code, in network byte order,
which is one of:
0 passed
1 dropped
2 scrubbed
a 2-byte reason code, in network byte order,
which is one of:
0 match
1 bad offset
2 fragment
3 short
4 normalize
5 memory
a 16-character interface name;
a 16-character ruleset name (only meaningful
if subrule is set);
a 4-byte rule number, in network byte order;
a 4-byte subrule number, in network byte
order;
a 1-byte direction, in network byte order,
which is one of:
0 incoming or outgoing
1 incoming
2 outgoing
DLT_PRISM_HEADER
Prism monitor mode information followed by an
802.11 header.
DLT_IP_OVER_FC RFC 2625 IP-over-Fibre Channel, with the link-layer
header being the Network_Header as described in
that RFC.
DLT_SUNATM SunATM devices; the link layer header contains, in
order:
a 1-byte flag field, containing a direction
flag in the uppermost bit, which is set for
packets transmitted by the machine and clear
for packets received by the machine, and a
4-byte traffic type in the low-order 4 bits,
which is one of:
0 raw traffic
1 LANE traffic
2 LLC-encapsulated traffic
3 MARS traffic
4 IFMP traffic
5 ILMI traffic
6 Q.2931 traffic
a 1-byte VPI value;
a 2-byte VCI field, in network byte order.
DLT_IEEE802_11_RADIO
radiotap(9) information followed by an 802.11
header.
DLT_ARCNET_LINUX
ARCNET, with no exception frames, reassembled pack-
ets rather than raw frames, and an extra 16-bit
offset field between the destination host and type
bytes.
DLT_LINUX_IRDA Linux-IrDA packets, with a DLT_LINUX_SLL header
followed by the IrLAP header.
pcap_list_datalinks() is used to get a list of the supported data link
types of the interface associated with the pcap descriptor.
pcap_list_datalinks() allocates an array to hold the list and sets
*dlt_buf. The caller is responsible for freeing the array. -1 is
returned on failure; otherwise, the number of data link types in the
array is returned.
pcap_set_datalink() is used to set the current data link type of the pcap
descriptor to the type specified by dlt. This operation is supported
only of the interface associated with the pcap descriptor supports multi-
ple data link types. -1 is returned on failure; 0 is returned on suc-
cess.
pcap_loop() is similar to pcap_dispatch() except it keeps reading packets
until cnt packets are processed or an error occurs. A negative cnt
causes pcap_loop() to loop forever (or at least until an error occurs).
pcap_next() returns a u_char pointer to the next packet.
pcap_datalink_name_to_val() translates a data link type name, which is a
DLT_ name with the DLT_ removed, to the corresponding data link type
value. The translation is case-insensitive. -1 is returned on failure.
pcap_datalink_val_to_name() translates a data link type value to the cor-
responding data link type name. NULL is returned on failure.
pcap_datalink_val_to_description() translates a data link type value to a
short description of that data link type. NULL is returned on failure.
pcap_snapshot() returns the snapshot length specified when
pcap_open_live() was called.
pcap_is_swapped() returns true if the current ``savefile'' uses a differ-
ent byte order than the current system.
pcap_major_version() returns the major number of the file format of the
savefile; pcap_minor_version() returns the minor number of the file for-
mat of the savefile. The version number is stored in the header of the
savefile.
pcap_file() returns the standard I/O stream of the ``savefile'', if a
``savefile'' was opened with pcap_open_offline(), or NULL, if a network
device was opened with pcap_open_live().
pcap_stats() returns 0 and fills in a pcap_stat struct. The values rep-
resent packet statistics from the start of the run to the time of the
call. If there is an error or the underlying packet capture doesn't sup-
port packet statistics, -1 is returned and the error text can be obtained
with pcap_perror() or pcap_geterr(). pcap_stats() is supported only on
live captures, not on ``savefiles''; no statistics are stored in
``savefiles'', so no statistics are available when reading from a
``savefile''.
pcap_fileno() returns the file descriptor number from which captured
packets are read if a network device was opened with pcap_open_live(), or
-1, if a ``savefile'' was opened with pcap_open_offline().
pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a
file descriptor on which one can do a select() or poll() to wait for it
to be possible to read packets without blocking, if such a descriptor
exists, or -1, if no such descriptor exists. Some network devices opened
with pcap_open_live() do not support select() or poll() (for example,
regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices),
so -1 is returned for those devices.
Note that on most versions of most BSDs (including Mac OS X) select() and
poll() do not work correctly on BPF devices; pcap_get_selectable_fd()
will return a file descriptor on most of those versions (the exceptions
being FreeBSD 4.3 and 4.4), a simple select() or poll() will not return
even after a timeout specified in pcap_open_live() expires. To work
around this, an application that uses select() or poll() to wait for
packets to arrive must put the pcap_t in non-blocking mode, and must
arrange that the select() or poll() have a timeout less than or equal to
the timeout specified in pcap_open_live(), and must try to read packets
after that timeout expires, regardless of whether select() or poll()
indicated that the file descriptor for the pcap_t is ready to be read or
not. (That workaround will not work in FreeBSD 4.3 and later; however,
in FreeBSD 4.6 and later, select() and poll() work correctly on BPF
devices, so the workaround isn't necessary, although it does no harm.)
pcap_get_selectable_fd() is not available on Windows.
pcap_perror() prints the text of the last pcap library error on stderr,
prefixed by prefix.
pcap_geterr() returns the error text pertaining to the last pcap library
error. NOTE: the pointer it returns will no longer point to a valid
error message string after the pcap_t passed to it is closed; you must
use or copy the string before closing the pcap_t.
pcap_strerror() is provided in case strerror(3) isn't available.
pcap_lib_version() returns a pointer to a string giving information about
the version of the libpcap library being used; note that it contains more
information than just a version number.
pcap_close() closes the files associated with p and deallocates
resources.
pcap_dump_file() returns the standard I/O stream of the ``savefile''
opened by pcap_dump_open().
pcap_dump_flush() flushes the output buffer to the ``savefile'', so that
any packets written with pcap_dump() but not yet written to the
``savefile'' will be written. -1 is returned on error, 0 on success.
pcap_dump_close() closes the ``savefile''.
SEE ALSO
tcpdump(8)
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berke-
ley National Laboratory, University of California, Berkeley, CA.
NetBSD 3.1 February 27, 2004 NetBSD 3.1
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