This file is a HOWTO for Wireshark developers interested in writing or working
on Wireshark protocol dissectors. It describes expected code patterns and the
use of some of the important functions and variables.

This file is compiled to give in depth information on Wireshark.
It is by no means all inclusive and complete. Please feel free to discuss on
the developer mailing list or upload merge requests to gitlab.
If you haven't read README.developer, read that first!

0. Prerequisites.

Before starting to develop a new dissector, a "running" Wireshark build
environment is required - there's no such thing as a standalone "dissector
build toolkit".

How to setup such an environment is platform dependent; detailed
information about these steps can be found in the "Developer's Guide"
(available from: https://www.wireshark.org) and in the INSTALL and
README.md files of the sources root dir.

0.1. Dissector related README files.

You'll find additional dissector related information in the following README
files:

- doc/README.heuristic      - what are heuristic dissectors and how to write them
- doc/README.request_response_tracking - how to track req./resp. times and such
- doc/README.wmem           - how to obtain "memory leak free" memory

0.2 Contributors

James Coe <jammer[AT]cin.net>
Gilbert Ramirez <gram[AT]alumni.rice.edu>
Jeff Foster <jfoste[AT]woodward.com>
Olivier Abad <oabad[AT]cybercable.fr>
Laurent Deniel <laurent.deniel[AT]free.fr>
Gerald Combs <gerald[AT]wireshark.org>
Guy Harris <guy[AT]alum.mit.edu>
Ulf Lamping <ulf.lamping[AT]web.de>
Barbu Paul - Gheorghe <barbu.paul.gheorghe[AT]gmail.com>

1. Setting up your protocol dissector code.

This section provides skeleton code for a protocol dissector. It also explains
the basic functions needed to enter values in the traffic summary columns,
add to the protocol tree, and work with registered header fields.

1.1 Skeleton code.

Wireshark requires certain things when setting up a protocol dissector.
We provide basic skeleton code for a dissector that you can copy to a new file
and fill in. Your dissector should follow the naming convention of "packet-"
followed by the abbreviated name for the protocol. It is recommended that where
possible you keep to the IANA abbreviated name for the protocol, if there is
one, or a commonly-used abbreviation for the protocol, if any.

The skeleton code lives in the file "packet-PROTOABBREV.c" in the same source
directory as this README.

If instead of using the skeleton you base your dissector on an existing real
dissector, please put a little note in the copyright header indicating which
dissector you started with.

Usually, you will put your newly created dissector file into the directory
epan/dissectors/, just like all the other packet-*.c files already in there.

Also, please add your dissector file to the corresponding makefiles,
described in section "1.8 Editing CMakeLists.txt to add your dissector" below.

Dissectors that use the dissector registration API to register with a lower
level protocol (this is the vast majority) don't need to define a prototype in
their .h file. For other dissectors the main dissector routine should have a
prototype in a header file whose name is "packet-", followed by the abbreviated
name for the protocol, followed by ".h"; any dissector file that calls your
dissector should be changed to include that file.

You may not need to include all the headers listed in the skeleton, and you may
need to include additional headers.

1.2 Explanation of needed substitutions in code skeleton.

In the skeleton sample code the following strings should be substituted with
your information.

YOUR_NAME       Your name, of course. You do want credit, don't you?
                It's the only payment you will receive...
YOUR_EMAIL_ADDRESS  Keep those cards and letters coming.
PROTONAME       The name of the protocol; this is displayed in the
                top-level protocol tree item for that protocol.
PROTOSHORTNAME  An abbreviated name for the protocol; this is displayed
                in the "Preferences" dialog box if your dissector has
                any preferences, in the dialog box of enabled protocols,
                and in the dialog box for filter fields when constructing
                a filter expression.
PROTOFILTERNAME A name for the protocol for use in filter expressions;
                it may contain only letters, digits, hyphens, underscores and
                periods. Names should use lower case only. (Support for
                upper/mixed case may be removed in the future.)
PROTOABBREV     An abbreviation for the protocol; this is used in code and
                must be a valid C identifier. Additionally it should follow
                any applicable C style guidelines. It is usually the same as
                PROTOFILTERNAME with all lower-case letters and
                non-alphanumerics replaced with underscores.
LICENSE         The license this dissector is under. Please use a SPDX License
                identifier.
YEARS           The years the above license is valid for.
FIELDNAME       The displayed name for the header field.
FIELDFILTERNAME A name for the header field for use in filter expressions;
                it may contain only letters, digits, hyphens, underscores and
                periods. It must start with PROTOFILTERNAME followed by a dot.
                Names should use lower case only. (Support for upper/mixed case
                may be removed in the future.)
FIELDABBREV     An abbreviation for the header field; this is used in code and
                must be a valid C identifier. Additionally it should follow
                any applicable C style guidelines. It is usually the same as
                FIELDFILTERNAME with all lower-case letters and
                non-alphanumerics replaced with underscores.
FIELDTYPE       FT_NONE, FT_BOOLEAN, FT_CHAR, FT_UINT8, FT_UINT16, FT_UINT24,
                FT_UINT32, FT_UINT40, FT_UINT48, FT_UINT56, FT_UINT64,
                FT_INT8, FT_INT16, FT_INT24, FT_INT32, FT_INT40, FT_INT48,
                FT_INT56, FT_INT64, FT_IEEE_11073_SFLOAT, FT_IEEE_11073_FLOAT,
                FT_FLOAT, FT_DOUBLE, FT_ABSOLUTE_TIME, FT_RELATIVE_TIME,
                FT_STRING, FT_STRINGZ, FT_STRINGZPAD, FT_STRINGZTRUNC,
                FT_UINT_STRING, FT_ETHER, FT_BYTES, FT_UINT_BYTES, FT_IPv4,
                FT_IPv6, FT_IPXNET, FT_FRAMENUM, FT_PROTOCOL, FT_EUI64, FT_GUID,
                FT_OID, FT_REL_OID, FT_AX25, FT_VINES, FT_SYSTEM_ID, FT_FCWWN
FIELDDISPLAY    --For FT_UINT{8,16,24,32,40,48,56,64} and
                       FT_INT{8,16,24,32,40,48,56,64):

                  BASE_DEC, BASE_HEX, BASE_OCT, BASE_DEC_HEX, BASE_HEX_DEC,
                  BASE_CUSTOM, or BASE_NONE, possibly ORed with
                  BASE_RANGE_STRING, BASE_EXT_STRING, BASE_VAL64_STRING,
                  BASE_ALLOW_ZERO, BASE_UNIT_STRING, BASE_SPECIAL_VALS,
                  BASE_NO_DISPLAY_VALUE, BASE_SHOW_ASCII_PRINTABLE, or
                  BASE_SHOW_UTF_8_PRINTABLE

                  BASE_NONE may be used with a non-NULL FIELDCONVERT when the
                  numeric value of the field itself is not of significance to
                  the user (for example, the number is a generated field).
                  When this is the case the numeric value is not shown to the
                  user in the protocol decode nor is it used when preparing
                  filters for the field in question.

                  BASE_NO_DISPLAY_VALUE will just display the field name with
                  no value. It is intended for byte arrays (FT_BYTES or
                  FT_UINT_BYTES) or header fields above a subtree. The
                  value will still be filterable, just not displayed.

                --For FT_UINT16:

                  BASE_PT_UDP, BASE_PT_TCP, BASE_PT_DCCP or BASE_PT_SCTP

                --For FT_UINT24:

                  BASE_OUI

                --For FT_CHAR:
                  BASE_HEX, BASE_OCT, BASE_CUSTOM, or BASE_NONE, possibly
                  ORed with BASE_RANGE_STRING, BASE_EXT_STRING or
                  BASE_VAL64_STRING.

                  BASE_NONE can be used in the same way as with FT_UINT8.

                --For FT_FLOAT, FT_DOUBLE:
                  BASE_NONE, BASE_DEC, BASE_HEX, BASE_EXP or BASE_CUSTOM,
                  possibly ORed with BASE_UNIT_STRING.

                  BASE_NONE uses BASE_DEC or BASE_EXP, similarly to the
                  %g double format for the printf() function.

                --For FT_IEEE_11073_SFLOAT, FT_IEEE_11073_FLOAT:
                  BASE_NONE, possibly ORed with BASE_UNIT_STRING.

                --For FT_ABSOLUTE_TIME:

                  ABSOLUTE_TIME_LOCAL, ABSOLUTE_TIME_UTC, ABSOLUTE_TIME_DOY_UTC,
                  ABSOLUTE_TIME_NTP_UTC, or ABSOLUTE_TIME_UNIX

                --For FT_BOOLEAN:

                  if BITMASK is non-zero:
                    Number of bits in the field containing the FT_BOOLEAN
                    bitfield.
                  otherwise:
                    (must be) BASE_NONE

                --For FT_STRING, FT_STRINGZ, FT_STRINGZPAD, FT_STRINGZTRUNC,
                  and FT_UINT_STRING:

                  BASE_STR_WSP or BASE_NONE

                --For FT_BYTES and FT_UINT_BYTES:

                  SEP_DOT, SEP_DASH, SEP_COLON, or SEP_SPACE to provide
                  a separator between bytes; BASE_NONE has no separator
                  between bytes.  These can be ORed with BASE_ALLOW_ZERO,
                  BASE_SHOW_ASCII_PRINTABLE, or BASE_SHOW_UTF_8_PRINTABLE.

                  BASE_ALLOW_ZERO displays <none> instead of <MISSING>
                  for a zero-sized byte array.
                  BASE_SHOW_ASCII_PRINTABLE will check whether the
                  field's value consists entirely of printable ASCII
                  characters and, if so, will display the field's value
                  as a string, in quotes.  The value will still be
                  filterable as a byte value.
                  BASE_SHOW_UTF_8_PRINTABLE will check whether the
                  field's value is valid UTF-8 consisting entirely of
                  printable characters and, if so, will display the field's
                  value as a string, in quotes.  The value will still be
                  filterable as a byte value.

                --For FT_IPv4:

                  BASE_NETMASK - Used for IPv4 address that should never
                                 attempted to be resolved (like netmasks)
                  otherwise:
                    (must be) BASE_NONE

                --For all other types:

                  BASE_NONE
FIELDCONVERT    VALS(x), VALS64(x), RVALS(x), TFS(x), CF_FUNC(x), NULL
BITMASK         Used to mask a field not 8-bit aligned or with a size other
                than a multiple of 8 bits
FIELDDESCR      A brief description of the field, or NULL. [Please do not use ""].

If, for example, PROTONAME is "Internet Bogosity Discovery Protocol",
PROTOSHORTNAME would be "IBDP", and PROTOFILTERNAME would be "ibdp". Try to
conform with IANA names.

1.2.1 Automatic substitution in code skeleton

Instead of manual substitutions in the code skeleton, a tool to automate it can
be found under the tools directory. The script is called tools/generate-dissector.py
and takes all the needed options to generate a compilable dissector. Look at the
above fields to know how to set them. Some assumptions have been made in the
generation to shorten the list of required options. The script patches the
CMakeLists.txt file adding the new dissector in the proper list, alphabetically
sorted.

1.3 The dissector and the data it receives.


1.3.1 Header file.

This is only needed if the dissector doesn't use self-registration to
register itself with the lower level dissector, or if the protocol dissector
wants/needs to expose code to other subdissectors.

The dissector must be declared exactly as follows in the file
packet-PROTOABBREV.h:

int
dissect_PROTOABBREV(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree);


1.3.2 Extracting data from packets.

NOTE: See the file /epan/tvbuff.h for more details.

The "tvb" argument to a dissector points to a buffer containing the raw
data to be analyzed by the dissector; for example, for a protocol
running atop UDP, it contains the UDP payload (but not the UDP header,
or any protocol headers above it). A tvbuffer is an opaque data
structure, the internal data structures are hidden and the data must be
accessed via the tvbuffer accessors.

The accessors are:

Bit accessors for a maximum of 8-bits, 16-bits 32-bits and 64-bits:

uint8_t tvb_get_bits8(tvbuff_t *tvb, int bit_offset, const int no_of_bits);
uint16_t tvb_get_bits16(tvbuff_t *tvb, unsigned bit_offset, const int no_of_bits, const unsigned encoding);
uint32_t tvb_get_bits32(tvbuff_t *tvb, unsigned bit_offset, const int no_of_bits, const unsigned encoding);
uint64_t tvb_get_bits64(tvbuff_t *tvb, unsigned bit_offset, const int no_of_bits, const unsigned encoding);

Single-byte accessors for 8-bit unsigned integers (uint8_t) and 8-bit
signed integers (int8_t):

uint8_t tvb_get_uint8(tvbuff_t *tvb, const int offset);
int8_t tvb_get_int8(tvbuff_t *tvb, const int offset);

These functions were previously named tvb_get_guint8 and tvb_get_gint8. You
can still use the old names but they have been deprecated.

Network-to-host-order accessors:

16-bit unsigned (uint16_t) and signed (int16_t) integers:

uint16_t tvb_get_ntohs(tvbuff_t *tvb, const int offset);
int16_t tvb_get_ntohis(tvbuff_t *tvb, const int offset);

24-bit unsigned and signed integers:

uint32_t tvb_get_ntoh24(tvbuff_t *tvb, const int offset);
int32_t tvb_get_ntohi24(tvbuff_t *tvb, const int offset);

32-bit unsigned (uint32_t) and signed (int32_t) integers:

uint32_t tvb_get_ntohl(tvbuff_t *tvb, const int offset);
int32_t tvb_get_ntohil(tvbuff_t *tvb, const int offset);

40-bit unsigned and signed integers:

uint64_t tvb_get_ntoh40(tvbuff_t *tvb, const int offset);
int64_t tvb_get_ntohi40(tvbuff_t *tvb, const int offset);

48-bit unsigned and signed integers:

uint64_t tvb_get_ntoh48(tvbuff_t *tvb, const int offset);
int64_t tvb_get_ntohi48(tvbuff_t *tvb, const int offset);

56-bit unsigned and signed integers:

uint64_t tvb_get_ntoh56(tvbuff_t *tvb, const int offset);
int64_t tvb_get_ntohi56(tvbuff_t *tvb, const int offset);

64-bit unsigned (uint64_t) and signed (int64_t) integers:

uint64_t tvb_get_ntoh64(tvbuff_t *tvb, const int offset);
int64_t tvb_get_ntohi64(tvbuff_t *tvb, const int offset);

Single-precision and double-precision IEEE floating-point numbers:

float tvb_get_ntohieee_float(tvbuff_t *tvb, const int offset);
double tvb_get_ntohieee_double(tvbuff_t *tvb, const int offset);

Little-Endian-to-host-order accessors:

16-bit unsigned (uint16_t) and signed (int16_t) integers:

uint16_t tvb_get_letohs(tvbuff_t *tvb, const int offset);
int16_t tvb_get_letohis(tvbuff_t *tvb, const int offset);

24-bit unsigned and signed integers:

uint32_t tvb_get_letoh24(tvbuff_t *tvb, const int offset);
int32_t tvb_get_letohi24(tvbuff_t *tvb, const int offset);

32-bit unsigned (uint32_t) and signed (int32_t) integers:

uint32_t tvb_get_letohl(tvbuff_t *tvb, const int offset);
int32_t tvb_get_letohil(tvbuff_t *tvb, const int offset);

40-bit unsigned and signed integers:

uint64_t tvb_get_letoh40(tvbuff_t *tvb, const int offset);
int64_t tvb_get_letohi40(tvbuff_t *tvb, const int offset);

48-bit unsigned and signed integers:

uint64_t tvb_get_letoh48(tvbuff_t *tvb, const int offset);
int64_t tvb_get_letohi48(tvbuff_t *tvb, const int offset);

56-bit unsigned and signed integers:

uint64_t tvb_get_letoh56(tvbuff_t *tvb, const int offset);
int64_t tvb_get_letohi56(tvbuff_t *tvb, const int offset);

64-bit unsigned (uint64_t) and signed (int64_t) integers:

uint64_t tvb_get_letoh64(tvbuff_t *tvb, const int offset);
int64_t tvb_get_letohi64(tvbuff_t *tvb, const int offset);

NOTE: Although each of the integer accessors above return types with
specific sizes, the returned values are subject to C's integer promotion
rules. It's often safer and more useful to use int or unsigned for 32-bit
and smaller types, and int64_t or uint64_t for 40-bit and larger types.
Just because a value occupied 16 bits on the wire or over the air
doesn't mean it will within Wireshark.

Single-precision and double-precision IEEE floating-point numbers:

float tvb_get_letohieee_float(tvbuff_t *tvb, const int offset);
double tvb_get_letohieee_double(tvbuff_t *tvb, const int offset);

Encoding-to_host-order accessors:

16-bit unsigned (uint16_t) and signed (int16_t) integers:

uint16_t tvb_get_uint16(tvbuff_t *tvb, const int offset, const unsigned encoding);
int16_t tvb_get_int16(tvbuff_t *tvb, const int offset, const unsigned encoding);

24-bit unsigned and signed integers:

uint32_t tvb_get_uint24(tvbuff_t *tvb, const int offset, const unsigned encoding);
int32_t tvb_get_int24(tvbuff_t *tvb, const int offset, const unsigned encoding);

32-bit unsigned (uint32_t) and signed (int32_t) integers:

uint32_t tvb_get_uint32(tvbuff_t *tvb, const int offset, const unsigned encoding);
int32_t tvb_get_int32(tvbuff_t *tvb, const int offset, const unsigned encoding);

40-bit unsigned and signed integers:

uint64_t tvb_get_uint40(tvbuff_t *tvb, const int offset, const unsigned encoding);
int64_t tvb_get_int40(tvbuff_t *tvb, const int offset, const unsigned encoding);

48-bit unsigned and signed integers:

uint64_t tvb_get_uint48(tvbuff_t *tvb, const int offset, const unsigned encoding);
int64_t tvb_get_int48(tvbuff_t *tvb, const int offset, const unsigned encoding);

56-bit unsigned and signed integers:

uint64_t tvb_get_uint56(tvbuff_t *tvb, const int offset, const unsigned encoding);
int64_t tvb_get_int56(tvbuff_t *tvb, const int offset, const unsigned encoding);

64-bit unsigned (uint64_t) and signed (int64_t) integers:

uint64_t tvb_get_uint64(tvbuff_t *tvb, const int offset, const unsigned encoding);
int64_t tvb_get_int64(tvbuff_t *tvb, const int offset, const unsigned encoding);

These functions were previously named tvb_get_guintXX and tvb_get_gintXX. You
can still use the old names but they have been deprecated.

Single-precision and double-precision IEEE floating-point numbers:

float tvb_get_ieee_float(tvbuff_t *tvb, const int offset, const unsigned encoding);
double tvb_get_ieee_double(tvbuff_t *tvb, const int offset, const unsigned encoding);

"encoding" should be ENC_BIG_ENDIAN for Network-to-host-order,
ENC_LITTLE_ENDIAN for Little-Endian-to-host-order, or ENC_HOST_ENDIAN
for host order.

Accessors for IPv4 and IPv6 addresses:

uint32_t tvb_get_ipv4(tvbuff_t *tvb, const int offset);
void tvb_get_ipv6(tvbuff_t *tvb, const int offset, ws_in6_addr *addr);

NOTE: IPv4 addresses are not to be converted to host byte order before
being passed to "proto_tree_add_ipv4()". You should use "tvb_get_ipv4()"
to fetch them, not "tvb_get_ntohl()" *OR* "tvb_get_letohl()" - don't,
for example, try to use "tvb_get_ntohl()", find that it gives you the
wrong answer on the PC on which you're doing development, and try
"tvb_get_letohl()" instead, as "tvb_get_letohl()" will give the wrong
answer on big-endian machines.

char *tvb_ip_to_str(wmem_allocator_t *scope, tvbuff_t *tvb, const int offset)
char *tvb_ip6_to_str(wmem_allocator_t *scope, tvbuff_t *tvb, const int offset)

Returns a null-terminated buffer containing a string with IPv4 or IPv6 Address
from the specified tvbuff, starting at the specified offset.

Accessors for GUID:

void tvb_get_ntohguid(tvbuff_t *tvb, const int offset, e_guid_t *guid);
void tvb_get_letohguid(tvbuff_t *tvb, const int offset, e_guid_t *guid);
void tvb_get_guid(tvbuff_t *tvb, const int offset, e_guid_t *guid, const unsigned encoding);

String accessors:

uint8_t *tvb_get_string_enc(wmem_allocator_t *scope, tvbuff_t *tvb, const int offset, const int length, const unsigned encoding);

Returns a null-terminated buffer allocated from the specified scope, containing
data from the specified tvbuff, starting at the specified offset, and containing
the specified length worth of characters. Reads data in the specified encoding
and produces UTF-8 in the buffer. See below for a list of input encoding values.

The buffer is allocated in the given wmem scope (see README.wmem for more
information).

uint8_t *tvb_get_stringz_enc(wmem_allocator_t *scope, tvbuff_t *tvb, const int offset, int *lengthp, const unsigned encoding);

Returns a null-terminated buffer allocated from the specified scope,
containing data from the specified tvbuff, starting at the specified
offset, and containing all characters from the tvbuff up to and
including a terminating null character in the tvbuff. Reads data in the
specified encoding and produces UTF-8 in the buffer. See below for a
list of input encoding values. "*lengthp" will be set to the length of
the string, including the terminating null.

The buffer is allocated in the given wmem scope (see README.wmem for more
information).

int tvb_get_nstringz(tvbuff_t *tvb, const int offset, const unsigned bufsize, uint8_t* buffer);
int tvb_get_nstringz0(tvbuff_t *tvb, const int offset, const unsigned bufsize, uint8_t* buffer);

Copies bufsize bytes, including the terminating NULL, to buffer. If a NULL
terminator is found before reaching bufsize, only the bytes up to and including
the NULL are copied. Returns the number of bytes copied (not including
terminating NULL), or -1 if the string was truncated in the buffer due to
not having reached the terminating NULL. In this case, the resulting
buffer is not NULL-terminated.
tvb_get_nstringz0() works like tvb_get_nstringz(), but never returns -1 since
the string is guaranteed to have a terminating NULL. If the string was truncated
when copied into buffer, a NULL is placed at the end of buffer to terminate it.

char *tvb_get_ts_23_038_7bits_string(wmem_allocator_t *scope, tvbuff_t *tvb,
                                     const int bit_offset, int no_of_chars);

tvb_get_ts_23_038_7bits_string() returns a string of a given number of
characters and encoded according to 3GPP TS 23.038 7 bits alphabet.

The buffer is allocated in the given wmem scope (see README.wmem for more
information).

Byte Array Accessors:

char *tvb_bytes_to_str(wmem_allocator_t *scope, tvbuff_t *tvb,
                       const int offset, const int len);

Formats a bunch of data from a tvbuff as bytes, returning a pointer
to the string with the data formatted as two hex digits for each byte.
The string pointed to is stored in an "wmem_alloc'd" buffer which will be freed
depending on its scope (typically pinfo->pool which is freed after the frame).
The formatted string will contain the hex digits for at most the first 16 bytes of
the data. If len is greater than 16 bytes, a trailing "..." will be added to the string.

char *tvb_bytes_to_str_punct(wmem_allocator_t *scope, tvbuff_t *tvb,
                             const int offset, const int len, const char punct);

This function is similar to tvb_bytes_to_str(...) except that 'punct' is inserted
between the hex representation of each byte.

GByteArray *tvb_get_string_bytes(tvbuff_t *tvb, const int offset, const int length,
                                 const unsigned encoding, GByteArray* bytes, int *endoff)

Given a tvbuff, an offset into the tvbuff, and a length that starts
at that offset (which may be -1 for "all the way to the end of the
tvbuff"), fetch the hex-decoded byte values of the tvbuff into the
passed-in 'bytes' array, based on the passed-in encoding. In other
words, convert from a hex-ascii string in tvbuff, into the supplied
GByteArray.

char *tvb_bcd_dig_to_wmem_packet_str(tvbuff_t *tvb, const int offset,
                                     const int len, dgt_set_t *dgt, bool skip_first);

Given a tvbuff, an offset into the tvbuff, and a length that starts
at that offset (which may be -1 for "all the way to the end of the
tvbuff"), fetch BCD encoded digits from a tvbuff starting from either
the low or high half byte, formatting the digits according to an input digit set,
if NUll a default digit set of 0-9 returning "?" for overdecadic digits will be used.
A pointer to the packet scope allocated string will be returned.
Note: a tvbuff content of 0xf is considered a 'filler' and will end the conversion.

Copying memory:
void *tvb_memcpy(tvbuff_t *tvb, void *target, const int offset, size_t length);

Copies into the specified target the specified length's worth of data
from the specified tvbuff, starting at the specified offset.

void *tvb_memdup(wmem_allocator_t *scope, tvbuff_t *tvb, const int offset, size_t length);

Returns a buffer containing a copy of the given TVB bytes. The buffer is
allocated in the given wmem scope (see README.wmem for more information).

Pointer-retrieval:
/* WARNING! Don't use this function. There is almost always a better way.
 * It's dangerous because once this pointer is given to the user, there's
 * no guarantee that the user will honor the 'length' and not overstep the
 * boundaries of the buffer. Also see the warning in the Portability section.
 */
const uint8_t *tvb_get_ptr(tvbuff_t *tvb, const int offset, const int length);

Length query:
Get amount of captured data in the buffer (which is *NOT* necessarily the
length of the packet). You probably want tvb_reported_length instead:

    unsigned tvb_captured_length(const tvbuff_t *tvb);

Get reported length of buffer:

    unsigned tvb_reported_length(const tvbuff_t *tvb);


1.4 Functions to handle columns in the traffic summary window.

The topmost pane of the main window is a list of the packets in the
capture, possibly filtered by a display filter.

Each line corresponds to a packet, and has one or more columns, as
configured by the user.

Many of the columns are handled by code outside individual dissectors;
most dissectors need only specify the value to put in the "Protocol" and
"Info" columns.

Columns are specified by COL_ values; the COL_ value for the "Protocol"
field, typically giving an abbreviated name for the protocol (but not
the all-lower-case abbreviation used elsewhere) is COL_PROTOCOL, and the
COL_ value for the "Info" field, giving a summary of the contents of the
packet for that protocol, is COL_INFO.

The value for a column can be specified with one of several functions,
all of which take the 'fd' argument to the dissector as their first
argument, and the COL_ value for the column as their second argument.

1.4.1 The col_set_str function.

'col_set_str' takes a string as its third argument, and sets the value
for the column to that value. It assumes that the pointer passed to it
points to a string constant or a static "const" array, not to a
variable, as it doesn't copy the string, it merely saves the pointer
value; the argument can itself be a variable, as long as it always
points to a string constant or a static "const" array.

It is more efficient than 'col_add_str' or 'col_add_fstr'; however, if
the dissector will be using 'col_append_str' or 'col_append_fstr" to
append more information to the column, the string will have to be copied
anyway, so it's best to use 'col_add_str' rather than 'col_set_str' in
that case.

For example, to set the "Protocol" column
to "PROTOFILTERNAME":

    col_set_str(pinfo->cinfo, COL_PROTOCOL, "PROTOFILTERNAME");


1.4.2 The col_add_str function.

'col_add_str' takes a string as its third argument, and sets the value
for the column to that value. It takes the same arguments as
'col_set_str', but copies the string, so that if the string is, for
example, an automatic variable that won't remain in scope when the
dissector returns, it's safe to use.


1.4.3 The col_add_fstr function.

'col_add_fstr' takes a 'printf'-style format string as its third
argument, and 'printf'-style arguments corresponding to '%' format
items in that string as its subsequent arguments. For example, to set
the "Info" field to "<XXX> request, <N> bytes", where "reqtype" is a
string containing the type of the request in the packet and "n" is an
unsigned integer containing the number of bytes in the request:

    col_add_fstr(pinfo->cinfo, COL_INFO, "%s request, %u bytes",
                 reqtype, n);

Don't use 'col_add_fstr' with a format argument of just "%s" -
'col_add_str', or possibly even 'col_set_str' if the string that matches
the "%s" is a static constant string, will do the same job more
efficiently.


1.4.4 The col_clear function.

If the Info column will be filled with information from the packet, that
means that some data will be fetched from the packet before the Info
column is filled in. If the packet is so small that the data in
question cannot be fetched, the routines to fetch the data will throw an
exception (see the comment at the beginning about tvbuffers improving
the handling of short packets - the tvbuffers keep track of how much
data is in the packet, and throw an exception on an attempt to fetch
data past the end of the packet, so that the dissector won't process
bogus data), causing the Info column not to be filled in.

This means that the Info column will have data for the previous
protocol, which would be confusing if, for example, the Protocol column
had data for this protocol.

Therefore, before a dissector fetches any data whatsoever from the
packet (unless it's a heuristic dissector fetching data to determine
whether the packet is one that it should dissect, in which case it
should check, before fetching the data, whether there's any data to
fetch; if there isn't, it should return false), it should set the
Protocol column and the Info column.

If the Protocol column will ultimately be set to, for example, a value
containing a protocol version number, with the version number being a
field in the packet, the dissector should, before fetching the version
number field or any other field from the packet, set it to a value
without a version number, using 'col_set_str', and should later set it
to a value with the version number after it's fetched the version
number.

If the Info column will ultimately be set to a value containing
information from the packet, the dissector should, before fetching any
fields from the packet, clear the column using 'col_clear' (which is
more efficient than clearing it by calling 'col_set_str' or
'col_add_str' with a null string), and should later set it to the real
string after it's fetched the data to use when doing that.


1.4.5 The col_append_str function.

Sometimes the value of a column, especially the "Info" column, can't be
conveniently constructed at a single point in the dissection process;
for example, it might contain small bits of information from many of the
fields in the packet. 'col_append_str' takes, as arguments, the same
arguments as 'col_add_str', but the string is appended to the end of the
current value for the column, rather than replacing the value for that
column. (Note that no blank separates the appended string from the
string to which it is appended; if you want a blank there, you must add
it yourself as part of the string being appended.)


1.4.6 The col_append_fstr function.

'col_append_fstr' is to 'col_add_fstr' as 'col_append_str' is to
'col_add_str' - it takes, as arguments, the same arguments as
'col_add_fstr', but the formatted string is appended to the end of the
current value for the column, rather than replacing the value for that
column.

1.4.7 The col_append_sep_str and col_append_sep_fstr functions.

In specific situations the developer knows that a column's value will be
created in a stepwise manner, where the appended values are listed. Both
'col_append_sep_str' and 'col_append_sep_fstr' functions will add an item
separator between two consecutive items, and will not add the separator at the
beginning of the column. The remainder of the work both functions do is
identical to what 'col_append_str' and 'col_append_fstr' do.

1.4.8 The col_set_fence and col_prepend_fence_fstr functions.

Sometimes a dissector may be called multiple times for different PDUs in the
same frame (for example in the case of SCTP chunk bundling: several upper
layer data packets may be contained in one SCTP packet). If the upper layer
dissector calls 'col_set_str()' or 'col_clear()' on the Info column when it
begins dissecting each of those PDUs then when the frame is fully dissected
the Info column would contain only the string from the last PDU in the frame.
The 'col_set_fence' function erects a "fence" in the column that prevents
subsequent 'col_...' calls from clearing the data currently in that column.
For example, the SCTP dissector calls 'col_set_fence' on the Info column
after it has called any subdissectors for that chunk so that subdissectors
of any subsequent chunks may only append to the Info column.
'col_prepend_fence_fstr' prepends data before a fence (moving it if
necessary). It will create a fence at the end of the prepended data if the
fence does not already exist.


1.4.9 The col_set_time function.

The 'col_set_time' function takes an nstime value as its third argument.
This nstime value is a relative value and will be added as such to the
column. The fourth argument is the filtername holding this value. This
way, rightclicking on the column makes it possible to build a filter
based on the time-value.

For example:

    col_set_time(pinfo->cinfo, COL_REL_TIME, &ts, "s4607.ploc.time");


1.5 Constructing the protocol tree.

The middle pane of the main window, and the topmost pane of a packet
popup window, are constructed from the "protocol tree" for a packet.

The protocol tree, or proto_tree, is an N-way tree structure of proto_nodes.
Of course the protocol dissectors don't care what a proto_tree really is;
they just pass the proto_tree pointer as an argument to the routines which
allow them to add items and new branches to the tree. (Some protocol dissectors
do access struct members directly instead of calling routines; this is
deprecated.)

When a packet is selected in the packet-list pane, or a packet popup
window is created, a new logical protocol tree (proto_tree) is created.
The pointer to the proto_tree (in this case, 'protocol tree'), is passed
to the top-level protocol dissector, and then to all subsequent protocol
dissectors for that packet, and then the GUI tree is drawn via
proto_tree_draw().

The logical proto_tree needs to know detailed information about the protocols
and fields about which information will be collected from the dissection
routines. By strictly defining (or "typing") the data that can be attached to a
proto tree, searching and filtering becomes possible. This means that for
every protocol and field (which I also call "header fields", since they are
fields in the protocol headers) which might be attached to a tree, some
information is needed.

Every dissector routine will need to register its protocols and fields
with the central protocol routines (in proto.c). At first I thought I
might keep all the protocol and field information about all the
dissectors in one file, but decentralization seemed like a better idea.
That one file would have gotten very large; one small change would have
required a re-compilation of the entire file. Also, by allowing
registration of protocols and fields at run-time, loadable modules of
protocol dissectors (perhaps even user-supplied) is feasible.

To do this, each protocol should have a register routine, which will be
called when Wireshark starts. The code to call the register routines is
generated automatically; to arrange that a protocol's register routine
be called at startup:

    the file containing a dissector's "register" routine must be
    added to "DISSECTOR_SRC" in "epan/dissectors/CMakeLists.txt";

    the "register" routine must have a name of the form
    "proto_register_XXX";

    the "register" routine must take no argument, and return no
    value;

    the "register" routine's name must appear in the source file
    either at the beginning of the line, or preceded only by "void "
    at the beginning of the line (that would typically be the
    definition) - other white space shouldn't cause a problem, e.g.:

void proto_register_XXX(void) {

    ...

}

and

void
proto_register_XXX( void )
{

    ...

}

    and so on should work.

For every protocol or field that a dissector wants to register, a variable of
type int needs to be used to keep track of the protocol. The IDs are
needed for establishing parent/child relationships between protocols and
fields, as well as associating data with a particular field so that it
can be stored in the logical tree and displayed in the GUI protocol
tree.

Some dissectors will need to create branches within their tree to help
organize header fields. These branches should be registered as header
fields. Only true protocols should be registered as protocols. This is
so that a display filter user interface knows how to distinguish
protocols from fields.

A protocol is registered with the name of the protocol and its
abbreviation.

Here is how the frame "protocol" is registered.

        int proto_frame;

        proto_frame = proto_register_protocol (
                /* name */            "Frame",
                /* short name */      "Frame",
                /* abbrev */          "frame" );

A header field is also registered with its name and abbreviation, but
information about its data type is needed. It helps to look at
the header_field_info struct to see what information is expected:

struct header_field_info {
    const char        *name;
    const char        *abbrev;
    enum ftenum        type;
    int                display;
    const void        *strings;
    uint64_t           bitmask;
    const char        *blurb;
    ...
};

name (FIELDNAME)
----------------
A string representing the name of the field. This is the name
that will appear in the graphical protocol tree. It must be a non-empty
string.

abbrev (FIELDFILTERNAME)
--------------------
A string with a filter name for the field. The name should start
with the filter name of the parent protocol followed by a period as a
separator. For example, the "src" field in an IP packet would have "ip.src"
as a filter name. It is acceptable to have multiple levels of periods if,
for example, you have fields in your protocol that are then subdivided into
subfields. For example, TRMAC has multiple error fields, so the names
follow this pattern: "trmac.errors.iso", "trmac.errors.noniso", etc.
It must be a non-empty string.

type (FIELDTYPE)
----------------
The type of value this field holds. The current field types are:

    FT_NONE     No field type. Used for fields that
                aren't given a value, and that can only
                be tested for presence or absence; a
                field that represents a data structure,
                with a subtree below it containing
                fields for the members of the structure,
                or that represents an array with a
                subtree below it containing fields for
                the members of the array, might be an
                FT_NONE field.
    FT_PROTOCOL Used for protocols which will be placing
                themselves as top-level items in the
                "Packet Details" pane of the UI.
    FT_BOOLEAN  0 means "false", any other value means
                "true".
    FT_FRAMENUM A frame number; if this is used, the "Go
                To Corresponding Frame" menu item can
                work on that field.
    FT_CHAR     An 8-bit ASCII character. It's treated similarly to an
                FT_UINT8, but is displayed as a C-style character
                constant.
    FT_UINT8    An 8-bit unsigned integer.
    FT_UINT16   A 16-bit unsigned integer.
    FT_UINT24   A 24-bit unsigned integer.
    FT_UINT32   A 32-bit unsigned integer.
    FT_UINT40   A 40-bit unsigned integer.
    FT_UINT48   A 48-bit unsigned integer.
    FT_UINT56   A 56-bit unsigned integer.
    FT_UINT64   A 64-bit unsigned integer.
    FT_INT8     An 8-bit signed integer.
    FT_INT16    A 16-bit signed integer.
    FT_INT24    A 24-bit signed integer.
    FT_INT32    A 32-bit signed integer.
    FT_INT40    A 40-bit signed integer.
    FT_INT48    A 48-bit signed integer.
    FT_INT56    A 56-bit signed integer.
    FT_INT64    A 64-bit signed integer.
    FT_IEEE_11073_SFLOAT    A 16-bit floating point number, consisting
                of an 4-bit exponent and 12-bit mantissa.
    FT_IEEE_11073_FLOAT     A 32-bit floating point number, consisting
                of an 8-bit exponent and 24-bit mantissa.
    FT_FLOAT    A single-precision floating point number.
    FT_DOUBLE   A double-precision floating point number.
    FT_ABSOLUTE_TIME    An absolute time from some fixed point in time,
                displayed as the date, followed by the time, as
                hours, minutes, and seconds with 9 digits after
                the decimal point.
    FT_RELATIVE_TIME    Seconds (4 bytes) and nanoseconds (4 bytes)
                of time relative to an arbitrary time.
                displayed as seconds and 9 digits
                after the decimal point.
    FT_STRING   A string of characters, not necessarily
                NULL-terminated, but possibly NULL-padded.
                This, and the other string-of-characters
                types, are to be used for text strings,
                not raw binary data.
    FT_STRINGZ  A NULL-terminated string of characters.
                The string length is normally the length
                given in the proto_tree_add_item() call.
                However if the length given in the call
                is -1, then the length used is that
                returned by calling tvb_strsize().
                This should only be used if the string,
                in the packet, is always terminated with
                a NULL character, either because the length
                isn't otherwise specified or because a
                character count *and* a NULL terminator are
                both used.
    FT_STRINGZPAD   A NULL-padded string of characters.
                The length is given in the proto_tree_add_item()
                call, but may be larger than the length of
                the string, with extra bytes being NULL padding.
                This is typically used for fixed-length fields
                that contain a string value that might be shorter
                than the fixed length.
    FT_STRINGZTRUNC  A NULL-truncated string of characters.
                The length is given in the proto_tree_add_item()
                call, but may be larger than the length of
                the string, with a NULL character after the last
                character of the string, and the remaining bytes
                being padding with unspecified contents.  This is
                typically used for fixed-length fields that contain
                a string value that might be shorter than the fixed
                length.
    FT_UINT_STRING  A counted string of characters, consisting
                of a count (represented as an integral value,
                of width given in the proto_tree_add_item()
                call) followed immediately by that number of
                characters.
    FT_ETHER    A six octet string displayed in
                Ethernet-address format.
    FT_BYTES    A string of bytes with arbitrary values;
                used for raw binary data.
    FT_UINT_BYTES   A counted string of bytes, consisting
                of a count (represented as an integral value,
                of width given in the proto_tree_add_item()
                call) followed immediately by that number of
                arbitrary values; used for raw binary data.
    FT_IPv4     A version 4 IP address (4 bytes) displayed
                in dotted-quad IP address format (4
                decimal numbers separated by dots).
    FT_IPv6     A version 6 IP address (16 bytes) displayed
                in standard IPv6 address format.
    FT_IPXNET   An IPX address displayed in hex as a 6-byte
                network number followed by a 6-byte station
                address.
    FT_GUID     A Globally Unique Identifier
    FT_OID      An ASN.1 Object Identifier
    FT_REL_OID  An ASN.1 Relative Object Identifier
    FT_EUI64    A EUI-64 Address
    FT_AX25     A AX-25 Address
    FT_VINES    A Vines Address
    FT_SYSTEM_ID  An OSI System-ID
    FT_FCWWN    A Fibre Channel WWN Address

Some of these field types are still not handled in the display filter
routines, but the most common ones are. The FT_UINT* variables all
represent unsigned integers, and the FT_INT* variables all represent
signed integers; the number on the end represent how many bits are used
to represent the number.

Some constraints are imposed on the header fields depending on the type
(e.g. FT_BYTES) of the field. Fields of type FT_ABSOLUTE_TIME must use
'ABSOLUTE_TIME_{LOCAL,UTC,DOY_UTC}, NULL, 0x0' as values for the
'display, 'strings', and 'bitmask' fields, and all other non-integral
types (i.e.. types that are _not_ FT_INT* and FT_UINT*) must use
'BASE_NONE, NULL, 0x0' as values for the 'display', 'strings', 'bitmask'
fields. The reason is simply that the type itself implicitly defines the
nature of 'display', 'strings', 'bitmask'.

display (FIELDDISPLAY)
----------------------
The display field has a couple of overloaded uses. This is unfortunate,
but since we're using C as an application programming language, this sometimes
makes for cleaner programs. Right now I still think that overloading
this variable was okay.

For integer fields (FT_UINT* and FT_INT*), this variable represents the
base in which you would like the value displayed. The acceptable bases
are:

    BASE_DEC,
    BASE_HEX,
    BASE_OCT,
    BASE_DEC_HEX,
    BASE_HEX_DEC,
    BASE_CUSTOM

BASE_DEC, BASE_HEX, and BASE_OCT are decimal, hexadecimal, and octal,
respectively. BASE_DEC_HEX and BASE_HEX_DEC display value in two bases
(the 1st representation followed by the 2nd in parenthesis).

BASE_CUSTOM allows one to specify a callback function pointer that will
format the value.

For 32-bit and smaller values, custom_fmt_func_t can be used to declare
the callback function pointer. Specifically, this is defined as:

    void func(char *, uint32_t);

For values larger than 32-bits, custom_fmt_func_64_t can be used to declare
the callback function pointer. Specifically, this is defined as:

    void func(char *, uint64_t);

For floating point values, custom_fmt_func_double_t can be used to declare
the callback function pointer. Specifically, this is defined as:

    void func(char *, double);

The first argument is a pointer to a buffer of the ITEM_LABEL_LENGTH size
and the second argument is the value to be formatted.

The callback function pointer types are defined in epan/proto.h.

For FT_UINT16 'display' can be used to select a transport layer protocol using one
of BASE_PT_UDP, BASE_PT_TCP, BASE_PT_DCCP or BASE_PT_SCTP. If transport name
resolution is enabled the port field label is displayed in decimal and as a well-known
service name (if one is available).

For FT_BOOLEAN fields that are also bitfields (i.e., 'bitmask' is non-zero),
'display' is used specify a "field-width" (i.e., tell the proto_tree how
wide the parent bitfield is). (If the FT_BOOLEAN 'bitmask' is zero, then
'display' must be BASE_NONE).

For integer fields a "field-width" is not needed since the type of
integer itself (FT_UINT8, FT_UINT16, FT_UINT24, FT_UINT32, FT_UINT40,
FT_UINT48, FT_UINT56, FT_UINT64, etc) tells the proto_tree how wide the
parent bitfield is. The same is true of FT_CHAR, as it's an 8-bit
character.

For FT_ABSOLUTE_TIME fields, 'display' is used to indicate whether the
time is to be displayed as a time in the time zone for the machine on
which Wireshark/TShark is running or as UTC and, for UTC, whether the
date should be displayed as "{monthname} {day_of_month}, {year}" or as
"{year/day_of_year}".

Additionally, BASE_NONE is used for 'display' as a NULL-value. That is, for
non-integers other than FT_ABSOLUTE_TIME fields, and non-bitfield
FT_BOOLEANs, you'll want to use BASE_NONE in the 'display' field. You may
not use BASE_NONE for integers.

It is possible that in the future we will record the endianness of
integers. If so, it is likely that we'll use a bitmask on the display field
so that integers would be represented as BEND|BASE_DEC or LEND|BASE_HEX.
But that has not happened yet; note that there are protocols for which
no endianness is specified, such as the X11 protocol and the DCE RPC
protocol, so it would not be possible to record the endianness of all
integral fields.

strings (FIELDCONVERT)
----------------------
-- value_string
Some integer fields, of type FT_UINT*, need labels to represent the true
value of a field. You could think of those fields as having an
enumerated data type, rather than an integral data type.

A 'value_string' structure is a way to map values to strings.

    typedef struct _value_string {
        uint32_t value;
        char    *strptr;
    } value_string;

For fields of that type, you would declare an array of "value_string"s:

    static const value_string valstringname[] = {
        { INTVAL1, "Descriptive String 1" },
        { INTVAL2, "Descriptive String 2" },
        { 0,       NULL }
    };

(the last entry in the array must have a NULL 'strptr' value, to
indicate the end of the array). The 'strings' field would be set to
'VALS(valstringname)'.

If the field has a numeric rather than an enumerated type, the 'strings'
field would be set to NULL.

If BASE_SPECIAL_VALS is also applied to the display bitmask, then if the
numeric value of a field doesn't match any values in the value_string
then just the numeric value is displayed (i.e. no "Unknown"). This is
intended for use when the value_string only gives special names for
certain field values and values not in the value_string are expected.

-- Extended value strings
You can also use an extended version of the value_string for faster lookups.
It requires a value_string array as input.
If all of a contiguous range of values from min to max are present in the array
in ascending order the value will be used as a direct index into a value_string array.

If the values in the array are not contiguous (ie: there are "gaps"), but are
in ascending order a binary search will be used.

Note: "gaps" in a value_string array can be filled with "empty" entries eg:
{value, "Unknown"} so that direct access to the array is possible.

Note: the value_string array values are *unsigned*; IOW: -1 is greater than 0.
      So:
      { -2, -1,  1,  2 }; wrong:   linear search will be used (note gap)
      {  1,  2, -2, -1 }; correct: binary search will be used

      As a special case:
      { -2, -1,  0,  1,  2 }; OK: direct(indexed) access will be used (note no gap)

The init macro (see below) will perform a check on the value string the first
time it is used to determine which search algorithm fits and fall back to a
linear search if the value_string does not meet the criteria above.

Use this macro to initialize the extended value_string at compile time:

static value_string_ext valstringname_ext = VALUE_STRING_EXT_INIT(valstringname);

Extended value strings can be created at run time by calling
   value_string_ext_new(<wmem scope for the value_string, most likely wmem_epan_scope()>
                        <ptr to value_string array>,
                        <total number of entries in the value_string_array>, /* include {0, NULL} entry */
                        <value_string_name>);

For hf[] array FT_(U)INT* fields that need a 'valstringname_ext' struct, the
'strings' field would be set to '&valstringname_ext'. Furthermore, the 'display'
field must be ORed with 'BASE_EXT_STRING' (e.g. BASE_DEC|BASE_EXT_STRING).

-- val64_string

val64_strings are like value_strings, except that the integer type
used is a uint64_t (instead of uint32_t). Instead of using the VALS()
macro for the 'strings' field in the header_field_info struct array,
'VALS64()' is used.

BASE_SPECIAL_VALS can also be used for val64_string.

-- val64_string_ext

val64_string_ext is like value_string_ext, except that the integer type
used is a uint64_t (instead of uint32_t).

Use this macro to initialize the extended val64_string at compile time:

static val64_string_ext val64stringname_ext = VAL64_STRING_EXT_INIT(val64stringname);

Extended val64 strings can be created at run time by calling
   val64_string_ext_new(<wmem scope for the value_string, most likely wmem_epan_scope()>
                        <ptr to val64_string array>,
                        <total number of entries in the val64_string_array>, /* include {0, NULL} entry */
                        <val64_string_name>);

For hf[] array FT_(U)INT* fields that need a 'val64stringname_ext' struct, the
'strings' field would be set to '&val64stringname_ext'. Furthermore, the 'display'
field must be ORed with both 'BASE_EXT_STRING' and 'BASE_VAL64_STRING'
(e.g. BASE_DEC|BASE_EXT_STRING|BASE_VAL64_STRING).

-- Unit string
Some integer fields, of type FT_UINT* and float fields, of type FT_FLOAT
or FT_DOUBLE, need units of measurement to help convey the field value.

A 'unit_name_string' structure is a way to add a unit suffix to a field.

    typedef struct unit_name_string {
        char *singular;     /* name to use for 1 unit */
        char *plural;       /* name to use for < 1 or > 1 units */
    } unit_name_string;

For fields with that unit name, you would declare a "unit_name_string":

    static const unit_name_string unitname[] =
        { "single item name" , "multiple item name" };

(the second entry can be NULL if there is no plural form of the unit name.
This is typically the case when abbreviations are used instead of full words.)

For hf[] array FT_(U)INT*, FT_FLOAT and FT_DOUBLE fields that need a
'unit_name_string' struct, the 'strings' field would be set to
'&unitname'. Furthermore, the 'display' field must be ORed
with 'BASE_UNIT_STRING' (e.g. BASE_DEC|BASE_UNIT_STRING).

There are several "common" unit name structures already defined in
epan/unit_strings.h, e.g. 'units_second_seconds'. Dissector authors may choose
to add the unit name structure there rather than locally in a dissector.

-- Ranges
If the field has a numeric type that might logically fit in ranges of values
one can use a range_string struct.

Thus a 'range_string' structure is a way to map ranges to strings.

    typedef struct _range_string {
        uint32_t       value_min;
        uint32_t       value_max;
        const char    *strptr;
    } range_string;

For fields of that type, you would declare an array of "range_string"s:

    static const range_string rvalstringname[] = {
        { INTVAL_MIN1, INTVALMAX1, "Descriptive String 1" },
        { INTVAL_MIN2, INTVALMAX2, "Descriptive String 2" },
        { 0,           0,          NULL                   }
    };

If INTVAL_MIN equals INTVAL_MAX for a given entry the range_string
behavior collapses to the one of value_string. Note that each range_string
within the array is tested in order, so any 'catch-all' entries need to come
after specific individual entries.

For FT_(U)INT* fields that need a 'range_string' struct, the 'strings' field
would be set to 'RVALS(rvalstringname)'. Furthermore, 'display' field must be
ORed with 'BASE_RANGE_STRING' (e.g. BASE_DEC|BASE_RANGE_STRING).

-- Booleans
FT_BOOLEANs have a default map of 0 = "False", 1 (or anything else) = "True".
Sometimes it is useful to change the labels for boolean values (e.g.,
to "Yes"/"No", "Fast"/"Slow", etc.). For these mappings, a struct called
true_false_string is used.

    typedef struct true_false_string {
        char    *true_string;
        char    *false_string;
    } true_false_string;

For Boolean fields for which "False" and "True" aren't the desired
labels, you would declare a "true_false_string"s:

    static const true_false_string boolstringname = {
        "String for True",
        "String for False"
    };

Its two fields are pointers to the string representing truth, and the
string representing falsehood. For FT_BOOLEAN fields that need a
'true_false_string' struct, the 'strings' field would be set to
'TFS(&boolstringname)'.

If the Boolean field is to be displayed as "False" or "True", the
'strings' field would be set to NULL.

Wireshark predefines a whole range of ready made "true_false_string"s
in tfs.h, included via packet.h.

-- Custom
Custom fields (BASE_CUSTOM) should use CF_FUNC(&custom_format_func) for the
'strings' field.

-- Frame numbers
FT_FRAMENUMs can use the 'strings' field to indicate their purpose by
setting the field to 'FRAMENUM_TYPE(x)', where x is one of the values of
the ft_framenum_type enum:

    FT_FRAMENUM_NONE
    FT_FRAMENUM_REQUEST
    FT_FRAMENUM_RESPONSE
    FT_FRAMENUM_ACK
    FT_FRAMENUM_DUP_ACK
    FT_FRAMENUM_RETRANS_PREV
    FT_FRAMENUM_RETRANS_NEXT

The packet list uses the value to determine the related packet symbol to draw.
Note that 'strings' field NULL is equal to FRAMENUM_TYPE(FT_FRAMENUM_NONE).

-- Note to plugin authors
Data cannot get exported from DLLs. For this reason plugin authors cannot use
existing fieldconvert strings (e.g. from existing dissectors or those from
epan/unit_strings.h). Plugins must define value_strings, unit_name_strings,
range_strings and true_false_strings locally.

bitmask (BITMASK)
-----------------
If the field is a bitfield, then the bitmask is the mask which will
leave only the bits needed to make the field when ANDed with a value.
The proto_tree routines will calculate 'bitshift' automatically
from 'bitmask', by finding the rightmost set bit in the bitmask.
This shift is applied before applying string mapping functions or
filtering.

If the field is not a bitfield, then bitmask should be set to 0.

blurb (FIELDDESCR)
------------------
This is a string giving a proper description of the field. It should be
at least one grammatically complete sentence, or NULL in which case the
name field is used. (Please do not use "").

It is meant to provide a more detailed description of the field than the
name alone provides. This information will be used in the man page, and
in a future GUI display-filter creation tool. We might also add tooltips
to the labels in the GUI protocol tree, in which case the blurb would
be used as the tooltip text.


1.5.1 Field Registration.

Protocol registration is handled by creating an instance of the
hf_register_info struct (or an array of such structs), and
calling the registration function along with the registration ID of
the protocol that is the parent of the fields. Here is a complete example:

    static int proto_eg;
    static int hf_field_a;
    static int hf_field_b;

    static hf_register_info hf[] = {
        { &hf_field_a,
            { "Field A", "proto.field_a", FT_UINT8, BASE_HEX, NULL,
                0xf0, "Field A represents Apples", HFILL }
        },

        { &hf_field_b,
            { "Field B", "proto.field_b", FT_UINT16, BASE_DEC, VALS(vs),
                0x0, "Field B represents Bananas", HFILL }
        }
    };

    proto_eg = proto_register_protocol("Example Protocol",
        "PROTO", "proto");
    proto_register_field_array(proto_eg, hf, array_length(hf));

Be sure that your array of hf_register_info structs is declared 'static',
since the proto_register_field_array() function does not create a copy
of the information in the array... it uses that static copy of the
information that the compiler created inside your array. Here's the
layout of the hf_register_info struct:

typedef struct hf_register_info {
    int               *p_id; /* pointer to parent variable */
    header_field_info  hfinfo;
} hf_register_info;

Also be sure to use the handy array_length() macro found in packet.h
to have the compiler compute the array length for you at compile time.

If you don't have any fields to register, do *NOT* create a zero-length
"hf" array; not all compilers used to compile Wireshark support them.
Just omit the "hf" array, and the "proto_register_field_array()" call,
entirely.

It is OK to have header fields with a different format be registered with
the same abbreviation. For instance, the following is valid:

    static hf_register_info hf[] = {
        { &hf_field_8bit, /* 8-bit version of proto.field */
            { "Field (8 bit)", "proto.field", FT_UINT8, BASE_DEC, NULL,
                0x00, "Field represents FOO", HFILL }
        },

        { &hf_field_32bit, /* 32-bit version of proto.field */
            { "Field (32 bit)", "proto.field", FT_UINT32, BASE_DEC, NULL,
                0x00, "Field represents FOO", HFILL }
        }
    };

This way a filter expression can match a header field, irrespective of the
representation of it in the specific protocol context. This is interesting
for protocols with variable-width header fields.

Note that the formats used must all belong to the same group as defined below:
- FT_INT8, FT_INT16, FT_INT24 and FT_INT32, FT_CHAR, FT_UINT8, FT_UINT16,
  FT_UINT24, FT_UINT32, FT_IPXNET, FT_FRAMENUM, FT_INT40, FT_INT48, FT_INT56,
  FT_INT64, FT_UINT40, FT_UINT48, FT_UINT56 and FT_UINT64
- FT_ABSOLUTE_TIME and FT_RELATIVE_TIME
- FT_STRING, FT_STRINGZ, FT_UINT_STRING, FT_STRINGZPAD, FT_STRINGZTRUNC and FT_AX25
- FT_FLOAT, FT_DOUBLE, FT_IEEE_11073_SFLOAT and FT_IEEE_11073_FLOAT
- FT_BYTES, FT_UINT_BYTES, FT_ETHER, FT_VINES, FT_FCWWN and FT_EUI64
- FT_OID, FT_REL_OID and FT_SYSTEM_ID

Any field not in a grouping above should *NOT* be used in duplicate field
abbreviations. The current code does not prevent it, but someday in the future
it might.

The HFILL macro at the end of the struct will set reasonable default values
for internally used fields.

1.5.2 Adding Items and Values to the Protocol Tree.

A protocol item is added to an existing protocol tree with one of a
handful of proto_XXX_DO_YYY() functions.

Subtrees can be made with the proto_item_add_subtree() function:

    item = proto_tree_add_item(...);
    new_tree = proto_item_add_subtree(item, tree_type);

This will add a subtree under the item in question; a subtree can be
created under an item made by any of the "proto_tree_add_XXX" functions,
so that the tree can be given an arbitrary depth.

Subtree types are integers, assigned by
"proto_register_subtree_array()". To register subtree types, pass an
array of pointers to "int" variables to hold the subtree type values to
"proto_register_subtree_array()":

    static int ett_eg;
    static int ett_field_a;

    static int *ett[] = {
        &ett_eg,
        &ett_field_a
    };

    proto_register_subtree_array(ett, array_length(ett));

in your "register" routine, just as you register the protocol and the
fields for that protocol.

The ett_ variables identify particular type of subtree so that if you expand
one of them, Wireshark keeps track of that and, when you click on
another packet, it automatically opens all subtrees of that type.
If you close one of them, all subtrees of that type will be closed when
you move to another packet.

There are many functions that the programmer can use to add either
protocol or field labels to the proto_tree, for example:

    proto_item *
    proto_tree_add_item(tree, id, tvb, start, length, encoding);

    proto_item *
    proto_tree_add_item_ret_int(tree, id, tvb, start, length, encoding,
        *retval);

    proto_item *
    proto_tree_add_subtree(tree, tvb, start, length, idx, tree_item,
        text);

    proto_item *
    proto_tree_add_int_format_value(tree, id, tvb, start, length,
        value, format, ...);

    proto_item *
    proto_tree_add_checksum(proto_tree *tree, tvbuff_t *tvb, const unsigned offset,
        const int hf_checksum, const int hf_checksum_status,
        struct expert_field* bad_checksum_expert, packet_info *pinfo,
        uint32_t computed_checksum, const unsigned encoding, const unsigned flags);

    proto_item *
    proto_tree_add_bitmask(tree, tvb, start, header, ett, fields,
        encoding);

    proto_item *
    proto_tree_add_bits_item(tree, id, tvb, bit_offset, no_of_bits,
        encoding);

The 'tree' argument is the tree to which the item is to be added. The
'tvb' argument is the tvbuff from which the item's value is being
extracted; the 'start' argument is the offset from the beginning of that
tvbuff of the item being added, and the 'length' argument is the length,
in bytes, of the item, bit_offset is the offset in bits and no_of_bits
is the length in bits.

The length of some items cannot be determined until the item has been
dissected; to add such an item, add it with a length of -1, and, when the
dissection is complete, set the length with 'proto_item_set_len()':

    void
    proto_item_set_len(ti, length);

The "ti" argument is the value returned by the call that added the item
to the tree, and the "length" argument is the length of the item.

All available protocol tree functions are declared in epan/proto.h, with
their documentation. The details of these functions and their parameters
are described below.

proto_tree_add_item()
---------------------
proto_tree_add_item is used when you wish to do no special formatting.
The item added to the GUI tree will contain the name (as passed in the
proto_register_*() function) and a value. The value will be fetched
from the tvbuff by proto_tree_add_item(), based on the type of the field
and the encoding of the value as specified by the "encoding" argument.

For FT_NONE, FT_BYTES, FT_ETHER, FT_IPv6, FT_IPXNET, FT_OID, FT_REL_OID,
FT_AX25, FT_VINES, FT_SYSTEM_ID, FT_FCWWN fields, and 'protocol' fields
the encoding is not relevant; the 'encoding' argument should be
ENC_NA (Not Applicable).

For FT_UINT_BYTES fields, the byte order of the count must be specified
as well as the 'encoding' for bytes which should be ENC_NA,
i.e. ENC_LITTLE_ENDIAN|ENC_NA

For integral, floating-point, Boolean, FT_GUID, and FT_EUI64 fields,
the encoding specifies the byte order of the value; the 'encoding'
argument should be ENC_LITTLE_ENDIAN if the value is little-endian
and ENC_BIG_ENDIAN if it is big-endian.

For FT_IPv4 fields, the encoding also specifies the byte order of the
value. In almost all cases, the encoding is in network byte order,
hence ENC_BIG_ENDIAN, but in at least one protocol dissected by Wireshark,
at least one IPv4 address is byte-swapped, so it's in little-endian
order, i.e. ENC_LITTLE_ENDIAN.

For string fields, the encoding specifies the character set used for the
string and the way individual code points in that character set are
encoded. For FT_UINT_STRING fields, the byte order of the count must be
specified. For UTF-16, UCS-2, and UCS-4, the byte order of the encoding
must be specified, and optionally ENC_BOM can also be indicated to detect
an initial BYTE ORDER MARK (the specified value is used if the field does
not begin with a BOM.) For counted UTF-16, UCS-2, and UCS-4 strings, the
byte order of the count and the characters in the string must be the same,
unless a BOM overrides the value for the characters. In other cases the
string encoding has no endianness or the endianness is implicitly specified
and nothing should be used. The character encodings that are currently
supported are:

    ENC_ASCII - ASCII (currently treated as UTF-8; in the future,
        all bytes with the 8th bit set will be treated as
        errors)
    ENC_UTF_8 - UTF-8-encoded Unicode
    ENC_UTF_16 - UTF-16-encoded Unicode, with surrogate pairs
    ENC_UCS_2 - UCS-2-encoded subset of Unicode, with no surrogate pairs
        and thus no code points above 0xFFFF
    ENC_UCS_4 - UCS-4-encoded Unicode (aka UTF-32)
    ENC_WINDOWS_1250 - Windows-1250 code page
    ENC_WINDOWS_1251 - Windows-1251 code page
    ENC_WINDOWS_1252 - Windows-1252 code page
    ENC_ISO_646_BASIC - ISO 646 "basic code table"
    ENC_ISO_8859_1 - ISO 8859-1
    ENC_ISO_8859_2 - ISO 8859-2
    ENC_ISO_8859_3 - ISO 8859-3
    ENC_ISO_8859_4 - ISO 8859-4
    ENC_ISO_8859_5 - ISO 8859-5
    ENC_ISO_8859_6 - ISO 8859-6
    ENC_ISO_8859_7 - ISO 8859-7
    ENC_ISO_8859_8 - ISO 8859-8
    ENC_ISO_8859_9 - ISO 8859-9
    ENC_ISO_8859_10 - ISO 8859-10
    ENC_ISO_8859_11 - ISO 8859-11
    ENC_ISO_8859_13 - ISO 8859-13
    ENC_ISO_8859_14 - ISO 8859-14
    ENC_ISO_8859_15 - ISO 8859-15
    ENC_ISO_8859_16 - ISO 8859-16
    ENC_3GPP_TS_23_038_7BITS - GSM 7 bits alphabet as described
        in 3GPP TS 23.038
    ENC_3GPP_TS_23_038_7BITS_UNPACKED - GSM 7 bits alphabet where each
        7 bit character occupies a distinct octet
    ENC_ETSI_TS_102_221_ANNEX_A - Coding scheme for SIM cards with GSM 7 bit
        alphabet, UCS-2 characters, or a mixture of the two as described
        in ETSI TS 102 221 Annex A
    ENC_EBCDIC - EBCDIC
    ENC_EBCDIC_CP037 - EBCDIC code page 037
    ENC_EBCDIC_CP500 - EBCDIC code page 500
    ENC_MAC_ROMAN - MAC ROMAN
    ENC_CP437 - DOS code page 437
    ENC_CP855 - DOS code page 855
    ENC_CP866 - DOS code page 866
    ENC_ASCII_7BITS - 7 bits ASCII
    ENC_T61 - ITU T.61
    ENC_BCD_DIGITS_0_9 - packed BCD (one digit per nibble), digits 0-9
    ENC_KEYPAD_ABC_TBCD - keypad-with-a/b/c "telephony packed BCD" = 0-9, *, #, a, b, c
    ENC_KEYPAD_BC_TBCD - keypad-with-B/C "telephony packed BCD" = 0-9, B, C, *, #
    ENC_GB18030 - GB 18030
    ENC_EUC_KR - EUC-KR
    ENC_DECT_STANDARD_8BITS - DECT standard 8 bit character set as defined in
        ETSI EN 300 175-5
    ENC_DECT_STANDARD_4BITS_TBCD - DECT standard 4 bit character set "telephony
        packet BCD" = 0-9, 0xb = SPACE

Other encodings will be added in the future.

For FT_ABSOLUTE_TIME fields, the encoding specifies the form in which
the time stamp is specified, as well as its byte order. The time stamp
encodings that are currently supported are:

    ENC_TIME_SECS_NSECS - 8, 12, or 16 bytes. For 8 bytes, the first 4
        bytes are seconds and the next 4 bytes are nanoseconds; for 12
        bytes, the first 8 bytes are seconds and the next 4 bytes are
        nanoseconds; for 16 bytes, the first 8 bytes are seconds and
        the next 8 bytes are nanoseconds. The seconds are seconds
        since the UN*X epoch (1970-01-01 00:00:00 UTC). (I.e., a UN*X
        struct timespec with a 4-byte or 8-byte time_t or a structure
        with an 8-byte time_t and an 8-byte nanoseconds field.)

    ENC_TIME_NTP - 8 bytes; the first 4 bytes are seconds since the NTP
        epoch (1900-01-01 00:00:00 GMT) and the next 4 bytes are 1/2^32's of
        a second since that second. (I.e., a 64-bit count of 1/2^32's of a
        second since the NTP epoch, with the upper 32 bits first and the
        lower 32 bits second, even when little-endian.)

    ENC_TIME_TOD - 8 bytes, as a count of microseconds since the System/3x0
        and z/Architecture epoch (1900-01-01 00:00:00 GMT).

    ENC_TIME_RTPS - 8 bytes; the first 4 bytes are seconds since the UN*X
        epoch and the next 4 bytes are 1/2^32's of a second since that
        second. (I.e., it's the offspring of a mating between UN*X time and
        NTP time). It's used by the Object Management Group's Real-Time
        Publish-Subscribe Wire Protocol for the Data Distribution Service.

    ENC_TIME_SECS_USECS - 8 bytes; the first 4 bytes are seconds since the
        UN*X epoch and the next 4 bytes are microseconds since that
        second. (I.e., a UN*X struct timeval with a 4-byte time_t.)

    ENC_TIME_SECS - 4 to 8 bytes, representing a value in seconds since
        the UN*X epoch.

    ENC_TIME_MSECS - 6 to 8 bytes, representing a value in milliseconds
        since the UN*X epoch.

    ENC_TIME_USECS - 8 bytes, representing a value in microseconds since
        the UN*X epoch.

    ENC_TIME_NSECS - 8 bytes, representing a value in nanoseconds since
        the UN*X epoch.

    ENC_TIME_SECS_NTP - 4 bytes, representing a count of seconds since
        the NTP epoch.

    ENC_TIME_RFC_3971 - 8 bytes, representing a count of 1/64ths of a
        second since the UN*X epoch; see section 5.3.1 "Timestamp Option"
        in RFC 3971.

    ENC_TIME_MSEC_NTP - 4-8 bytes, representing a count of milliseconds since
        the NTP epoch.

    ENC_TIME_MIP6 - 8 bytes; the first 48 bits are seconds since the UN*X epoch
        and the remaining 16 bits indicate the number of 1/65536's of a second
        since that second.

    ENC_TIME_CLASSIC_MAC_OS_SECS - 4-8 bytes, representing a count of seconds
        since January 1, 1904, 00:00:00 UTC.

For FT_RELATIVE_TIME fields, the encoding specifies the form in which
the time stamp is specified, as well as its byte order. The time stamp
encodings that are currently supported are:

    ENC_TIME_SECS_NSECS - 8, 12, or 16 bytes. For 8 bytes, the first 4
        bytes are seconds and the next 4 bytes are nanoseconds; for 12
        bytes, the first 8 bytes are seconds and the next 4 bytes are
        nanoseconds; for 16 bytes, the first 8 bytes are seconds and
        the next 8 bytes are nanoseconds.

    ENC_TIME_SECS_USECS - 8 bytes; the first 4 bytes are seconds and the
        next 4 bytes are microseconds.

    ENC_TIME_SECS - 4 to 8 bytes, representing a value in seconds.

    ENC_TIME_MSECS - 6 to 8 bytes, representing a value in milliseconds.

    ENC_TIME_USECS - 8 bytes, representing a value in microseconds.

    ENC_TIME_NSECS - 8 bytes, representing a value in nanoseconds.

For other types, there is no support for proto_tree_add_item().

Now that definitions of fields have detailed information about bitfield
fields, you can use proto_tree_add_item() with no extra processing to
add bitfield values to your tree. Here's an example. Take the Format
Identifier (FID) field in the Transmission Header (TH) portion of the SNA
protocol. The FID is the high nibble of the first byte of the TH. The
FID would be registered like this:

    name        = "Format Identifier"
    abbrev      = "sna.th.fid"
    type        = FT_UINT8
    display     = BASE_HEX
    strings     = sna_th_fid_vals
    bitmask     = 0xf0

The bitmask contains the value which would leave only the FID if bitwise-ANDed
against the parent field, the first byte of the TH.

The code to add the FID to the tree would be;

    proto_tree_add_item(bf_tree, hf_sna_th_fid, tvb, offset, 1,
        ENC_BIG_ENDIAN);

The definition of the field already has the information about bitmasking
and bitshifting, so it does the work of masking and shifting for us!
This also means that you no longer have to create value_string structs
with the values bitshifted. The value_string for FID looks like this,
even though the FID value is actually contained in the high nibble.
(You'd expect the values to be 0x0, 0x10, 0x20, etc.)

/* Format Identifier */
static const value_string sna_th_fid_vals[] = {
    { 0x0, "SNA device <--> Non-SNA Device" },
    { 0x1, "Subarea Node <--> Subarea Node" },
    { 0x2, "Subarea Node <--> PU2" },
    { 0x3, "Subarea Node or SNA host <--> Subarea Node" },
    { 0x4, "?" },
    { 0x5, "?" },
    { 0xf, "Adjacent Subarea Nodes" },
    { 0, NULL }
};

The final implication of this is that display filters work the way you'd
naturally expect them to. You'd type "sna.th.fid == 0xf" to find Adjacent
Subarea Nodes. The user does not have to shift the value of the FID to
the high nibble of the byte ("sna.th.fid == 0xf0") as was necessary
in the past.

proto_tree_add_item_ret_XXX()
------------------------------
proto_tree_add_item_ret_XXX is used when you want the displayed value returned
for further processing only integer and unsigned integer types up to 32 bits are
supported usage of proper FT_ is checked.

proto_tree_add_XXX_item()
---------------------
proto_tree_add_XXX_item is used when you wish to do no special formatting,
but also either wish for the retrieved value from the tvbuff to be handed
back (to avoid doing tvb_get_...), and/or wish to have the value be decoded
from the tvbuff in a string-encoded format.

The item added to the GUI tree will contain the name (as passed in the
proto_register_*() function) and a value. The value will be fetched
from the tvbuff, based on the type of the XXX name and the encoding of
the value as specified by the "encoding" argument.

This function retrieves the value even if the passed-in tree param is NULL,
so that it can be used by dissectors at all times to both get the value
and set the tree item to it.

Like other proto_tree_add functions, if there is a tree and the value cannot
be decoded from the tvbuff, then an expert info error is reported. For string
encoding, this means that a failure to decode the hex value from the string
results in an expert info error being added to the tree.

For string-decoding, the passed-in encoding argument needs to specify the
string encoding (e.g., ENC_ASCII, ENC_UTF_8) as well as the format. For
some XXX types, the format is constrained - for example for the encoding format
for proto_tree_add_time_item() can only be one of the ENC_ISO_8601_* ones
or ENC_IMF_DATE_TIME. For proto_tree_add_bytes_item() if ENC_STR_HEX is used
it must be bit-or'ed with one or more of the ENC_SEP_* separator types.

proto_tree_add_protocol_format()
--------------------------------
proto_tree_add_protocol_format is used to add the top-level item for the
protocol when the dissector routine wants complete control over how the
field and value will be represented on the GUI tree. The ID value for
the protocol is passed in as the "id" argument; the rest of the
arguments are a "printf"-style format and any arguments for that format.
The caller must include the name of the protocol in the format; it is
not added automatically as in proto_tree_add_item().

proto_tree_add_none_format()
----------------------------
proto_tree_add_none_format is used to add an item of type FT_NONE.
The caller must include the name of the field in the format; it is
not added automatically as in proto_tree_add_item().

proto_tree_add_bytes()
proto_tree_add_time()
proto_tree_add_ipxnet()
proto_tree_add_ipv4()
proto_tree_add_ipv6()
proto_tree_add_ether()
proto_tree_add_string()
proto_tree_add_boolean()
proto_tree_add_float()
proto_tree_add_double()
proto_tree_add_uint()
proto_tree_add_uint64()
proto_tree_add_int()
proto_tree_add_int64()
proto_tree_add_guid()
proto_tree_add_oid()
proto_tree_add_eui64()
------------------------
These routines are used to add items to the protocol tree if either:

    the value of the item to be added isn't just extracted from the
    packet data, but is computed from data in the packet;

    the value was fetched into a variable.

The 'value' argument has the value to be added to the tree.

NOTE: in all cases where the 'value' argument is a pointer, a copy is
made of the object pointed to; if you have dynamically allocated a
buffer for the object, that buffer will not be freed when the protocol
tree is freed - you must free the buffer yourself when you don't need it
any more.

For proto_tree_add_bytes(), the 'value_ptr' argument is a pointer to a
sequence of bytes.


proto_tree_add_bytes_with_length() is similar to proto_tree_add_bytes,
except that the length is not derived from the tvb length. Instead,
the displayed data size is controlled by 'ptr_length'.

For proto_tree_add_bytes_format() and proto_tree_add_bytes_format_value(), the
'value_ptr' argument is a pointer to a sequence of bytes or NULL if the bytes
should be taken from the given TVB using the given offset and length.

For proto_tree_add_time(), the 'value_ptr' argument is a pointer to an
"nstime_t", which is a structure containing the time to be added; it has
'secs' and 'nsecs' members, giving the integral part and the fractional
part of a time in units of seconds, with 'nsecs' being the number of
nanoseconds. For absolute times, "secs" is a UNIX-style seconds since
January 1, 1970, 00:00:00 GMT value.

For proto_tree_add_ipxnet(), the 'value' argument is a 32-bit IPX
network address.

For proto_tree_add_ipv4(), the 'value' argument is a 32-bit IPv4
address, in network byte order.

For proto_tree_add_ipv6(), the 'value_ptr' argument is a pointer to a
128-bit IPv6 address.

For proto_tree_add_ether(), the 'value_ptr' argument is a pointer to a
48-bit MAC address.

For proto_tree_add_string(), the 'value_ptr' argument is a pointer to a
text string; this string must be NULL terminated even if the string in the
TVB is not (as may be the case with FT_STRINGs).

For proto_tree_add_boolean(), the 'value' argument is a 32-bit integer.
It is masked and shifted as defined by the field info after which zero
means "false", and non-zero means "true".

For proto_tree_add_float(), the 'value' argument is a 'float' in the
host's floating-point format.

For proto_tree_add_double(), the 'value' argument is a 'double' in the
host's floating-point format.

For proto_tree_add_uint(), the 'value' argument is a 32-bit unsigned
integer value, in host byte order. (This routine cannot be used to add
64-bit integers.)

For proto_tree_add_uint64(), the 'value' argument is a 64-bit unsigned
integer value, in host byte order.

For proto_tree_add_int(), the 'value' argument is a 32-bit signed
integer value, in host byte order. (This routine cannot be used to add
64-bit integers.)

For proto_tree_add_int64(), the 'value' argument is a 64-bit signed
integer value, in host byte order.

For proto_tree_add_guid(), the 'value_ptr' argument is a pointer to an
e_guid_t structure.

For proto_tree_add_oid(), the 'value_ptr' argument is a pointer to an
ASN.1 Object Identifier.

For proto_tree_add_eui64(), the 'value' argument is a 64-bit unsigned
integer value, in host byte order.

proto_tree_add_bytes_format()
proto_tree_add_time_format()
proto_tree_add_ipxnet_format()
proto_tree_add_ipv4_format()
proto_tree_add_ipv6_format()
proto_tree_add_ether_format()
proto_tree_add_string_format()
proto_tree_add_boolean_format()
proto_tree_add_float_format()
proto_tree_add_double_format()
proto_tree_add_uint_format()
proto_tree_add_uint64_format()
proto_tree_add_int_format()
proto_tree_add_int64_format()
proto_tree_add_guid_format()
proto_tree_add_oid_format()
proto_tree_add_eui64_format()
----------------------------
These routines are used to add items to the protocol tree when the
dissector routine wants complete control over how the field and value
will be represented on the GUI tree. The argument giving the value is
the same as the corresponding proto_tree_add_XXX() function; the rest of
the arguments are a "printf"-style format and any arguments for that
format. The caller must include the name of the field in the format; it
is not added automatically as in the proto_tree_add_XXX() functions.

proto_tree_add_bytes_format_value()
proto_tree_add_time_format_value()
proto_tree_add_ipxnet_format_value()
proto_tree_add_ipv4_format_value()
proto_tree_add_ipv6_format_value()
proto_tree_add_ether_format_value()
proto_tree_add_string_format_value()
proto_tree_add_boolean_format_value()
proto_tree_add_float_format_value()
proto_tree_add_double_format_value()
proto_tree_add_uint_format_value()
proto_tree_add_uint64_format_value()
proto_tree_add_int_format_value()
proto_tree_add_int64_format_value()
proto_tree_add_guid_format_value()
proto_tree_add_oid_format_value()
proto_tree_add_eui64_format_value()
------------------------------------

These routines are used to add items to the protocol tree when the
dissector routine wants complete control over how the value will be
represented on the GUI tree. The argument giving the value is the same
as the corresponding proto_tree_add_XXX() function; the rest of the
arguments are a "printf"-style format and any arguments for that format.
With these routines, unlike the proto_tree_add_XXX_format() routines,
the name of the field is added automatically as in the
proto_tree_add_XXX() functions; only the value is added with the format.
One use case for this would be to add a unit of measurement string to
the value of the field, however using BASE_UNIT_STRING in the hf_
definition is now preferred.

proto_tree_add_checksum()
----------------------------
proto_tree_add_checksum is used to add a checksum field. The hf field
provided must be the correct size of the checksum (FT_UINT, FT_UINT16,
FT_UINT32, etc). Additional parameters are there to provide "status"
and expert info depending on whether the checksum matches the provided
value. The "status" and expert info can be used in cases except
where PROTO_CHECKSUM_NO_FLAGS is used.

proto_tree_add_subtree()
---------------------
proto_tree_add_subtree() is used to add a label to the GUI tree and create
a subtree for other fields. It will contain no value, so it is not searchable
in the display filter process.

This should only be used for items with subtrees, which may not
have values themselves - the items in the subtree are the ones with values.

For a subtree, the label on the subtree might reflect some of the items
in the subtree. This means the label can't be set until at least some
of the items in the subtree have been dissected. To do this, use
'proto_item_set_text()' or 'proto_item_append_text()':

    void
    proto_item_set_text(proto_item *ti, ...);

    void
    proto_item_append_text(proto_item *ti, ...);

'proto_item_set_text()' takes as an argument the proto_item value returned by
one of the parameters in 'proto_tree_add_subtree()', a 'printf'-style format
string, and a set of arguments corresponding to '%' format items in that string,
and replaces the text for the item created by 'proto_tree_add_subtree()' with the result
of applying the arguments to the format string.

'proto_item_append_text()' is similar, but it appends to the text for
the item the result of applying the arguments to the format string.

For example, early in the dissection, one might do:

    subtree = proto_tree_add_subtree(tree, tvb, offset, length, ett, &ti, <label>);

and later do

    proto_item_set_text(ti, "%s: %s", type, value);

after the "type" and "value" fields have been extracted and dissected.
<label> would be a label giving what information about the subtree is
available without dissecting any of the data in the subtree.

Note that an exception might be thrown when trying to extract the values of
the items used to set the label, if not all the bytes of the item are
available. Thus, one should create the item with text that is as
meaningful as possible, and set it or append additional information to
it as the values needed to supply that information are extracted.

proto_tree_add_subtree_format()
----------------------------
This is like proto_tree_add_subtree(), but uses printf-style arguments to
create the label; it is used to allow routines that take a printf-like
variable-length list of arguments to add a text item to the protocol
tree.

proto_tree_add_bits_item()
--------------------------
Adds a number of bits to the protocol tree which does not have to be byte
aligned. The offset and length is in bits.
Output format:

..10 1010 10.. .... "value" (formatted as FT_ indicates).

proto_tree_add_bits_ret_val()
-----------------------------
Works in the same way but also returns the value of the read bits.

proto_tree_add_split_bits_item_ret_val()
-----------------------------------
Similar, but is used for items that are made of 2 or more smaller sets of bits (crumbs)
which are not contiguous, but are concatenated to form the actual value. The size of
the crumbs and the order of assembly are specified in an array of crumb_spec structures.

proto_tree_add_split_bits_crumb()
---------------------------------
Helper function for the above, to add text for each crumb as it is encountered.

proto_tree_add_ts_23_038_7bits_item()
-------------------------------------
Adds a string of a given number of characters and encoded according to 3GPP TS 23.038 7 bits
alphabet.

proto_tree_add_bitmask() et al.
-------------------------------
These functions provide easy to use and convenient dissection of many types of common
bitmasks into individual fields.

header is an integer type and must be of type FT_[U]INT{8|16|24|32||40|48|56|64} and
represents the entire dissectable width of the bitmask.

'header' and 'ett' are the hf fields and ett field respectively to create an
expansion that covers the bytes of the bitmask.

'fields' is a NULL terminated array of pointers to hf fields representing
the individual subfields of the bitmask. These fields must either be integers
(usually of the same byte width as 'header') or of the type FT_BOOLEAN.
Each of the entries in 'fields' will be dissected as an item under the
'header' expansion and also IF the field is a boolean and IF it is set to 1,
then the name of that boolean field will be printed on the 'header' expansion
line. For integer type subfields that have a value_string defined, the
matched string from that value_string will be printed on the expansion line
as well.

Example: (from the SCSI dissector)
    static int hf_scsi_inq_peripheral;
    static int hf_scsi_inq_qualifier;
    static int hf_scsi_inq_devtype;
    ...
    static int ett_scsi_inq_peripheral;
    ...
    static int * const peripheral_fields[] = {
        &hf_scsi_inq_qualifier,
        &hf_scsi_inq_devtype,
        NULL
    };
    ...
    /* Qualifier and DeviceType */
    proto_tree_add_bitmask(tree, tvb, offset, hf_scsi_inq_peripheral,
        ett_scsi_inq_peripheral, peripheral_fields, ENC_BIG_ENDIAN);
    offset += 1;
    ...
        { &hf_scsi_inq_peripheral,
          {"Peripheral", "scsi.inquiry.peripheral", FT_UINT8, BASE_HEX,
           NULL, 0, NULL, HFILL}
        },
        { &hf_scsi_inq_qualifier,
          {"Qualifier", "scsi.inquiry.qualifier", FT_UINT8, BASE_HEX,
           VALS (scsi_qualifier_val), 0xE0, NULL, HFILL}
        },
        { &hf_scsi_inq_devtype,
          {"Device Type", "scsi.inquiry.devtype", FT_UINT8, BASE_HEX,
           VALS (scsi_devtype_val), SCSI_DEV_BITS, NULL, HFILL}
        },
    ...

Which provides very pretty dissection of this one byte bitmask.

    Peripheral: 0x05, Qualifier: Device type is connected to logical unit, Device Type: CD-ROM
        000. .... = Qualifier: Device type is connected to logical unit (0x00)
        ...0 0101 = Device Type: CD-ROM (0x05)

The proto_tree_add_bitmask_text() function is an extended version of
the proto_tree_add_bitmask() function. In addition, it allows to:
- Provide a leading text (e.g. "Flags: ") that will appear before
  the comma-separated list of field values
- Provide a fallback text (e.g. "None") that will be appended if
  no fields warranted a change to the top-level title.
- Using flags, specify which fields will affect the top-level title.

There are the following flags defined:

  BMT_NO_APPEND - the title is taken "as-is" from the 'name' argument.
  BMT_NO_INT - only boolean flags are added to the title.
  BMT_NO_FALSE - boolean flags are only added to the title if they are set.
  BMT_NO_TFS - only add flag name to the title, do not use true_false_string

The proto_tree_add_bitmask_with_flags() function is an extended version
of the proto_tree_add_bitmask() function. It allows using flags to specify
which fields will affect the top-level title. The flags are the
same BMT_NO_* flags as used in the proto_tree_add_bitmask_text() function.

The proto_tree_add_bitmask() behavior can be obtained by providing
both 'name' and 'fallback' arguments as NULL, and a flags of
(BMT_NO_FALSE|BMT_NO_TFS).

The proto_tree_add_bitmask_len() function is intended for protocols where
bitmask length is permitted to vary, so a length is specified explicitly
along with the bitmask value. USB Video "bmControl" and "bControlSize"
fields follow this pattern. The primary intent of this is "forward
compatibility," enabling an interpreter coded for version M of a structure
to comprehend fields in version N of the structure, where N > M and
bControlSize increases from version M to version N.

proto_tree_add_bitmask_len() is an extended version of proto_tree_add_bitmask()
that uses an explicitly specified (rather than inferred) length to control
dissection. Because of this, it may encounter two cases that
proto_tree_add_bitmask() and proto_tree_add_bitmask_text() may not:
- A length that exceeds that of the 'header' and bitmask subfields.
  In this case the least-significant bytes of the bitmask are dissected.
  An expert warning is generated in this case, because the dissection code
  likely needs to be updated for a new revision of the protocol.
- A length that is shorter than that of the 'header' and bitmask subfields.
  In this case, subfields whose data is fully present are dissected,
  and other subfields are not. No warning is generated in this case,
  because the dissection code is likely for a later revision of the protocol
  than the packet it was called to interpret.


proto_item_set_generated()
--------------------------
proto_item_set_generated is used to mark fields as not being read from the
captured data directly, but inferred from one or more values.

One of the primary uses of this is the presentation of verification of
checksums. Every IP packet has a checksum line, which can present the result
of the checksum verification, if enabled in the preferences. The result is
presented as a subtree, where the result is enclosed in square brackets
indicating a generated field.

  Header checksum: 0x3d42 [correct]
  [Checksum Status: Good (1)]

proto_item_set_hidden()
-----------------------
proto_item_set_hidden is used to hide fields, which have already been added
to the tree, from being visible in the displayed tree.

NOTE that creating hidden fields is actually quite a bad idea from a UI design
perspective because the user (someone who did not write nor has ever seen the
code) has no way of knowing that hidden fields are there to be filtered on
thus defeating the whole purpose of putting them there. A Better Way might
be to add the fields (that might otherwise be hidden) to a subtree where they
won't be seen unless the user opens the subtree--but they can be found if the
user wants.

One use for hidden fields (which would be better implemented using visible
fields in a subtree) follows: The caller may want a value to be
included in a tree so that the packet can be filtered on this field, but
the representation of that field in the tree is not appropriate. An
example is the token-ring routing information field (RIF). The best way
to show the RIF in a GUI is by a sequence of ring and bridge numbers.
Rings are 3-digit hex numbers, and bridges are single hex digits:

    RIF: 001-A-013-9-C0F-B-555

In the case of RIF, the programmer should use a field with no value and
use proto_tree_add_none_format() to build the above representation. The
programmer can then add the ring and bridge values, one-by-one, with
proto_tree_add_item() and hide them with proto_item_set_hidden() so that the
user can then filter on or search for a particular ring or bridge. Here's a
skeleton of how the programmer might code this.

    char *rif;
    rif = create_rif_string(...);

    proto_tree_add_none_format(tree, hf_tr_rif_label, ..., "RIF: %s", rif);

    for (i = 0; i < num_rings; i++) {
        proto_item *pi;

        pi = proto_tree_add_item(tree, hf_tr_rif_ring, ..., ENC_BIG_ENDIAN);
        proto_item_set_hidden(pi);
    }
    for (i = 0; i < num_rings - 1; i++) {
        proto_item *pi;

        pi = proto_tree_add_item(tree, hf_tr_rif_bridge, ..., ENC_BIG_ENDIAN);
        proto_item_set_hidden(pi);
    }

The logical tree has these items:

    hf_tr_rif_label, text="RIF: 001-A-013-9-C0F-B-555", value = NONE
    hf_tr_rif_ring,  hidden, value=0x001
    hf_tr_rif_bridge, hidden, value=0xA
    hf_tr_rif_ring,  hidden, value=0x013
    hf_tr_rif_bridge, hidden, value=0x9
    hf_tr_rif_ring,  hidden, value=0xC0F
    hf_tr_rif_bridge, hidden, value=0xB
    hf_tr_rif_ring,  hidden, value=0x555

GUI or print code will not display the hidden fields, but a display
filter or "packet grep" routine will still see the values. The possible
filter is then possible:

    tr.rif_ring eq 0x013

proto_item_set_url
------------------
proto_item_set_url is used to mark fields as containing a URL. This can only
be done with fields of type FT_STRING(Z). If these fields are presented they
are underlined, as could be done in a browser. These fields are sensitive to
clicks as well, launching the configured browser with this URL as parameter.

1.6 Utility routines.

1.6.1 val_to_str, val_to_str_const, try_val_to_str and try_val_to_str_idx

A dissector may need to convert a value to a string, using a
'value_string' structure, by hand, rather than by declaring a field with
an associated 'value_string' structure; this might be used, for example,
to generate a COL_INFO line for a frame.

val_to_str() handles the most common case:

    const char *
    val_to_str(wmem_allocator_t* scope, uint32_t val, const value_string *vs, const char *fmt)

If the value 'val' is found in the 'value_string' table pointed to by
'vs', 'val_to_str' will return the corresponding string; otherwise, it
will use 'fmt' as an 'sprintf'-style format, with 'val' as an argument,
to generate a string, and will return a pointer to that string.
You can use it in a call to generate a COL_INFO line for a frame such as

    col_add_fstr(COL_INFO, ", %s", val_to_str(pinfo->pool, val, table, "Unknown %d"));

If you don't need to display 'val' in your fmt string, you can use
val_to_str_const() which just takes a string constant instead and returns it
unmodified when 'val' isn't found.

If you need to handle the failure case in some custom way, try_val_to_str()
will return NULL if val isn't found:

    const char *
    try_val_to_str(uint32_t val, const value_string *vs)

Note that, you must check whether 'try_val_to_str()' returns NULL, and arrange
that its return value not be dereferenced if it's NULL. 'try_val_to_str_idx()'
behaves similarly, except it also returns an index into the value_string array,
or -1 if 'val' was not found.

The *_ext functions are "extended" versions of those already described. They
should be used for large value-string arrays which contain many entries. They
implement value to string conversions which will do either a direct access or
a binary search of the value string array if possible. See
"Extended Value Strings" under section 1.6 "Constructing the protocol tree" for
more information.

See epan/value_string.h for detailed information on the various value_string
functions.

To handle 64-bit values, there are an equivalent set of functions. These are:

    const char *
    val64_to_str(const uint64_t val, const val64_string *vs, const char *fmt)

    const char *
    val64_to_str_const(const uint64_t val, const val64_string *vs, const char *unknown_str);

    const char *
    try_val64_to_str(const uint64_t val, const val64_string *vs);

    const char *
    try_val64_to_str_idx(const uint64_t val, const val64_string *vs, int *idx);


1.6.2 rval_to_str, try_rval_to_str and try_rval_to_str_idx

A dissector may need to convert a range of values to a string, using a
'range_string' structure.

Most of the same functions exist as with regular value_strings (see section
1.6.1) except with the names 'rval' instead of 'val'.


1.7 Calling Other Dissectors.

As each dissector completes its portion of the protocol analysis, it
is expected to create a new tvbuff of type TVBUFF_SUBSET which
contains the payload portion of the protocol (that is, the bytes
that are relevant to the next dissector).

To create a new TVBUFF_SUBSET that begins at a specified offset in a
parent tvbuff, and runs to the end of the parent tvbuff, the routine
tvbuff_new_subset_remaining() is used:

    next_tvb = tvb_new_subset_remaining(tvb, offset);

Where:
    tvb is the tvbuff that the dissector has been working on. It
    can be a tvbuff of any type.

    next_tvb is the new TVBUFF_SUBSET.

    offset is the byte offset of 'tvb' at which the new tvbuff
    should start. The first byte is the byte at offset 0.

To create a new TVBUFF_SUBSET that begins at a specified offset in a
parent tvbuff, with a specified number of bytes in the payload, the
routine tvbuff_new_subset_length() is used:

    next_tvb = tvb_new_subset_length(tvb, offset, reported_length);

Where:
    tvb is the tvbuff that the dissector has been working on. It
    can be a tvbuff of any type.

    next_tvb is the new TVBUFF_SUBSET.

    offset is the byte offset of 'tvb' at which the new tvbuff
    should start. The first byte is the byte at offset 0.

    reported_length is the number of bytes that the current protocol
    says should be in the payload.

In the few cases where the number of bytes available in the new subset
must be explicitly specified, rather than being calculated based on the
number of bytes in the payload, the routine tvb_new_subset_length_caplen()
is used:

    next_tvb = tvb_new_subset_length_caplen(tvb, offset, length, reported_length);

Where:
    tvb is the tvbuff that the dissector has been working on. It
    can be a tvbuff of any type.

    next_tvb is the new TVBUFF_SUBSET.

    offset is the byte offset of 'tvb' at which the new tvbuff
    should start. The first byte is the byte at offset 0.

    length is the number of bytes in the new TVBUFF_SUBSET. A length
    argument of -1 says to use as many bytes as are available in
    'tvb'.

    reported_length is the number of bytes that the current protocol
    says should be in the payload. A reported_length of -1 says that
    the protocol doesn't say anything about the size of its payload.

To call a dissector you need to get the handle of the dissector using
find_dissector(), passing it the string name of the dissector. The setting
of the handle is usually done once at startup during the proto_reg_handoff
function within the calling dissector.

1.7.1 Dissector Tables

Another way to call a subdissector is to setup a dissector table. A dissector
table is a list of subdissectors grouped by a common identifier (integer or
string) in a dissector. Subdissectors will register themselves with the dissector
table using their unique identifier using one of the following APIs:

    void dissector_add_uint(const char *abbrev, const uint32_t pattern,
                            dissector_handle_t handle);

    void dissector_add_uint_range(const char *abbrev, struct epan_range *range,
                                  dissector_handle_t handle);

    void dissector_add_string(const char *name, const char *pattern,
                              dissector_handle_t handle);

    void dissector_add_for_decode_as(const char *name,
                                     dissector_handle_t handle);

    dissector_add_for_decode_as doesn't add a unique identifier in the dissector
    table, but it lets the user add it from the command line or, in Wireshark,
    through the "Decode As" UI.

Then when the dissector hits the common identifier field, it will use one of the
following APIs to invoke the subdissector:

    int dissector_try_uint(dissector_table_t sub_dissectors,
        const uint32_t uint_val, tvbuff_t *tvb, packet_info *pinfo,
        proto_tree *tree);

    int dissector_try_uint_with_data(dissector_table_t sub_dissectors,
        const uint32_t uint_val, tvbuff_t *tvb, packet_info *pinfo,
        proto_tree *tree, const bool add_proto_name, void *data);

    int dissector_try_string_with_data(dissector_table_t sub_dissectors, const char *string,
        tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, true, void *data);

These pass a subset of the remaining packet (typically the rest of the
packet) for the dissector table to determine which subdissector is called.
This allows dissection of a packet to be expanded outside of dissector without
having to modify the dissector directly.


1.8 Editing CMakeLists.txt to add your dissector.

To arrange that your dissector will be built as part of Wireshark, you
must add the name of the source file for your dissector to the DISSECTOR_SRC
section of epan/dissectors/CMakeLists.txt


1.9 Using the git source code tree.

  See <https://www.wireshark.org/develop.html>


1.10 Submitting code for your new dissector.

  See <https://www.wireshark.org/docs/wsdg_html_chunked/ChSrcContribute.html>
  and <https://wiki.wireshark.org/Development/SubmittingPatches>.

  - VERIFY that your dissector code does not use prohibited or deprecated APIs
    as follows:
    perl <wireshark_root>/tools/checkAPIs.pl <source-filename(s)>

  - VERIFY that your dissector code does not contain any header field related
    problems:
    perl <wireshark_root>/tools/checkhf.pl <source-filename(s)>

  - VERIFY that your dissector code does not contain any display filter related
    problems:
    perl <wireshark_root>/tools/checkfiltername.pl <source-filename(s)>

  - CHECK your dissector with CppCheck (http://cppcheck.sourceforge.net/) using
    Wireshark's customized configuration. This is particularly important on
    Windows, since Microsoft's compiler warnings are quite thin:
    ./tools/cppcheck/cppcheck.sh <source-filename(s)>

  - TEST YOUR DISSECTOR BEFORE SUBMITTING IT.
    Use fuzz-test.sh and/or randpkt against your dissector. These are
    described at <https://wiki.wireshark.org/FuzzTesting>.

  - Subscribe to <mailto:wireshark-dev[AT]wireshark.org> by sending an email to
    <mailto:wireshark-dev-request[AT]wireshark.org?body="help"> or visiting
    <https://lists.wireshark.org/>.

  - 'git diff' to verify all your changes look good.

  - 'git add' all the files you changed.

  - 'git commit' to commit (locally) your changes. First line of commit message
    should be a summary of the changes followed by an empty line and a more
    verbose description.

  - 'git push downstream HEAD' to push the changes to GitLab. (This assumes
    that you have a remote named "downstream" that points to a fork of
    https://gitlab.com/wireshark/wireshark.)

  - Create a Wiki page on the protocol at <https://gitlab.com/wireshark/editor-wiki>.
    (You'll need to request access to https://gitlab.com/wireshark/wiki-editors.)
    A template is provided so it is easy to setup in a consistent style.
      See: <https://wiki.wireshark.org/HowToEdit>
      and  <https://wiki.wireshark.org/ProtocolReference>

  - If possible, add sample capture files to the sample captures page at
    <https://wiki.wireshark.org/SampleCaptures>. These
    files are used by the automated build system for fuzz testing.

  - If you don't think the wiki is the right place for your sample capture,
    submit a bug report to the Wireshark issue database, found at
    <https://gitlab.com/wireshark/wireshark/-/issues>, qualified as an
    enhancement and attach your sample capture there. Normally a new
    dissector won't be accepted without a sample capture! If you open a
    bug be sure to cross-link your GitLab merge request.

2. Advanced dissector topics.

2.1 Introduction.

Some of the advanced features are being worked on constantly. When using them
it is wise to check the relevant header and source files for additional details.

2.2 Following "conversations".

In Wireshark a conversation is defined as a series of data packets between two
address:port combinations. A conversation is not sensitive to the direction of
the packet. The same conversation will be returned for a packet bound from
ServerA:1000 to ClientA:2000 and the packet from ClientA:2000 to ServerA:1000.

2.2.1 Conversation Routines

There are nine routines that you will use to work with a conversation:
conversation_new, conversation_new_full, find_conversation,
find_conversation_full, find_or_create_conversation,
conversation_add_proto_data, conversation_get_proto_data,
conversation_delete_proto_data, and conversation_set_dissector.

2.2.1.1 The conversation_init function.

This is an internal routine for the conversation code. As such you
will not have to call this routine. Just be aware that this routine is
called at the start of each capture and before the packets are filtered
with a display filter. The routine will destroy all stored
conversations. This routine does NOT clean up any data pointers that are
passed in the conversation_add_proto_data 'data' variable. You are
responsible for this clean up if you pass a malloc'ed pointer
in this variable.

See item 2.2.1.5 for more information about use of the 'data' pointer.


2.2.1.2 The conversation_new function.

This routine will create a new conversation based upon two address/port
pairs. If you want to associate with the conversation a pointer to a
private data structure you must use the conversation_add_proto_data
function. The ctype variable is used to differentiate between
conversations over different protocols, i.e. TCP and UDP. The options
variable is used to define a conversation that will accept any destination
address and/or port. Set options = 0 if the destination port and address
are known when conversation_new is called. See section 2.4 for more
information on usage of the options parameter.

The conversation_new prototype:
    conversation_t *conversation_new(uint32_t setup_frame, address *addr1,
        address *addr2, conversation_type ctype, uint32_t port1, uint32_t port2,
        unsigned options);

Where:
    uint32_t setup_frame     = The lowest numbered frame for this conversation
    address* addr1           = first data packet address
    address* addr2           = second data packet address
    conversation_type ctype  = conversation type, defined in conversation.h
    uint32_t port1           = first data packet port
    uint32_t port2           = second data packet port
    unsigned options         = conversation options, NO_ADDR2 and/or NO_PORT2

setup_frame indicates the first frame for this conversation, and is used to
distinguish multiple conversations with the same addr1/port1 and addr2/port2
pair that occur within the same capture session.

"addr1" and "port1" are the first address/port pair; "addr2" and "port2"
are the second address/port pair. A conversation doesn't have source
and destination address/port pairs - packets in a conversation go in
both directions - so "addr1"/"port1" may be the source or destination
address/port pair; "addr2"/"port2" would be the other pair.

If NO_ADDR2 is specified, the conversation is set up so that a
conversation lookup will match only the "addr1" address; if NO_PORT2 is
specified, the conversation is set up so that a conversation lookup will
match only the "port1" port; if both are specified, i.e.
NO_ADDR2|NO_PORT2, the conversation is set up so that the lookup will
match only the "addr1"/"port1" address/port pair. This can be used if a
packet indicates that, later in the capture, a conversation will be
created using certain addresses and ports, in the case where the packet
doesn't specify the addresses and ports of both sides.

2.2.1.3 The conversation_new_full function.

This routine will create a new conversation based upon an arbitrary
lists of elements. Elements can be addresses, strings, unsigned
integers, or unsigned 64-bit integers. Unlike conversation_new, element
lists are matched strictly; wildcards aren't (yet) supported.

The conversation_new_full prototype:
    conversation_t *conversation_new_full(const uint32_t setup_frame,
        conversation_element_t *elements);

Where:
    uint32_t setup_frame             = The lowest numbered frame for
        this conversation
    conversation_element_t *elements = An array of data types and
        values which identify this conversation. The array MUST be
        terminated with a CE_ENDPOINT element.

2.2.1.4 The find_conversation function.

Call this routine to look up a conversation. If no conversation is found,
the routine will return a NULL value.

The find_conversation prototype:

    conversation_t *find_conversation(uint32_t frame_num, address *addr_a,
        address *addr_b, conversation_type ctype, uint32_t port_a, uint32_t port_b,
        unsigned options);

Where:
    uint32_t frame_num       = a frame number to match
    address* addr_a          = first address
    address* addr_b          = second address
    conversation_type ctype  = conversation type
    uint32_t port_a          = first data packet port
    uint32_t port_b          = second data packet port
    unsigned options         = conversation options, NO_ADDR_B and/or NO_PORT_B

frame_num is a frame number to match. The conversation returned is where
    (frame_num >= conversation->setup_frame
    && frame_num < conversation->next->setup_frame)
Suppose there are a total of 3 conversations (A, B, and C) that match
addr_a/port_a and addr_b/port_b, where the setup_frame used in
conversation_new() for A, B and C are 10, 50, and 100 respectively. The
frame_num passed in find_conversation is compared to the setup_frame of each
conversation. So if (frame_num >= 10 && frame_num < 50), conversation A is
returned. If (frame_num >= 50 && frame_num < 100), conversation B is returned.
If (frame_num >= 100) conversation C is returned.

"addr_a" and "port_a" are the first address/port pair; "addr_b" and
"port_b" are the second address/port pair. Again, as a conversation
doesn't have source and destination address/port pairs, so
"addr_a"/"port_a" may be the source or destination address/port pair;
"addr_b"/"port_b" would be the other pair. The search will match the
"a" address/port pair against both the "1" and "2" address/port pairs,
and match the "b" address/port pair against both the "2" and "1"
address/port pairs; you don't have to worry about which side the "a" or
"b" pairs correspond to.

If the NO_ADDR_B flag was specified to "find_conversation()", the
"addr_b" address will be treated as matching any "wildcarded" address;
if the NO_PORT_B flag was specified, the "port_b" port will be treated
as matching any "wildcarded" port. If both flags are specified, i.e.
NO_ADDR_B|NO_PORT_B, the "addr_b" address will be treated as matching
any "wildcarded" address and the "port_b" port will be treated as
matching any "wildcarded" port.

2.2.1.5 The find_conversation_full function.

Call this routine to look up a conversation based on an element list. If
no conversation is found, the routine will return a NULL value.

The find_conversation_full prototype:

    conversation_t *find_conversation_full(uint32_t frame_num,
        conversation_element_t *elements);

Where:
    uint32_t setup_frame             = The lowest numbered frame for
        this conversation
    conversation_element_t *elements = An array of data types and
        values which identify this conversation. The array MUST be
        terminated with a CE_ENDPOINT element.

2.2.1.6 The find_conversation_pinfo function.

This convenience function will find an existing conversation (by calling
find_conversation())

The find_conversation_pinfo prototype:

    extern conversation_t *find_conversation_pinfo(packet_info *pinfo,
        const unsigned options);

Where:
    packet_info *pinfo  = the packet_info structure
    const unsigned options = conversation options, NO_ADDR_B and/or NO_PORT_B

The frame number and the addresses necessary for find_conversation() are
taken from the addresses and ports in the pinfo structure,
pinfo->conv_addr_port_endpoints if pinfo->use_conv_addr_port_endpoints is set,
or pinfo->conv_elements if it is set.

2.2.1.7 The find_or_create_conversation function.

This convenience function will find an existing conversation (by calling
find_conversation()) and, if a conversation does not already exist, create a
new conversation by calling conversation_new().

The find_or_create_conversation prototype:

    extern conversation_t *find_or_create_conversation(packet_info *pinfo);

Where:
    packet_info *pinfo = the packet_info structure

The frame number and the addresses necessary for find_conversation() and
conversation_new() are taken from the pinfo structure (as is commonly done)
and no 'options' are used.

2.2.1.8 The conversation_add_proto_data function.

Once you have created a conversation with conversation_new, you can
associate data with it using this function.

The conversation_add_proto_data prototype:

    void conversation_add_proto_data(conversation_t *conv, int proto,
        void *proto_data);

Where:
    conversation_t *conv    = the conversation in question
    int proto               = registered protocol number
    void *data              = dissector data structure

"conversation" is the value returned by conversation_new. "proto" is a
unique protocol number created with proto_register_protocol. Protocols
are typically registered in the proto_register_XXXX section of your
dissector. "data" is a pointer to the data you wish to associate with the
conversation. "data" usually points to "wmem_alloc'd" memory; the
memory will be automatically freed each time a new dissection begins
and thus need not be managed (freed) by the dissector.
Using the protocol number allows several dissectors to
associate data with a given conversation.


2.2.1.9 The conversation_get_proto_data function.

After you have located a conversation with find_conversation, you can use
this function to retrieve any data associated with it.

The conversation_get_proto_data prototype:

    void *conversation_get_proto_data(conversation_t *conv, int proto);

Where:
    conversation_t *conv    = the conversation in question
    int proto               = registered protocol number

"conversation" is the conversation created with conversation_new. "proto"
is a unique protocol number created with proto_register_protocol,
typically in the proto_register_XXXX portion of a dissector. The function
returns a pointer to the data requested, or NULL if no data was found.


2.2.1.10 The conversation_delete_proto_data function.

After you are finished with a conversation, you can remove your association
with this function. Please note that ONLY the conversation entry is
removed. If you have allocated any memory for your data (other than with wmem_alloc),
 you must free it as well.

The conversation_delete_proto_data prototype:

    void conversation_delete_proto_data(conversation_t *conv, int proto);

Where:
    conversation_t *conv = the conversation in question
    int proto            = registered protocol number

"conversation" is the conversation created with conversation_new. "proto"
is a unique protocol number created with proto_register_protocol,
typically in the proto_register_XXXX portion of a dissector.

2.2.1.11 The conversation_set_dissector function

This function sets the protocol dissector to be invoked whenever
conversation parameters (addresses, port_types, ports, etc) are matched
during the dissection of a packet.

The conversation_set_dissector prototype:

    void conversation_set_dissector(conversation_t *conversation, const dissector_handle_t handle);

Where:
    conversation_t *conv = the conversation in question
    const dissector_handle_t handle = the dissector handle.


2.2.2 Using timestamps relative to the conversation

There is a framework to calculate timestamps relative to the start of the
conversation. First of all the timestamp of the first packet that has been
seen in the conversation must be kept in the protocol data to be able
to calculate the timestamp of the current packet relative to the start
of the conversation. The timestamp of the last packet that was seen in the
conversation should also be kept in the protocol data. This way the
delta time between the current packet and the previous packet in the
conversation can be calculated.

So add the following items to the struct that is used for the protocol data:

  nstime_t ts_first;
  nstime_t ts_prev;

The ts_prev value should only be set during the first run through the
packets (ie PINFO_FD_VISITED(pinfo) is false).

Next step is to use the per-packet information (described in section 2.5)
to keep the calculated delta timestamp, as it can only be calculated
on the first run through the packets. This is because a packet can be
selected in random order once the whole file has been read.

After calculating the conversation timestamps, it is time to put them in
the appropriate columns with the function 'col_set_time' (described in
section 1.5.9). The column used for relative timestamps is:

COL_REL_TIME, /* Delta time to last frame in conversation */

Last but not least, there MUST be a preference in each dissector that
uses conversation timestamps that makes it possible to enable and
disable the calculation of conversation timestamps. The main argument
for this is that a higher level conversation is able to overwrite
the values of lower level conversations in these two columns. Being
able to actively select which protocols may overwrite the conversation
timestamp columns gives the user the power to control these columns.
(A second reason is that conversation timestamps use the per-packet
data structure which uses additional memory, which should be avoided
if these timestamps are not needed)

Have a look at the differences to packet-tcp.[ch] in SVN 22966 and
SVN 23058 to see the implementation of conversation timestamps for
the tcp-dissector.


2.2.3 The example conversation code using wmem_file_scope memory.

For a conversation between two IP addresses and ports you can use this as an
example. This example uses wmem_alloc() with wmem_file_scope() to allocate
memory and stores the data pointer in the conversation 'data' variable.

/************************ Global values ************************/

/* define your structure here */
typedef struct {

} my_entry_t;

/* Registered protocol number */
static int my_proto;

/********************* in the dissector routine *********************/

/* the local variables in the dissector */

conversation_t *conversation;
my_entry_t *data_ptr;


/* look up the conversation */

conversation = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst,
        conversation_pt_to_conversation_type(pinfo->ptype),
        pinfo->srcport, pinfo->destport, 0);

/* if conversation found get the data pointer that you stored */
if (conversation) {
    data_ptr = (my_entry_t*)conversation_get_proto_data(conversation, my_proto);
} else {

    /* new conversation create local data structure */

    data_ptr = wmem_alloc(wmem_file_scope(), sizeof(my_entry_t));

    /*** add your code here to setup the new data structure ***/

    /* create the conversation with your data pointer  */

    conversation = conversation_new(pinfo->num,  &pinfo->src, &pinfo->dst,
        conversation_pt_to_conversation_type(pinfo->ptype),
        pinfo->srcport, pinfo->destport, 0);
    conversation_add_proto_data(conversation, my_proto, (void *)data_ptr);
}

/* at this point the conversation data is ready */

/***************** in the protocol register routine *****************/

my_proto = proto_register_protocol("My Protocol", "My Protocol", "my_proto");


2.2.4 An example conversation code that starts at a specific frame number.

Sometimes a dissector has determined that a new conversation is needed that
starts at a specific frame number, when a capture session encompasses multiple
conversation that reuse the same src/dest ip/port pairs. You can use the
conversation->setup_frame returned by find_conversation with
pinfo->num to determine whether or not there already exists a conversation
that starts at the specific frame number.

/* in the dissector routine */

    conversation = find_conversation(pinfo->num, &pinfo->src, &pinfo->dst,
        conversation_pt_to_conversation_type(pinfo->ptype),
        pinfo->srcport, pinfo->destport, 0);
    if (conversation == NULL || (conversation->setup_frame != pinfo->num)) {
        /* It's not part of any conversation or the returned
         * conversation->setup_frame doesn't match the current frame
         * create a new one.
         */
        conversation = conversation_new(pinfo->num, &pinfo->src, &pinfo->dst,
            conversation_pt_to_conversation_type(pinfo->ptype),
            pinfo->srcport, pinfo->destport, 0);
    }


2.2.5 The example conversation code using conversation index field.

Sometimes the conversation isn't enough to define a unique data storage
value for the network traffic. For example if you are storing information
about requests carried in a conversation, the request may have an
identifier that is used to  define the request. In this case the
conversation and the identifier are required to find the data storage
pointer. You can use the conversation data structure index value to
uniquely define the conversation.

See packet-afs.c for an example of how to use the conversation index. In
this dissector multiple requests are sent in the same conversation. To store
information for each request the dissector has an internal hash table based
upon the conversation index and values inside the request packets.


    /* in the dissector routine */

    /* to find a request value, first lookup conversation to get index */
    /* then used the conversation index, and request data to find data */
    /* in the local hash table */

    conversation = find_or_create_conversation(pinfo);

    request_key.conversation = conversation->index;
    request_key.service = pntohu16(&rxh->serviceId);
    request_key.callnumber = pntohu32(&rxh->callNumber);

    request_val = (struct afs_request_val *)g_hash_table_lookup(
        afs_request_hash, &request_key);

    /* only allocate a new hash element when it's a request */
    opcode = 0;
    if (!request_val && !reply) {
        new_request_key = wmem_alloc(wmem_file_scope(), sizeof(struct afs_request_key));
        *new_request_key = request_key;

        request_val = wmem_alloc(wmem_file_scope(), sizeof(struct afs_request_val));
        request_val -> opcode = pntohu32(&afsh->opcode);
        opcode = request_val->opcode;

        g_hash_table_insert(afs_request_hash, new_request_key, request_val);
    }


2.2.6 Registering your dissector for conversation filtering

In cases where protocols define a conversation, you can register them through
the UI menu of Wireshark by locating "Analyze -> Conversation Filter". This can
reduce the task of isolating sessions and viewing them to a single click. For users
that frequent a specific protocol in a conversation, this may save time and reduce errors.

When you have a protocol to add to the UI, use the add_conversation_filter_protocol() function.
Using this built-in function will allow you to add your dissector to a conversation, providing
only the packets relevant to the specified protocol in a conversation.


Example using the "example_protocol" protocol:
    Use the add_conversation_filter_protocol() function with the conversation and your "example_protocol":
        add_conversation_filter_protocol(example_protocol)



2.2.7 Initialization and Cleanup in Dissector Plugins

- proto_register_foo() should be used to register protocol fields and can
  also perform global (once-only) initialization.
- proto_reg_handoff_foo() is called after registration and is typically used
  to register the dissector with heuristic or static tables.

There are no built-in dissect_init() or dissect_cleanup() hooks. If your plugin
needs to allocate temporary memory per-packet or per-frame, use the pinfo->pool
which is an ephemeral memory pool managed by Wireshark and reset after each packet.

Example:

    static int proto_foo = -1;

    void proto_register_foo(void) {
        static hf_register_info hf[] = {
            { &hf_foo_field,
              { "Foo Field", "foo.field",
                FT_UINT8, BASE_DEC, NULL, 0x0,
                NULL, HFILL }
            }
        };
        proto_foo = proto_register_protocol("Foo Protocol", "FOO", "foo");
        proto_register_field_array(proto_foo, hf, array_length(hf));
    }

    void proto_reg_handoff_foo(void) {
        dissector_handle_t foo_handle;

        foo_handle = create_dissector_handle(dissect_foo, proto_foo);
        dissector_add_uint("udp.port", FOO_PORT, foo_handle);
    }

2.2.8 Exported PDUs

Wireshark supports Exported PDUs (Protocol Data Units), which allow pre-processed or extracted protocol payloads
to be re-encapsulated and interpreted as individual packets. This is useful for cases where an external tool extracts
sessions (e.g., VoIP, HTTP) and reinjects them into a capture for post-analysis.

To handle Exported PDUs, Wireshark uses the `exported_pdu` dissector, which can wrap protocols like TCP, UDP, or any
other payload into a new frame.

An Exported PDU typically includes the following fields:

- `pdu_type`: The protocol type of the encapsulated PDU.
- `tvb`: The data buffer containing the payload.
- `metadata`: (Optional) Custom information passed to the dissector.

Plugins or dissectors can create and register exported PDUs using the internal API or by writing their data in PCAPNG
format with `block_type` `0x00000BAD`.

This mechanism improves modular analysis and allows tools to reuse Wireshark’s decoding logic.

For more, see: `epan/exported_pdu.h`


2.3 Dynamic conversation dissector registration.


NOTE:   This sections assumes that all information is available to
    create a complete conversation, source port/address and
    destination port/address. If either the destination port or
    address is known, see section 2.4 Dynamic server port dissector
    registration.

For protocols that negotiate a secondary port connection, for example
packet-msproxy.c, a conversation can install a dissector to handle
the secondary protocol dissection. After the conversation is created
for the negotiated ports use the conversation_set_dissector to define
the dissection routine.
Before we create these conversations or assign a dissector to them we should
first check that the conversation does not already exist and if it exists
whether it is registered to our protocol or not.
We should do this because it is uncommon but it does happen that multiple
different protocols can use the same socketpair during different stages of
an application cycle. By keeping track of the frame number a conversation
was started in Wireshark can still tell these different protocols apart.

The second argument to conversation_set_dissector is a dissector handle,
which is created with a call to create_dissector_handle,
register_dissector, or register_dissector_with_description.

register_dissector_with_description takes as arguments a string giving a name
for the dissector, a string with a human-readable summary of the dissector, a
pointer to the dissector function, and a protocol ID as returned by
proto_register_protocol.

register_dissector takes as arguments a string giving a name for the
dissector, a pointer to the dissector function, and a protocol ID
as returned by proto_register_protocol.

create_dissector_handle takes as arguments a pointer to the dissector
function and a protocol ID as returned by proto_register_protocol.
It is recommended to use one of the above two functions instead of this one,
since they allow the dissector to be referenced by name from the command line,
by other dissectors via calls to find_dissector, etc.

The protocol ID is the ID for the protocol dissected by the function.
The function will not be called if the protocol has been disabled by the
user; instead, the data for the protocol will be dissected as raw data.

An example -

/* the handle for the dynamic dissector *
static dissector_handle_t sub_dissector_handle;

/* prototype for the dynamic dissector */
static void sub_dissector(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree);

/* in the main protocol dissector, where the next dissector is setup */

/* if conversation has a data field, create it and load structure */

/* First check if a conversation already exists for this socketpair */
    conversation = find_conversation(pinfo->num,
                                     &pinfo->src, &pinfo->dst, conversation_type,
                                     src_port, dst_port,  0);

/* If there is no such conversation, or if there is one but for
   someone else's protocol then we just create a new conversation
   and assign our protocol to it.
*/
    if ((conversation == NULL) ||
        (conversation->dissector_handle != sub_dissector_handle)) {
        new_conv_info = wmem_alloc(wmem_file_scope(), sizeof(struct _new_conv_info));
        new_conv_info->data1 = value1;

        /* create the conversation for the dynamic port */
        conversation = conversation_new(pinfo->num,
                                        &pinfo->src, &pinfo->dst, protocol,
                                        src_port, dst_port, new_conv_info, 0);

        /* set the dissector for the new conversation */
        conversation_set_dissector(conversation, sub_dissector_handle);
    }
    ...

void
proto_register_PROTOABBREV(void)
{
    ...

    sub_dissector_handle = register_dissector("PROTOABBREV", sub_dissector,
        proto);

    ...
}

2.4 Dynamic server port dissector registration.

NOTE: While this example used both NO_ADDR2 and NO_PORT2 to create a
conversation with only one port and address set, this isn't a
requirement. Either the second port or the second address can be set
when the conversation is created.

For protocols that define a server address and port for a secondary
protocol, a conversation can be used to link a protocol dissector to
the server port and address. The key is to create the new
conversation with the second address and port set to the "accept
any" values.

Some server applications can use the same port for different protocols during
different stages of a transaction. For example it might initially use SNMP
to perform some discovery and later switch to use TFTP using the same port.
In order to handle this properly we must first check whether such a
conversation already exists or not and if it exists we also check whether the
registered dissector_handle for that conversation is "our" dissector or not.
If not we create a new conversation on top of the previous one and set this new
conversation to use our protocol.
Since Wireshark keeps track of the frame number where a conversation started
wireshark will still be able to keep the packets apart even though they do use
the same socketpair.
        (See packet-tftp.c and packet-snmp.c for examples of this)

There are two support routines that will allow the second port and/or
address to be set later.

conversation_set_port2(conversation_t *conv, uint32_t port);
conversation_set_addr2(conversation_t *conv, address addr);

These routines will change the second address or port for the
conversation. So, the server port conversation will be converted into a
more complete conversation definition. Don't use these routines if you
want to create a conversation between the server and client and retain the
server port definition, you must create a new conversation.


An example -

/* the handle for the dynamic dissector *
static dissector_handle_t sub_dissector_handle;

    ...

/* in the main protocol dissector, where the next dissector is setup */

/* if conversation has a data field, create it and load structure */

    new_conv_info = wmem_alloc(wmem_file_scope(), sizeof(struct _new_conv_info));
    new_conv_info->data1 = value1;

/* create the conversation for the dynamic server address and port      */
/* NOTE: The second address and port values don't matter because the    */
/* NO_ADDR2 and NO_PORT2 options are set.                               */

/* First check if a conversation already exists for this
    IP/protocol/port
*/
    conversation = find_conversation(pinfo->num,
                                     &server_src_addr, 0, protocol,
                                     server_src_port, 0, NO_ADDR2 | NO_PORT_B);

/* If there is no such conversation, or if there is one but for
   someone else's protocol then we just create a new conversation
   and assign our protocol to it.
*/
    if ((conversation == NULL) ||
        (conversation->dissector_handle != sub_dissector_handle)) {
        conversation = conversation_new(pinfo->num,
                                        &server_src_addr, 0, conversation_type,
                                        server_src_port, 0, new_conv_info, NO_ADDR2 | NO_PORT2);

        /* set the dissector for the new conversation */
        conversation_set_dissector(conversation, sub_dissector_handle);
    }

2.5 Per-packet information.

Information can be stored for each data packet that is processed by the
dissector. The information is added with the p_add_proto_data function and
retrieved with the p_get_proto_data function. The data pointers passed into
the p_add_proto_data are not managed by the proto_data routines, however the
data pointer memory scope must match that of the scope parameter.
The two most common use cases for p_add_proto_data/p_get_proto_data are for
persistent data about the packet for the lifetime of the capture (file scope)
and to exchange data between dissectors across a single packet (packet scope).
It is also used to provide packet data for Decode As dialog (packet scope).

These functions are declared in <epan/proto_data.h>.

void
p_add_proto_data(wmem_allocator_t *scope, packet_info *pinfo, int proto, uint32_t key, void *proto_data)
void *
p_get_proto_data(wmem_allocator_t *scope, packet_info *pinfo, int proto, uint32_t key)

Where:
    scope      - Lifetime of the data to be stored, typically wmem_file_scope()
                 or pinfo->pool (packet scope). Must match scope of data
                 allocated.
    pinfo      - The packet info pointer.
    proto      - Protocol id returned by the proto_register_protocol call
                 during initialization
    key        - key associated with 'proto_data'
    proto_data - pointer to the dissector data.


2.6 User Preferences.

If the dissector has user options, there is support for adding these preferences
to a configuration dialog.

You must register the module with the preferences routine with -

       module_t *prefs_register_protocol(proto_id, void (*apply_cb)(void))
       or
       module_t *prefs_register_protocol_subtree(const char *subtree, int id,
              void (*apply_cb)(void));


Where: proto_id   - the value returned by "proto_register_protocol()" when
                    the protocol was registered.
       apply_cb   - Callback routine that is called when preferences are
                    applied. It may be NULL, which inhibits the callback.
       subtree    - grouping preferences tree node name (several protocols can
                    be grouped under one preferences subtree)

Then you can register the fields that can be configured by the user with these
routines -

    /* Register a preference with an unsigned integral value. */
    void prefs_register_uint_preference(module_t *module, const char *name,
        const char *title, const char *description, unsigned base, unsigned *var);

    /* Register a preference with an Boolean value. */
    void prefs_register_bool_preference(module_t *module, const char *name,
        const char *title, const char *description, bool *var);

    /* Register a preference with an enumerated value. */
    void prefs_register_enum_preference(module_t *module, const char *name,
        const char *title, const char *description, int *var,
        const enum_val_t *enumvals, bool radio_buttons)

    /* Register a preference with a character-string value. */
    void prefs_register_string_preference(module_t *module, const char *name,
        const char *title, const char *description, char **var)

    /* Register a preference with a password (a character-string) value. */
    /* The value is hold during runtime, only in memory. It is never written to disk */
    void prefs_register_password_preference(module_t *module, const char *name,
        const char *title, const char *description, char **var)

    /* Register a preference with a file name (string) value.
    * File name preferences are basically like string preferences
    * except that the GUI gives the user the ability to browse for the
    * file. Set for_writing true to show a Save dialog instead of normal Open.
    */
    void prefs_register_filename_preference(module_t *module, const char *name,
        const char *title, const char *description, char **var,
        bool for_writing)

    /* Register a preference with a range of unsigned integers (e.g.,
     * "1-20,30-40").
     */
    void prefs_register_range_preference(module_t *module, const char *name,
        const char *title, const char *description, range_t *var,
        uint32_t max_value)

Where: module - Returned by the prefs_register_protocol routine
     name     - This is appended to the name of the protocol, with a
            "." between them, to construct a name that identifies
            the field in the preference file; the name itself
            should not include the protocol name, as the name in
            the preference file will already have it. Make sure that
            only lower-case ASCII letters, numbers, underscores and
            dots appear in the preference name.
     title    - Field title in the preferences dialog
     description - Comments added to the preference file above the
               preference value and shown as tooltip in the GUI, or NULL
     var      - pointer to the storage location that is updated when the
            field is changed in the preference dialog box. Note that
            with string preferences the given pointer is overwritten
            with a pointer to a new copy of the string during the
            preference registration. The passed-in string may be
            freed, but you must keep another pointer to the string
            in order to free it.
     base      - Base that the unsigned integer is expected to be in,
            see strtoul(3).
     enumvals - an array of enum_val_t structures. This must be
            NULL-terminated; the members of that structure are:

            a short name, to be used with the "-o" flag - it
            should not contain spaces or upper-case letters,
            so that it's easier to put in a command line;

            a description, which is used in the GUI (and
            which, for compatibility reasons, is currently
            what's written to the preferences file) - it can
            contain spaces, capital letters, punctuation,
            etc.;

            the numerical value corresponding to that name
            and description
     radio_buttons - true if the field is to be displayed in the
             preferences dialog as a set of radio buttons,
             false if it is to be displayed as an option
             menu
     max_value - The maximum allowed value for a range (0 is the minimum).

These functions are declared in <epan/prefs.h>.

An example from packet-rtpproxy.c -

    proto_rtpproxy = proto_register_protocol ( "Sippy RTPproxy Protocol", "RTPproxy", "rtpproxy");

    ...

    rtpproxy_module = prefs_register_protocol(proto_rtpproxy, proto_reg_handoff_rtpproxy);

    prefs_register_bool_preference(rtpproxy_module, "establish_conversation",
                                   "Establish Media Conversation",
                                   "Specifies that RTP/RTCP/T.38/MSRP/etc streams are decoded based "
                                   "upon port numbers found in RTPproxy answers",
                                   &rtpproxy_establish_conversation);

    prefs_register_uint_preference(rtpproxy_module, "reply.timeout",
                                   "RTPproxy reply timeout", /* Title */
                                   "Maximum timeout value in waiting for reply from RTPProxy (in milliseconds).", /* Descr */
                                   10,
                                   &rtpproxy_timeout);

This will create preferences "rtpproxy.establish_conversation" and
"rtpproxy.reply.timeout", the first of which is an Boolean and the
second of which is a unsigned integer.

Note that a warning will pop up if you've saved such preference to the
preference file and you subsequently take the code out. The way to make
a preference obsolete is to register it as such:

/* Register a preference that used to be supported but no longer is. */
    void prefs_register_obsolete_preference(module_t *module,
                                            const char *name);

2.7 Reassembly/desegmentation for protocols running atop TCP.

There are two main ways of reassembling a Protocol Data Unit (PDU) which
spans across multiple TCP segments. The first approach is simpler, but
assumes you are running atop of TCP when this occurs (but your dissector
might run atop of UDP, too, for example), and that your PDUs consist of a
fixed amount of data that includes enough information to determine the PDU
length, possibly followed by additional data. The second method is more
generic but requires more code and is less efficient.

2.7.1 Using tcp_dissect_pdus().

For the first method, you register two different dissection methods, one
for the TCP case, and one for the other cases. It is a good idea to
also have a dissect_PROTO_common function which will parse the generic
content that you can find in all PDUs which is called from
dissect_PROTO_tcp when the reassembly is complete and from
dissect_PROTO_udp (or dissect_PROTO_other).

To register the distinct dissector functions, consider the following
example, stolen from packet-hartip.c:

    #include "packet-tcp.h"

    dissector_handle_t hartip_tcp_handle;
    dissector_handle_t hartip_udp_handle;

    hartip_udp_handle = register_dissector_with_description("hart_ip", "HART-IP over UDP", dissect_hartip_udp, proto_hartip);
    hartip_tcp_handle = register_dissector_with_description("hart_ip.tcp", "HART-IP over TCP", dissect_hartip_tcp, proto_hartip);

    dissector_add_uint_with_preference("udp.port", HARTIP_PORT, hartip_udp_handle);
    dissector_add_uint_with_preference("tcp.port", HARTIP_PORT, hartip_tcp_handle);

The dissect_hartip_udp function does very little work and calls
dissect_hartip_common, while dissect_hartip_tcp calls tcp_dissect_pdus with a
reference to a callback which will be called with reassembled data:

    static int
    dissect_hartip_tcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree,
                   void *data)
    {
        if (!tvb_bytes_exist(tvb, 0, HARTIP_HEADER_LENGTH))
            return 0;

        tcp_dissect_pdus(tvb, pinfo, tree, hartip_desegment, HARTIP_HEADER_LENGTH,
                         get_dissect_hartip_len, dissect_hartip_pdu, data);
        return tvb_reported_length(tvb);
    }

(The dissect_hartip_pdu function acts similarly to dissect_hartip_udp.)
The arguments to tcp_dissect_pdus are:

    the tvbuff pointer, packet_info pointer, and proto_tree pointer
    passed to the dissector;

    a bool flag indicating whether desegmentation is enabled for
    your protocol;

    the number of bytes of PDU data required to determine the length
    of the PDU;

    a routine that takes as arguments a packet_info pointer, a tvbuff
    pointer and an offset value representing the offset into the tvbuff
    at which a PDU begins, and a void pointer for user data, and should
    return the total length of the PDU in bytes (or 0 if more bytes are
    needed to determine the message length).
    The routine must not throw exceptions (it is guaranteed that the
    number of bytes specified by the previous argument to
    tcp_dissect_pdus is available, but more data might not be available,
    so don't refer to any data past that);

    a new_dissector_t routine to dissect the pdu that's passed a tvbuff
    pointer, packet_info pointer, proto_tree pointer and a void pointer for
    user data, with the tvbuff containing a possibly-reassembled PDU. (The
    "reported_length" of the tvbuff will be the length of the PDU);

    a void pointer to user data that is passed to the length-determining
    routine, and the dissector routine referenced in the previous parameter.

2.7.2 Modifying the pinfo struct.

The second reassembly mode is preferred when the dissector cannot determine
how many bytes it will need to read in order to determine the size of a PDU.
It may also be useful if your dissector needs to support reassembly from
protocols other than TCP.

Your dissect_PROTO will initially be passed a tvbuff containing the payload of
the first packet. It should dissect as much data as it can, noting that it may
contain more than one complete PDU. If the end of the provided tvbuff coincides
with the end of a PDU then all is well and your dissector can just return as
normal. (If it is a new-style dissector, it should return the number of bytes
successfully processed.)

If the dissector discovers that the end of the tvbuff does /not/ coincide with
the end of a PDU, (ie, there is half of a PDU at the end of the tvbuff), it can
indicate this to the parent dissector, by updating the pinfo struct. The
desegment_offset field is the offset in the tvbuff at which the dissector will
continue processing when next called. The desegment_len field should contain
the estimated number of additional bytes required for completing the PDU. Next
time your dissect_PROTO is called, it will be passed a tvbuff composed of the
end of the data from the previous tvbuff together with desegment_len more bytes.

If the dissector cannot tell how many more bytes it will need, it should set
desegment_len=DESEGMENT_ONE_MORE_SEGMENT; it will then be called again as soon
as any more data becomes available. Dissectors should set the desegment_len to a
reasonable value when possible rather than always setting
DESEGMENT_ONE_MORE_SEGMENT as it will generally be more efficient. Also, you
*must not* set desegment_len=1 in this case, in the hope that you can change
your mind later: once you return a positive value from desegment_len, your PDU
boundary is set in stone.

static hf_register_info hf[] = {
    { &hf_cstring,
        { "C String", "c.string", FT_STRING, BASE_NONE, NULL, 0x0, NULL, HFILL }
    }
};

/**
*   Dissect a buffer containing ASCII C strings.
*
*   @param  tvb     The buffer to dissect.
*   @param  pinfo   Packet Info.
*   @param  tree    The protocol tree.
*   @param  data    Optional data parameter given by parent dissector.
**/
static int dissect_cstr(tvbuff_t * tvb, packet_info * pinfo, proto_tree * tree, void *data _U_)
{
    unsigned offset = 0;

    while (offset < tvb_reported_length(tvb)) {
        int available = tvb_reported_length_remaining(tvb, offset);
        int len = tvb_strnlen(tvb, offset, available);

        if (-1 == len) {
            /* we ran out of data: ask for more */
            pinfo->desegment_offset = offset;
            pinfo->desegment_len = DESEGMENT_ONE_MORE_SEGMENT;
            return (offset + available);
        }

        col_set_str(pinfo->cinfo, COL_INFO, "C String");

        len += 1; /* Add one for the '\0' */

        proto_tree_add_item(tree, hf_cstring, tvb, offset, len, ENC_ASCII);
        offset += (unsigned)len;
    }

    /* if we get here, then the end of the tvb coincided with the end of a
       string. Happy days. */
    return tvb_captured_length(tvb);
}

This simple dissector will repeatedly return DESEGMENT_ONE_MORE_SEGMENT
requesting more data until the tvbuff contains a complete C string. The C string
will then be added to the protocol tree. Note that there may be more
than one complete C string in the tvbuff, so the dissection is done in a
loop.

2.8 Using udp_dissect_pdus().

As noted in section 2.7.1, TCP has an API to dissect its PDU that can handle
a PDU spread across multiple packets or multiple PDUs spread across a single
packet. This section describes a similar mechanism for UDP, but is only
applicable for one or more PDUs in a single packet. If a protocol runs on top
of TCP as well as UDP, a common PDU dissection function can be created for both.

To register the distinct dissector functions, consider the following
example using UDP and TCP dissection, stolen from packet-dnp.c:

    #include "packet-tcp.h"
    #include "packet-udp.h"

    dissector_handle_t dnp3_tcp_handle;
    dissector_handle_t dnp3_udp_handle;

    dnp3_tcp_handle = register_dissector("dnp3.tcp", dissect_dnp3_tcp, proto_dnp3);
    dnp3_udp_handle = register_dissector("dnp3.udp", dissect_dnp3_udp, proto_dnp3);

    dissector_add_uint_with_preference("tcp.port", TCP_PORT_DNP, dnp3_tcp_handle);
    dissector_add_uint_with_preference("udp.port", UDP_PORT_DNP, dnp3_udp_handle);

Both dissect_dnp3_tcp and dissect_dnp3_udp call tcp_dissect_pdus and
udp_dissect_pdus respectively, with a reference to the same callbacks which
are called to handle PDU data.

    static int
    dissect_dnp3_udp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
    {
        return udp_dissect_pdus(tvb, pinfo, tree, DNP_HDR_LEN, dnp3_udp_check_header,
                                get_dnp3_message_len, dissect_dnp3_message, data);
    }

    static int
    dissect_dnp3_tcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
    {
        if (!check_dnp3_header(tvb, false)) {
            return 0;
        }

        tcp_dissect_pdus(tvb, pinfo, tree, true, DNP_HDR_LEN,
                         get_dnp3_message_len, dissect_dnp3_message, data);

        return tvb_captured_length(tvb);
    }

(udp_dissect_pdus has an option of a heuristic check function within it while
tcp_dissect_pdus does not, so it's done outside)

The arguments to udp_dissect_pdus are:

    the tvbuff pointer, packet_info pointer, and proto_tree pointer
    passed to the dissector;

    the number of bytes of PDU data required to determine the length
    of the PDU;

    an optional routine (passing NULL is okay) that takes as arguments a
    packet_info pointer, a tvbuff pointer and an offset value representing the
    offset into the tvbuff at which a PDU begins, and a void pointer for user
    data, and should return true if the packet belongs to the dissector.
    The routine must not throw exceptions (it is guaranteed that the
    number of bytes specified by the previous argument to
    udp_dissect_pdus is available, but more data might not be available,
    so don't refer to any data past that);

    a routine that takes as arguments a packet_info pointer, a tvbuff
    pointer and an offset value representing the offset into the tvbuff
    at which a PDU begins, and a void pointer for user data, and should
    return the total length of the PDU in bytes. If return value is 0,
    it's treated the same as a failed heuristic.
    The routine must not throw exceptions (it is guaranteed that the
    number of bytes specified by the previous argument to
    tcp_dissect_pdus is available, but more data might not be available,
    so don't refer to any data past that);

    a new_dissector_t routine to dissect the pdu that's passed a tvbuff
    pointer, packet_info pointer, proto_tree pointer and a void pointer for
    user data, with the tvbuff containing a possibly-reassembled PDU. (The
    "reported_length" of the tvbuff will be the length of the PDU);

    a void pointer to user data that is passed to the length-determining
    routine, and the dissector routine referenced in the previous parameter.

2.9 PINOs (Protocols in name only)

For the typical dissector there is a 1-1 relationship between it and its
protocol. However, there are times when a protocol needs multiple "names"
because it has multiple dissection functions going into the same dissector
table. The multiple names removes confusion when picking dissection through
Decode As functionality.

Once the "main" protocol name has been created through proto_register_protocol,
additional "pinos" can be created with proto_register_protocol_in_name_only.
These pinos have all of the naming conventions of a protocol, but are stored
separately as to remove confusion from real protocols. "pinos" the main
protocol's properties for things like enable/disable. i.e. If the "main"
protocol has been disabled, all of its pinos will be disabled as well.
Pinos should not have any fields registered with them or heuristic tables
associated with them.

Another use case for pinos is when a protocol contains a TLV design and it
wants to create a dissector table to handle dissection of the "V". Dissector
tables require a "protocol", but the dissection functions for that table
typically aren't a protocol. In this case proto_register_protocol_in_name_only
creates the necessary placeholder for the dissector table. In addition, because
a dissector table exists, "V"s of the TLVs can be dissected outside of the
original dissector file.

2.10 Creating Decode As functionality.

While the Decode As functionality is available through the GUI, the underlying
functionality is controlled by dissectors themselves. To create Decode As
functionality for a dissector, two things are required:
    1. A dissector table
    2. A series of structures to assist the GUI in how to present the dissector
       table data.

Consider the following example using IP dissection, stolen from packet-ip.c:

    static build_valid_func ip_da_build_value[1] = {ip_value};
    static decode_as_value_t ip_da_values = {ip_prompt, 1, ip_da_build_value};
    static decode_as_t ip_da = {"ip", "ip.proto", 1, 0, &ip_da_values, NULL, NULL,
                                decode_as_default_populate_list, decode_as_default_reset, decode_as_default_change, NULL};
    ...
    ip_dissector_table = register_dissector_table("ip.proto", "IP protocol", ip_proto, FT_UINT8, BASE_DEC);
    ...
    register_decode_as(&ip_da);

ip_da_build_value contains all of the function pointers (typically just 1) that
can be used to retrieve the value(s) that go into the dissector table. This is
usually data saved by the dissector during packet dissector with an API like
p_add_proto_data and retrieved in the "value" function with p_get_proto_data.

ip_da_values contains all of the function pointers (typically just 1) that
provide the text explaining the name and use of the value field that will
be passed to the dissector table to change the dissection output.

ip_da pulls everything together including the dissector (protocol) name, the
"layer type" of the dissector, the dissector table name, the function pointer
values as well as handlers for populating, applying and resetting the changes
to the dissector table through Decode As GUI functionality. For dissector
tables that are an integer or string type, the provided "default" handling
functions shown in the example should suffice.

All entries into a dissector table that use Decode As must have a unique
protocol ID. If a protocol wants multiple entries into a dissector table,
a pino should be used (see section 2.9)

2.11 ptvcursors.

The ptvcursor API allows a simpler approach to writing dissectors for
simple protocols. The ptvcursor API works best for protocols whose fields
are static and whose format does not depend on the value of other fields.
However, even if only a portion of your protocol is statically defined,
then that portion could make use of ptvcursors.

The ptvcursor API lets you extract data from a tvbuff, and add it to a
protocol tree in one step. It also keeps track of the position in the
tvbuff so that you can extract data again without having to compute any
offsets --- hence the "cursor" name of the API.

The three steps for a simple protocol are:
    1. Create a new ptvcursor with ptvcursor_new()
    2. Add fields with multiple calls of ptvcursor_add()
    3. Delete the ptvcursor with ptvcursor_free()

ptvcursor offers the possibility to add subtrees in the tree as well. It can be
done in very simple steps :
    1. Create a new subtree with ptvcursor_push_subtree(). The old subtree is
       pushed in a stack and the new subtree will be used by ptvcursor.
    2. Add fields with multiple calls of ptvcursor_add(). The fields will be
       added in the new subtree created at the previous step.
    3. Pop the previous subtree with ptvcursor_pop_subtree(). The previous
       subtree is again used by ptvcursor.
Note that at the end of the parsing of a packet you must have popped each
subtree you pushed. If it's not the case, the dissector will generate an error.

To use the ptvcursor API, include the "ptvcursor.h" file. The PGM dissector
is an example of how to use it. You don't need to look at it as a guide;
instead, the API description here should be good enough.

2.11.1 ptvcursor API.

ptvcursor_t*
ptvcursor_new(proto_tree* tree, tvbuff_t* tvb, int offset)
    This creates a new ptvcursor_t object for iterating over a tvbuff.
You must call this and use this ptvcursor_t object so you can use the
ptvcursor API.

proto_item*
ptvcursor_add(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding)
    This will extract 'length' bytes from the tvbuff and place it in
the proto_tree as field 'hf', which is a registered header_field. The
pointer to the proto_item that is created is passed back to you. Internally,
the ptvcursor advances its cursor so the next call to ptvcursor_add
starts where this call finished. The 'encoding' parameter is relevant for
certain type of fields (See above under proto_tree_add_item()).

proto_item *
ptvcursor_add_ret_uint(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding, uint32_t *retval);
    Like ptvcursor_add, but returns uint value retrieved

proto_item *
ptvcursor_add_ret_int(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding, int32_t *retval);
    Like ptvcursor_add, but returns int value retrieved

proto_item *
ptvcursor_add_ret_string(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding, wmem_allocator_t *scope, const uint8_t **retval);
    Like ptvcursor_add, but returns string retrieved

proto_item *
ptvcursor_add_ret_boolean(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding, bool *retval);
    Like ptvcursor_add, but returns boolean value retrieved

proto_item *
ptvcursor_add_no_advance(ptvcursor_t* ptvc, int hf, int length, const unsigned encoding)
    Like ptvcursor_add, but does not advance the internal cursor.

void
ptvcursor_advance(ptvcursor_t* ptvc, int length)
    Advances the internal cursor without adding anything to the proto_tree.

void
ptvcursor_free(ptvcursor_t* ptvc)
    Frees the memory associated with the ptvcursor. You must call this
after your dissection with the ptvcursor API is completed.


proto_tree *
ptvcursor_push_subtree(ptvcursor_t* ptvc, proto_item* it, int ett_subtree)
    Pushes the current subtree in the tree stack of the cursor, creates a new
one and sets this one as the working tree.

void
ptvcursor_pop_subtree(ptvcursor_t* ptvc);
    Pops a subtree in the tree stack of the cursor

proto_tree *
ptvcursor_add_with_subtree(ptvcursor_t* ptvc, int hfindex, int length,
                            const unsigned encoding, int ett_subtree);
    Adds an item to the tree and creates a subtree.
If the length is unknown, length may be defined as SUBTREE_UNDEFINED_LENGTH.
In this case, at the next pop, the item length will be equal to the advancement
of the cursor since the creation of the subtree.

proto_tree *
ptvcursor_add_text_with_subtree(ptvcursor_t* ptvc, int length,
                                int ett_subtree, const char *format, ...);
    Add a text node to the tree and create a subtree.
If the length is unknown, length may be defined as SUBTREE_UNDEFINED_LENGTH.
In this case, at the next pop, the item length will be equal to the advancement
of the cursor since the creation of the subtree.

2.11.2 Miscellaneous functions.

tvbuff_t *
ptvcursor_tvbuff(ptvcursor_t* ptvc)
    Returns the tvbuff associated with the ptvcursor.

int
ptvcursor_current_offset(ptvcursor_t* ptvc)
    Returns the current offset.

proto_tree *
ptvcursor_tree(ptvcursor_t* ptvc)
    Returns the proto_tree associated with the ptvcursor.

void
ptvcursor_set_tree(ptvcursor_t* ptvc, proto_tree *tree)
    Sets a new proto_tree for the ptvcursor.

proto_tree *
ptvcursor_set_subtree(ptvcursor_t* ptvc, proto_item* it, int ett_subtree);
    Creates a subtree and adds it to the cursor as the working tree but does
not save the old working tree.

2.12 Optimizations

2.12.1 Null Protocol Trees

A protocol dissector may be called in 2 different ways - with, or
without a non-null "tree" argument.

If the proto_tree argument is null, Wireshark does not need to use
the protocol tree information from your dissector, and therefore is
passing the dissector a null "tree" argument so that it doesn't
need to do work necessary to build the protocol tree.

In the interest of speed, if "tree" is NULL, one can avoid building a
protocol tree and adding stuff to it, or even looking at any packet
data needed only if you're building the protocol tree, if possible.
In particular, string operations and complex calculations that produce
values which will be added as text to the tree and used nowhere else
should be omitted when the tree is NULL.

Note, however, that you must fill in column information, create
conversations, reassemble packets, do calls to "expert" functions,
build any other persistent state needed for dissection, and call
subdissectors regardless of whether "tree" is NULL or not. Note
also that there is no guarantee, the first time the dissector is
called, whether "tree" will be null or not; your dissector must work
correctly, building or updating whatever state information is
necessary, in either case.

Doing this properly generally means structuring the dissection so that
the functions for adding items to the protocol tree are interleaved with
the functions that must be called even when the tree is NULL. This
allows dissection to occur without backtracking to an earlier offset,
which ensures that in the case of invalid or truncated data that aborts
dissection as much information is added to the protocol tree as possible.

This can be inconvenient. Fortunately, the routines for adding items to
the protocol tree can be passed a null protocol tree pointer, in which
case they'll return a null item pointer, and "proto_item_add_subtree()"
returns a null tree pointer if passed a null item pointer. Functions
like "proto_tree_add_item_ret_XXX()" still return the requested value
if there is a null protocol tree, but skip the rest of the work.

So, if you call the API routines and are careful not to dereference any
null tree or item pointers directly, you can ignore whether or not "tree"
is null and do the dissection normally and still get some optimization
benefits. This almost certainly results in easier to read code and lowers
the chance of making subtle errors, and is thus recommended for initial
implementations. It might not be as efficient as skipping that work if
you're not building a protocol tree, but if the code would have a lot of
tests whether "tree" is null to skip that work, you might still be better
off just doing all that work regardless.

2.12.2 Faked Protocol Items

If the "tree" parameter is not NULL, Wireshark needs to use protocol tree
information from your dissector. However, it may not need to use protocol
tree information from *all* fields, because the full set of tree details
will not be displayed or printed (the tree is not "visible"). Wireshark
may be dissecting the packet in order to see if a particular filter matches,
or to determine the value of custom columns that depend on field values,
or to tap values from certain fields (on packets that only match a filter).

In such cases, Wireshark will keep track of which fields are referenced,
and when the API routines are called avoid actually creating a field_info
struct and extracting data from the frame, instead returning a "faked"
protocol item. Passing this faked protocol item to other API routines
like proto_item_set_len() or proto_item_append_text() will have no effect.
API routines that return a value, such as proto_tree_add_item_ret_string(),
will still extract data from the string as necessary to return. Thus the
typical dissector can ignore whether items are faked or not and simply
benefit from the optimization. Dissectors that examine the protocol tree
and protocol items and their field_info structs after creation directly
may have unexpected results; this is best avoided in favor of using the
API routines.

/*
 * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
 *
 * Local variables:
 * c-basic-offset: 4
 * tab-width: 4
 * indent-tabs-mode: nil
 * End:
 *
 * vi: set shiftwidth=4 tabstop=4 expandtab:
 * :indentSize=4:tabSize=4:noTabs=true:
 */
