compile a document about heuristic dissectors, following:

[obnox/wireshark/wip.git] / doc / README.developer

$Revision$
$Date$
$Author$

This file is a HOWTO for Wireshark developers. It describes how to start coding
a Wireshark protocol dissector 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 send
remarks and patches to the developer mailing list.

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:
http://www.wireshark.org) and in the INSTALL and README files of the sources
root dir.

0.1. General README files.

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

- README.capture - the capture engine internals
- README.design - Wireshark software design - incomplete
- README.developer - this file
- README.display_filter - Display Filter Engine
- README.idl2wrs - CORBA IDL converter
- README.packaging - how to distribute a software package containing WS
- README.regression - regression testing of WS and TS
- README.stats_tree - a tree statistics counting specific packets
- README.tapping - "tap" a dissector to get protocol specific events
- README.xml-output - how to work with the PDML exported output
- wiretap/README.developer - how to add additional capture file types to
  Wiretap

0.2. Dissector related README files.

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

- README.binarytrees - fast access to large data collections
- README.heuristic - what are heuristic dissectors and how to write them
- README.malloc - how to obtain "memory leak free" memory
- README.plugins - how to "pluginize" a dissector
- README.request_response_tracking - how to track req./resp. times and such

0.3 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>

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 Code style.

1.1.1 Portability.

Wireshark runs on many platforms, and can be compiled with a number of
different compilers; here are some rules for writing code that will work
on multiple platforms.

Don't use C++-style comments (comments beginning with "//" and running
to the end of the line); Wireshark's dissectors are written in C, and
thus run through C rather than C++ compilers, and not all C compilers
support C++-style comments (GCC does, but IBM's C compiler for AIX, for
example, doesn't do so by default).

Don't initialize variables in their declaration with non-constant
values. Not all compilers support this. E.g. don't use
        guint32 i = somearray[2];
use
        guint32 i;
        i = somearray[2];
instead.

Don't use zero-length arrays; not all compilers support them.  If an
array would have no members, just leave it out.

Don't declare variables in the middle of executable code; not all C
compilers support that.  Variables should be declared outside a
function, or at the beginning of a function or compound statement.

Don't use anonymous unions; not all compilers support it. 
Example:
typedef struct foo {
  guint32 foo;
  union {
    guint32 foo_l;
    guint16 foo_s;
  } u;  /* have a name here */
} foo_t;

Don't use "uchar", "u_char", "ushort", "u_short", "uint", "u_int",
"ulong", "u_long" or "boolean"; they aren't defined on all platforms.
If you want an 8-bit unsigned quantity, use "guint8"; if you want an
8-bit character value with the 8th bit not interpreted as a sign bit,
use "guchar"; if you want a 16-bit unsigned quantity, use "guint16";
if you want a 32-bit unsigned quantity, use "guint32"; and if you want
an "int-sized" unsigned quantity, use "guint"; if you want a boolean,
use "gboolean".  Use "%d", "%u", "%x", and "%o" to print those types;
don't use "%ld", "%lu", "%lx", or "%lo", as longs are 64 bits long on
many platforms, but "guint32" is 32 bits long.

Don't use "long" to mean "signed 32-bit integer", and don't use
"unsigned long" to mean "unsigned 32-bit integer"; "long"s are 64 bits
long on many platforms.  Use "gint32" for signed 32-bit integers and use
"guint32" for unsigned 32-bit integers.

Don't use "long" to mean "signed 64-bit integer" and don't use "unsigned
long" to mean "unsigned 64-bit integer"; "long"s are 32 bits long on
many other platforms.  Don't use "long long" or "unsigned long long",
either, as not all platforms support them; use "gint64" or "guint64",
which will be defined as the appropriate types for 64-bit signed and
unsigned integers.

When printing or displaying the values of 64-bit integral data types,
don't use "%lld", "%llu", "%llx", or "%llo" - not all platforms
support "%ll" for printing 64-bit integral data types.  Instead, for
GLib routines, and routines that use them, such as all the routines in
Wireshark that take format arguments, use G_GINT64_MODIFIER, for example:

    proto_tree_add_text(tree, tvb, offset, 8,
                        "Sequence Number: %" G_GINT64_MODIFIER "u",
                        sequence_number);

When using standard C routines, such as printf and scanf, use
PRId64, PRIu64, PRIx64, PRIX64, and PRIo64, for example:

   printf("Sequence Number: %" PRIu64 "\n", sequence_number);

When specifying an integral constant that doesn't fit in 32 bits, don't
use "LL" at the end of the constant - not all compilers use "LL" for
that.  Instead, put the constant in a call to the "G_GINT64_CONSTANT()"
macro, e.g.

        G_GINT64_CONSTANT(11644473600U)

rather than

        11644473600ULL

Don't use a label without a statement following it.  For example,
something such as

        if (...) {

                ...

        done:
        }

will not work with all compilers - you have to do

        if (...) {

                ...

        done:
                ;
        }

with some statement, even if it's a null statement, after the label.

Don't use "bzero()", "bcopy()", or "bcmp()"; instead, use the ANSI C
routines

        "memset()" (with zero as the second argument, so that it sets
        all the bytes to zero);

        "memcpy()" or "memmove()" (note that the first and second
        arguments to "memcpy()" are in the reverse order to the
        arguments to "bcopy()"; note also that "bcopy()" is typically
        guaranteed to work on overlapping memory regions, while
        "memcpy()" isn't, so if you may be copying from one region to a
        region that overlaps it, use "memmove()", not "memcpy()" - but
        "memcpy()" might be faster as a result of not guaranteeing
        correct operation on overlapping memory regions);

        and "memcmp()" (note that "memcmp()" returns 0, 1, or -1, doing
        an ordered comparison, rather than just returning 0 for "equal"
        and 1 for "not equal", as "bcmp()" does).

Not all platforms necessarily have "bzero()"/"bcopy()"/"bcmp()", and
those that do might not declare them in the header file on which they're
declared on your platform.

Don't use "index()" or "rindex()"; instead, use the ANSI C equivalents,
"strchr()" and "strrchr()".  Not all platforms necessarily have
"index()" or "rindex()", and those that do might not declare them in the
header file on which they're declared on your platform.

Don't fetch data from packets by getting a pointer to data in the packet
with "tvb_get_ptr()", casting that pointer to a pointer to a structure,
and dereferencing that pointer.  That pointer won't necessarily be aligned
on the proper boundary, which can cause crashes on some platforms (even
if it doesn't crash on an x86-based PC); furthermore, the data in a
packet is not necessarily in the byte order of the machine on which
Wireshark is running.  Use the tvbuff routines to extract individual
items from the packet, or use "proto_tree_add_item()" and let it extract
the items for you.

Don't use structures that overlay packet data, or into which you copy
packet data; the C programming language does not guarantee any
particular alignment of fields within a structure, and even the
extensions that try to guarantee that are compiler-specific and not
necessarily supported by all compilers used to build Wireshark.  Using
bitfields in those structures is even worse; the order of bitfields
is not guaranteed.

Don't use "ntohs()", "ntohl()", "htons()", or "htonl()"; the header
files required to define or declare them differ between platforms, and
you might be able to get away with not including the appropriate header
file on your platform but that might not work on other platforms.
Instead, use "g_ntohs()", "g_ntohl()", "g_htons()", and "g_htonl()";
those are declared by <glib.h>, and you'll need to include that anyway,
as Wireshark header files that all dissectors must include use stuff from
<glib.h>.

Don't fetch a little-endian value using "tvb_get_ntohs() or
"tvb_get_ntohl()" and then using "g_ntohs()", "g_htons()", "g_ntohl()",
or "g_htonl()" on the resulting value - the g_ routines in question
convert between network byte order (big-endian) and *host* byte order,
not *little-endian* byte order; not all machines on which Wireshark runs
are little-endian, even though PCs are.  Fetch those values using
"tvb_get_letohs()" and "tvb_get_letohl()".

Don't put a comma after the last element of an enum - some compilers may
either warn about it (producing extra noise) or refuse to accept it.

Don't include <unistd.h> without protecting it with

        #ifdef HAVE_UNISTD_H

                ...

        #endif

and, if you're including it to get routines such as "open()", "close()",
"read()", and "write()" declared, also include <io.h> if present:

        #ifdef HAVE_IO_H
        #include <io.h>
        #endif

in order to declare the Windows C library routines "_open()",
"_close()", "_read()", and "_write()".  Your file must include <glib.h>
- which many of the Wireshark header files include, so you might not have
to include it explicitly - in order to get "open()", "close()",
"read()", "write()", etc. mapped to "_open()", "_close()", "_read()",
"_write()", etc..

Do not use "open()", "rename()", "mkdir()", "stat()", "unlink()", "remove()",
"fopen()", "freopen()" directly.  Instead use "ws_open()", "ws_rename()",
"ws_mkdir()", "ws_stat()", "ws_unlink()", "ws_remove()", "ws_fopen()",
"ws_freopen()": these wrapper functions change the path and file name from
UTF8 to UTF16 on Windows allowing the functions to work correctly when the
path or file name contain non-ASCII characters.

When opening a file with "ws_fopen()", "ws_freopen()", or "ws_fdopen()", if
the file contains ASCII text, use "r", "w", "a", and so on as the open mode
- but if it contains binary data, use "rb", "wb", and so on.  On
Windows, if a file is opened in a text mode, writing a byte with the
value of octal 12 (newline) to the file causes two bytes, one with the
value octal 15 (carriage return) and one with the value octal 12, to be
written to the file, and causes bytes with the value octal 15 to be
discarded when reading the file (to translate between C's UNIX-style
lines that end with newline and Windows' DEC-style lines that end with
carriage return/line feed).

In addition, that also means that when opening or creating a binary
file, you must use "ws_open()" (with O_CREAT and possibly O_TRUNC if the
file is to be created if it doesn't exist), and OR in the O_BINARY flag.
That flag is not present on most, if not all, UNIX systems, so you must
also do

        #ifndef O_BINARY
        #define O_BINARY        0
        #endif

to properly define it for UNIX (it's not necessary on UNIX).

Don't use forward declarations of static arrays without a specified size
in a fashion such as this:

        static const value_string foo_vals[];

                ...

        static const value_string foo_vals[] = {
                { 0,            "Red" },
                { 1,            "Green" },
                { 2,            "Blue" },
                { 0,            NULL }
        };

as some compilers will reject the first of those statements.  Instead,
initialize the array at the point at which it's first declared, so that
the size is known.

Don't put a comma after the last tuple of an initializer of an array.

For #define names and enum member names, prefix the names with a tag so
as to avoid collisions with other names - this might be more of an issue
on Windows, as it appears to #define names such as DELETE and
OPTIONAL.

Don't use the "numbered argument" feature that many UNIX printf's
implement, e.g.:

        g_snprintf(add_string, 30, " - (%1$d) (0x%1$04x)", value);

as not all UNIX printf's implement it, and Windows printf doesn't appear
to implement it.  Use something like

        g_snprintf(add_string, 30, " - (%d) (0x%04x)", value, value);

instead.

Don't use "variadic macros", such as

        #define DBG(format, args...)    fprintf(stderr, format, ## args)

as not all C compilers support them.  Use macros that take a fixed
number of arguments, such as

        #define DBG0(format)            fprintf(stderr, format)
        #define DBG1(format, arg1)      fprintf(stderr, format, arg1)
        #define DBG2(format, arg1, arg2) fprintf(stderr, format, arg1, arg2)

                ...

or something such as

        #define DBG(args)               printf args

Don't use

        case N ... M:

as that's not supported by all compilers.

snprintf() -> g_snprintf()
snprintf() is not available on all platforms, so it's a good idea to use the
g_snprintf() function declared by <glib.h> instead.

tmpnam() -> mkstemp()
tmpnam is insecure and should not be used any more. Wireshark brings its
own mkstemp implementation for use on platforms that lack mkstemp.
Note: mkstemp does not accept NULL as a parameter.

The pointer returned by a call to "tvb_get_ptr()" is not guaranteed to be
aligned on any particular byte boundary; this means that you cannot
safely cast it to any data type other than a pointer to "char",
"unsigned char", "guint8", or other one-byte data types.  You cannot,
for example, safely cast it to a pointer to a structure, and then access
the structure members directly; on some systems, unaligned accesses to
integral data types larger than 1 byte, and floating-point data types,
cause a trap, which will, at best, result in the OS slowly performing an
unaligned access for you, and will, on at least some platforms, cause
the program to be terminated.

Wireshark supports platforms with GLib 2.4[.x]/GTK+ 2.4[.x] or newer. 
If a Glib/GTK+ mechanism is available only in Glib/GTK+ versions 
newer than 2.4/2.4 then use "#if GTK_CHECK_VERSION(...)" to conditionally
compile code using that mechanism. 

When different code must be used on UN*X and Win32, use a #if or #ifdef
that tests _WIN32, not WIN32.  Try to write code portably whenever
possible, however; note that there are some routines in Wireshark with
platform-dependent implementations and platform-independent APIs, such
as the routines in epan/filesystem.c, allowing the code that calls it to
be written portably without #ifdefs.

1.1.2 String handling

Do not use functions such as strcat() or strcpy().
A lot of work has been done to remove the existing calls to these functions and
we do not want any new callers of these functions.

Instead use g_snprintf() since that function will if used correctly prevent
buffer overflows for large strings.

When using a buffer to create a string, do not use a buffer stored on the stack.
I.e. do not use a buffer declared as
   char buffer[1024];
instead allocate a buffer dynamically using the emem routines (see
README.malloc) such as
   char *buffer=NULL;
   ...
   #define MAX_BUFFER 1024
   buffer=ep_alloc(MAX_BUFFER);
   buffer[0]='\0';
   ...
   g_snprintf(buffer, MAX_BUFFER, ...

This avoids the stack from being corrupted in case there is a bug in your code
that accidentally writes beyond the end of the buffer.


If you write a routine that will create and return a pointer to a filled in
string and if that buffer will not be further processed or appended to after
the routine returns (except being added to the proto tree),
do not preallocate the buffer to fill in and pass as a parameter instead
pass a pointer to a pointer to the function and return a pointer to an
emem allocated buffer that will be automatically freed. (see README.malloc)

I.e. do not write code such as
  static void
  foo_to_str(char *string, ... ){
     <fill in string>
  }
  ...
     char buffer[1024];
     ...
     foo_to_str(buffer, ...
     proto_tree_add_text(... buffer ...

instead write the code as
  static void
  foo_to_str(char **buffer, ...
    #define MAX_BUFFER x
    *buffer=ep_alloc(MAX_BUFFER);
    <fill in *buffer>
  }
  ...
    char *buffer;
    ...
    foo_to_str(&buffer, ...
    proto_tree_add_text(... *buffer ...

Use ep_ allocated buffers. They are very fast and nice. These buffers are all
automatically free()d when the dissection of the current packet ends so you
don't have to worry about free()ing them explicitly in order to not leak memory.
Please read README.malloc.

1.1.3 Robustness.

Wireshark is not guaranteed to read only network traces that contain correctly-
formed packets. Wireshark is commonly used to track down networking
problems, and the problems might be due to a buggy protocol implementation
sending out bad packets.

Therefore, protocol dissectors not only have to be able to handle
correctly-formed packets without, for example, crashing or looping
infinitely, they also have to be able to handle *incorrectly*-formed
packets without crashing or looping infinitely.

Here are some suggestions for making dissectors more robust in the face
of incorrectly-formed packets:

Do *NOT* use "g_assert()" or "g_assert_not_reached()" in dissectors.
*NO* value in a packet's data should be considered "wrong" in the sense
that it's a problem with the dissector if found; if it cannot do
anything else with a particular value from a packet's data, the
dissector should put into the protocol tree an indication that the
value is invalid, and should return. You can use the DISSECTOR_ASSERT
macro for that purpose.

If you are allocating a chunk of memory to contain data from a packet,
or to contain information derived from data in a packet, and the size of
the chunk of memory is derived from a size field in the packet, make
sure all the data is present in the packet before allocating the buffer.
Doing so means that

        1) Wireshark won't leak that chunk of memory if an attempt to
           fetch data not present in the packet throws an exception

and

        2) it won't crash trying to allocate an absurdly-large chunk of
           memory if the size field has a bogus large value.

If you're fetching into such a chunk of memory a string from the buffer,
and the string has a specified size, you can use "tvb_get_*_string()",
which will check whether the entire string is present before allocating
a buffer for the string, and will also put a trailing '\0' at the end of
the buffer.

If you're fetching into such a chunk of memory a 2-byte Unicode string
from the buffer, and the string has a specified size, you can use
"tvb_get_ephemeral_faked_unicode()", which will check whether the entire
string is present before allocating a buffer for the string, and will also
put a trailing '\0' at the end of the buffer.  The resulting string will be
a sequence of single-byte characters; the only Unicode characters that
will be handled correctly are those in the ASCII range.  (Wireshark's
ability to handle non-ASCII strings is limited; it needs to be
improved.)

If you're fetching into such a chunk of memory a sequence of bytes from
the buffer, and the sequence has a specified size, you can use
"tvb_memdup()", which will check whether the entire sequence is present
before allocating a buffer for it.

Otherwise, you can check whether the data is present by using
"tvb_ensure_bytes_exist()" or by getting a pointer to the data by using
"tvb_get_ptr()", although note that there might be problems with using
the pointer from "tvb_get_ptr()" (see the item on this in the
Portability section above, and the next item below).

Note also that you should only fetch string data into a fixed-length
buffer if the code ensures that no more bytes than will fit into the
buffer are fetched ("the protocol ensures" isn't good enough, as
protocol specifications can't ensure only packets that conform to the
specification will be transmitted or that only packets for the protocol
in question will be interpreted as packets for that protocol by
Wireshark).  If there's no maximum length of string data to be fetched,
routines such as "tvb_get_*_string()" are safer, as they allocate a buffer
large enough to hold the string.  (Note that some variants of this call
require you to free the string once you're finished with it.)

If you have gotten a pointer using "tvb_get_ptr()", you must make sure
that you do not refer to any data past the length passed as the last
argument to "tvb_get_ptr()"; while the various "tvb_get" routines
perform bounds checking and throw an exception if you refer to data not
available in the tvbuff, direct references through a pointer gotten from
"tvb_get_ptr()" do not do any bounds checking.

If you have a loop that dissects a sequence of items, each of which has
a length field, with the offset in the tvbuff advanced by the length of
the item, then, if the length field is the total length of the item, and
thus can be zero, you *MUST* check for a zero-length item and abort the
loop if you see one.  Otherwise, a zero-length item could cause the
dissector to loop infinitely.  You should also check that the offset,
after having the length added to it, is greater than the offset before
the length was added to it, if the length field is greater than 24 bits
long, so that, if the length value is *very* large and adding it to the
offset causes an overflow, that overflow is detected.

If you are fetching a length field from the buffer, corresponding to the
length of a portion of the packet, and subtracting from that length a
value corresponding to the length of, for example, a header in the
packet portion in question, *ALWAYS* check that the value of the length
field is greater than or equal to the length you're subtracting from it,
and report an error in the packet and stop dissecting the packet if it's
less than the length you're subtracting from it.  Otherwise, the
resulting length value will be negative, which will either cause errors
in the dissector or routines called by the dissector, or, if the value
is interpreted as an unsigned integer, will cause the value to be
interpreted as a very large positive value.

Any tvbuff offset that is added to as processing is done on a packet
should be stored in a 32-bit variable, such as an "int"; if you store it
in an 8-bit or 16-bit variable, you run the risk of the variable
overflowing.

sprintf() -> g_snprintf()
Prevent yourself from using the sprintf() function, as it does not test the
length of the given output buffer and might be writing into unintended memory
areas. This function is one of the main causes of security problems like buffer
exploits and many other bugs that are very hard to find. It's much better to
use the g_snprintf() function declared by <glib.h> instead.

You should test your dissector against incorrectly-formed packets.  This
can be done using the randpkt and editcap utilities that come with the
Wireshark distribution.  Testing using randpkt can be done by generating
output at the same layer as your protocol, and forcing Wireshark/TShark
to decode it as your protocol, e.g. if your protocol sits on top of UDP:

    randpkt -c 50000 -t dns randpkt.pcap
    tshark -nVr randpkt.pcap -d udp.port==53,<myproto>

Testing using editcap can be done using preexisting capture files and the
"-E" flag, which introduces errors in a capture file.  E.g.:

    editcap -E 0.03 infile.pcap outfile.pcap
    tshark -nVr outfile.pcap

The script fuzz-test.sh is available to help automate these tests.

1.1.4 Name convention.

Wireshark uses the underscore_convention rather than the InterCapConvention for
function names, so new code should probably use underscores rather than
intercaps for functions and variable names. This is especially important if you
are writing code that will be called from outside your code.  We are just
trying to keep things consistent for other developers.

1.1.5 White space convention.

Avoid using tab expansions different from 8 column widths, as not all
text editors in use by the developers support this. For a detailed
discussion of tabs, spaces, and indentation, see

    http://www.jwz.org/doc/tabs-vs-spaces.html

When creating a new file, you are free to choose an indentation logic.
Most of the files in Wireshark tend to use 2-space or 4-space
indentation. You are encouraged to write a short comment on the
indentation logic at the beginning of this new file, especially if
you're using non-mod-8 tabs.  The tabs-vs-spaces document above provides
examples of Emacs and vi modelines for this purpose.

When editing an existing file, try following the existing indentation
logic and even if it very tempting, never ever use a restyler/reindenter
utility on an existing file.  If you run across wildly varying
indentation styles within the same file, it might be helpful to send a
note to wireshark-dev for guidance.

1.1.6 Compiler warnings

You should write code that is free of compiler warnings. Such warnings will
often indicate questionable code and sometimes even real bugs, so it's best
to avoid warnings at all.

The compiler flags in the Makefiles are set to "treat warnings as errors",
so your code won't even compile when warnings occur.

1.2 Skeleton code.

Wireshark requires certain things when setting up a protocol dissector.
Below is skeleton code for a dissector that you can copy to a 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.

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 makefile,
described in section "1.9 Editing Makefile.common to add your dissector" below.

Dissectors that use the dissector registration to register with a lower level
dissector don't need to define a prototype in the .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
below, and you may need to include additional headers.  For example, the
code inside

        #ifdef HAVE_LIBPCRE

                ...

        #endif

is needed only if you are using a function from libpcre, e.g. the
"pcre_compile()" function.

The "$Id$" in the comment will be updated by Subversion when the file is
checked in.

When creating a new file, it is fine to just write "$Id$" as Subversion will
automatically fill in the identifier at the time the file will be added to the
SVN repository (committed).

------------------------------------Cut here------------------------------------
/* packet-PROTOABBREV.c
 * Routines for PROTONAME dissection
 * Copyright 200x, YOUR_NAME <YOUR_EMAIL_ADDRESS>
 *
 * $Id$
 *
 * Wireshark - Network traffic analyzer
 * By Gerald Combs <gerald@wireshark.org>
 * Copyright 1998 Gerald Combs
 *
 * Copied from WHATEVER_FILE_YOU_USED (where "WHATEVER_FILE_YOU_USED"
 * is a dissector file; if you just copied this from README.developer,
 * don't bother with the "Copied from" - you don't even need to put
 * in a "Copied from" if you copied an existing dissector, especially
 * if the bulk of the code in the new dissector is your code)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <glib.h>

#include <epan/packet.h>
#include <epan/prefs.h>

/* IF PROTO exposes code to other dissectors, then it must be exported
   in a header file. If not, a header file is not needed at all. */
#include "packet-PROTOABBREV.h"

/* Forward declaration we need below */
void proto_reg_handoff_PROTOABBREV(void);

/* Initialize the protocol and registered fields */
static int proto_PROTOABBREV = -1;
static int hf_PROTOABBREV_FIELDABBREV = -1;

/* Global sample preference ("controls" display of numbers) */
static gboolean gPREF_HEX = FALSE;

/* Initialize the subtree pointers */
static gint ett_PROTOABBREV = -1;

/* Code to actually dissect the packets */
static int
dissect_PROTOABBREV(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{

/* Set up structures needed to add the protocol subtree and manage it */
        proto_item *ti;
        proto_tree *PROTOABBREV_tree;

/*  First, if at all possible, do some heuristics to check if the packet cannot
 *  possibly belong to your protocol.  This is especially important for
 *  protocols directly on top of TCP or UDP where port collisions are
 *  common place (e.g., even though your protocol uses a well known port,
 *  someone else may set up, for example, a web server on that port which,
 *  if someone analyzed that web server's traffic in Wireshark, would result
 *  in Wireshark handing an HTTP packet to your dissector).  For example:
 */
        /* Check that there's enough data */
        if (tvb_length(tvb) < /* your protocol's smallest packet size */)
                return 0;

        /* Get some values from the packet header, probably using tvb_get_*() */
        if ( /* these values are not possible in PROTONAME */ )
                /*  This packet does not appear to belong to PROTONAME.
                 *  Return 0 to give another dissector a chance to dissect it.
                 */
                return 0;

/* Make entries in Protocol column and Info column on summary display */
        if (check_col(pinfo->cinfo, COL_PROTOCOL))
                col_set_str(pinfo->cinfo, COL_PROTOCOL, "PROTOABBREV");

/* This field shows up as the "Info" column in the display; you should use
   it, if possible, to summarize what's in the packet, so that a user looking
   at the list of packets can tell what type of packet it is. See section 1.5
   for more information.

   Before changing the contents of a column you should make sure the column is
   active by calling "check_col(pinfo->cinfo, COL_*)". If it is not active
   don't bother setting it.

   If you are setting the column to a constant string, use "col_set_str()",
   as it's more efficient than the other "col_set_XXX()" calls.

   If you're setting it to a string you've constructed, or will be
   appending to the column later, use "col_add_str()".

   "col_add_fstr()" can be used instead of "col_add_str()"; it takes
   "printf()"-like arguments.  Don't use "col_add_fstr()" with a format
   string of "%s" - just use "col_add_str()" or "col_set_str()", as it's
   more efficient than "col_add_fstr()".

   If you will be fetching any data from the packet before filling in
   the Info column, clear that column first, in case the calls to fetch
   data from the packet throw an exception because they're fetching data
   past the end of the packet, so that the Info column doesn't have data
   left over from the previous dissector; do

        if (check_col(pinfo->cinfo, COL_INFO))
                col_clear(pinfo->cinfo, COL_INFO);

   */

        if (check_col(pinfo->cinfo, COL_INFO))
                col_set_str(pinfo->cinfo, COL_INFO, "XXX Request");

/* A protocol dissector can be called in 2 different ways:

        (a) Operational dissection

                In this mode, Wireshark is only interested in the way protocols
                interact, protocol conversations are created, packets are
                reassembled and handed over to higher-level protocol dissectors.
                In this mode Wireshark does not build a so-called "protocol
                tree".

        (b) Detailed dissection

                In this mode, Wireshark is also interested in all details of
                a given protocol, so a "protocol tree" is created.

   Wireshark distinguishes between the 2 modes with the proto_tree pointer:
        (a) <=> tree == NULL
        (b) <=> tree != NULL

   In the interest of speed, if "tree" is NULL, 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.

   Note, however, that you must fill in column information, create
   conversations, reassemble packets, build any other persistent state
   needed for dissection, and call subdissectors regardless of whether
   "tree" is NULL or not.  This might be inconvenient to do without
   doing most of the dissection work; 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, so, if you're careful not to dereference any null
   tree or item pointers, you can accomplish this by doing all the
   dissection work.  This 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 if you skipped that work,
   you might still be better off just doing all that work regardless of
   whether "tree" is null or not. */
        if (tree) {

/* NOTE: The offset and length values in the call to
   "proto_tree_add_item()" define what data bytes to highlight in the hex
   display window when the line in the protocol tree display
   corresponding to that item is selected.

   Supplying a length of -1 is the way to highlight all data from the
   offset to the end of the packet. */

/* create display subtree for the protocol */
                ti = proto_tree_add_item(tree, proto_PROTOABBREV, tvb, 0, -1, FALSE);

                PROTOABBREV_tree = proto_item_add_subtree(ti, ett_PROTOABBREV);

/* add an item to the subtree, see section 1.6 for more information */
                proto_tree_add_item(PROTOABBREV_tree,
                    hf_PROTOABBREV_FIELDABBREV, tvb, offset, len, FALSE);


/* Continue adding tree items to process the packet here */


        }

/* If this protocol has a sub-dissector call it here, see section 1.8 */

/* Return the amount of data this dissector was able to dissect */
        return tvb_length(tvb);
}


/* Register the protocol with Wireshark */

/* this format is require because a script is used to build the C function
   that calls all the protocol registration.
*/

void
proto_register_PROTOABBREV(void)
{
        module_t *PROTOABBREV_module;

/* Setup list of header fields  See Section 1.6.1 for details*/
        static hf_register_info hf[] = {
                { &hf_PROTOABBREV_FIELDABBREV,
                        { "FIELDNAME",           "PROTOABBREV.FIELDABBREV",
                        FIELDTYPE, FIELDBASE, FIELDCONVERT, BITMASK,
                        "FIELDDESCR", HFILL }
                }
        };

/* Setup protocol subtree array */
        static gint *ett[] = {
                &ett_PROTOABBREV
        };

/* Register the protocol name and description */
        proto_PROTOABBREV = proto_register_protocol("PROTONAME",
            "PROTOSHORTNAME", "PROTOABBREV");

/* Required function calls to register the header fields and subtrees used */
        proto_register_field_array(proto_PROTOABBREV, hf, array_length(hf));
        proto_register_subtree_array(ett, array_length(ett));

/* Register preferences module (See Section 2.6 for more on preferences) */
        PROTOABBREV_module = prefs_register_protocol(proto_PROTOABBREV,
            proto_reg_handoff_PROTOABBREV);

/* Register a sample preference */
        prefs_register_bool_preference(PROTOABBREV_module, "show_hex",
             "Display numbers in Hex",
             "Enable to display numerical values in hexadecimal.",
             &gPREF_HEX);
}


/* If this dissector uses sub-dissector registration add a registration routine.
   This exact format is required because a script is used to find these
   routines and create the code that calls these routines.

   This function is also called by preferences whenever "Apply" is pressed
   (see prefs_register_protocol above) so it should accommodate being called
   more than once.
*/
void
proto_reg_handoff_PROTOABBREV(void)
{
        static gboolean inited = FALSE;

        if (!inited) {

            dissector_handle_t PROTOABBREV_handle;

/*  Use new_create_dissector_handle() to indicate that dissect_PROTOABBREV()
 *  returns the number of bytes it dissected (or 0 if it thinks the packet
 *  does not belong to PROTONAME).
 */
            PROTOABBREV_handle = new_create_dissector_handle(dissect_PROTOABBREV,
                proto_PROTOABBREV);
            dissector_add("PARENT_SUBFIELD", ID_VALUE, PROTOABBREV_handle);

            inited = TRUE;
        }

        /*
          If you perform registration functions which are dependent upon
          prefs the you should de-register everything which was associated
          with the previous settings and re-register using the new prefs
          settings here. In general this means you need to keep track of 
          the PROTOABBREV_handle and the value the preference had at the time 
          you registered.  The PROTOABBREV_handle value and the value of the preference 
          can be saved using local statics in this function (proto_reg_handoff). ie.

          static PROTOABBREV_handle;
          static int currentPort = -1;

          ...

          if (currentPort != -1) {
              dissector_delete("tcp.port", currentPort, PROTOABBREV_handle);
          }

          currentPort = gPortPref;

          dissector_add("tcp.port", currentPort, PROTOABBREV_handle);

        */
}

------------------------------------Cut here------------------------------------

1.3 Explanation of needed substitutions in code skeleton.

In the above code block 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.
WHATEVER_FILE_YOU_USED  Add this line if you are using another file as a
                starting point.
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.
PROTOABBREV     A name for the protocol for use in filter expressions;
                it shall contain only lower-case letters, digits, and
                hyphens.
FIELDNAME       The displayed name for the header field.
FIELDABBREV     The abbreviated name for the header field. (NO SPACES)
FIELDTYPE       FT_NONE, FT_BOOLEAN, FT_UINT8, FT_UINT16, FT_UINT24,
                FT_UINT32, FT_UINT64, FT_INT8, FT_INT16, FT_INT24, FT_INT32,
                FT_INT64, FT_FLOAT, FT_DOUBLE, FT_ABSOLUTE_TIME,
                FT_RELATIVE_TIME, FT_STRING, FT_STRINGZ, FT_EBCDIC,
                FT_UINT_STRING, FT_ETHER, FT_BYTES, FT_IPv4, FT_IPv6, FT_IPXNET,
                FT_FRAMENUM, FT_PROTOCOL, FT_GUID, FT_OID
FIELDBASE       BASE_NONE, BASE_DEC, BASE_HEX, BASE_OCT, BASE_DEC_HEX,
                BASE_HEX_DEC, BASE_RANGE_STRING, BASE_CUSTOM
FIELDCONVERT    VALS(x), RVALS(x), TFS(x), NULL
BITMASK         Usually 0x0 unless using the TFS(x) field conversion.
FIELDDESCR      A brief description of the field, or NULL.
PARENT_SUBFIELD Lower level protocol field used for lookup, i.e. "tcp.port"
ID_VALUE        Lower level protocol field value that identifies this protocol
                For example the TCP or UDP port number

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

1.4 The dissector and the data it receives.


1.4.1 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);


1.4.2 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:

guint8 tvb_get_bits8(tvbuff_t *tvb, gint bit_offset, gint no_of_bits);
guint16 tvb_get_bits16(tvbuff_t *tvb, gint bit_offset, gint no_of_bits,gboolean little_endian);
guint32 tvb_get_bits32(tvbuff_t *tvb, gint bit_offset, gint no_of_bits,gboolean little_endian);
guint64 tvb_get_bits64(tvbuff_t *tvb, gint bit_offset, gint no_of_bits,gboolean little_endian);

Single-byte accessor:

guint8  tvb_get_guint8(tvbuff_t*, gint offset);

Network-to-host-order accessors for 16-bit integers (guint16), 24-bit
integers, 32-bit integers (guint32), and 64-bit integers (guint64):

guint16 tvb_get_ntohs(tvbuff_t*, gint offset);
guint32 tvb_get_ntoh24(tvbuff_t*, gint offset);
guint32 tvb_get_ntohl(tvbuff_t*, gint offset);
guint64 tvb_get_ntoh64(tvbuff_t*, gint offset);

Network-to-host-order accessors for single-precision and
double-precision IEEE floating-point numbers:

gfloat tvb_get_ntohieee_float(tvbuff_t*, gint offset);
gdouble tvb_get_ntohieee_double(tvbuff_t*, gint offset);

Little-Endian-to-host-order accessors for 16-bit integers (guint16),
24-bit integers, 32-bit integers (guint32), and 64-bit integers
(guint64):

guint16 tvb_get_letohs(tvbuff_t*, gint offset);
guint32 tvb_get_letoh24(tvbuff_t*, gint offset);
guint32 tvb_get_letohl(tvbuff_t*, gint offset);
guint64 tvb_get_letoh64(tvbuff_t*, gint offset);

Little-Endian-to-host-order accessors for single-precision and
double-precision IEEE floating-point numbers:

gfloat tvb_get_letohieee_float(tvbuff_t*, gint offset);
gdouble tvb_get_letohieee_double(tvbuff_t*, gint offset);

Accessors for IPv4 and IPv6 addresses:

guint32 tvb_get_ipv4(tvbuff_t*, gint offset);
void tvb_get_ipv6(tvbuff_t*, gint offset, struct e_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.

Accessors for GUID:

void tvb_get_ntohguid(tvbuff_t *, gint offset, e_guid_t *guid);
void tvb_get_letohguid(tvbuff_t *, gint offset, e_guid_t *guid);

String accessors:

guint8 *tvb_get_string(tvbuff_t*, gint offset, gint length);
guint8 *tvb_get_ephemeral_string(tvbuff_t*, gint offset, gint length);

Returns a null-terminated buffer containing data from the specified
tvbuff, starting at the specified offset, and containing the specified
length worth of characters (the length of the buffer will be length+1,
as it includes a null character to terminate the string).

tvb_get_string() returns a buffer allocated by g_malloc() so you must
g_free() it when you are finished with the string. Failure to g_free() this
buffer will lead to memory leaks.
tvb_get_ephemeral_string() returns a buffer allocated from a special heap
with a lifetime until the next packet is dissected. You do not need to
free() this buffer, it will happen automatically once the next packet is
dissected.


guint8 *tvb_get_stringz(tvbuff_t *tvb, gint offset, gint *lengthp);
guint8 *tvb_get_ephemeral_stringz(tvbuff_t *tvb, gint offset, gint *lengthp);

Returns a null-terminated buffer, allocated with "g_malloc()",
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.  "*lengthp"
will be set to the length of the string, including the terminating null.

tvb_get_stringz() returns a buffer allocated by g_malloc() so you must
g_free() it when you are finished with the string. Failure to g_free() this
buffer will lead to memory leaks.
tvb_get_ephemeral_stringz() returns a buffer allocated from a special heap
with a lifetime until the next packet is dissected. You do not need to
free() this buffer, it will happen automatically once the next packet is
dissected.


guint8 *tvb_fake_unicode(tvbuff_t*, gint offset, gint length, gboolean little_endian);
guint8 *tvb_get_ephemeral_faked_unicode(tvbuff_t*, gint offset, gint length, gboolean little_endian);

Converts a 2-byte unicode string to an ASCII string.
Returns a null-terminated buffer containing data from the specified
tvbuff, starting at the specified offset, and containing the specified
length worth of characters (the length of the buffer will be length+1,
as it includes a null character to terminate the string).

tvb_fake_unicode() returns a buffer allocated by g_malloc() so you must
g_free() it when you are finished with the string. Failure to g_free() this
buffer will lead to memory leaks.
tvb_get_ephemeral_faked_unicode() returns a buffer allocated from a special
heap with a lifetime until the next packet is dissected. You do not need to
free() this buffer, it will happen automatically once the next packet is
dissected.


Copying memory:
guint8* tvb_memcpy(tvbuff_t*, guint8* target, gint offset, gint length);

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

guint8* tvb_memdup(tvbuff_t*, gint offset, gint length);
guint8* ep_tvb_memdup(tvbuff_t*, gint offset, gint length);

Returns a buffer, allocated with "g_malloc()", containing the specified
length's worth of data from the specified tvbuff, starting at the
specified offset. The ephemeral variant is freed automatically after the
packet is dissected.

Pointer-retrieval:
/* WARNING! This function is possibly expensive, temporarily allocating
 * another copy of the packet data. Furthermore, 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.
 */
guint8* tvb_get_ptr(tvbuff_t*, gint offset, gint length);

The reason that tvb_get_ptr() might have to allocate a copy of its data
only occurs with TVBUFF_COMPOSITES, data that spans multiple tvbuffers.
If the user requests a pointer to a range of bytes that span the member
tvbuffs that make up the TVBUFF_COMPOSITE, the data will have to be
copied to another memory region to assure that all the bytes are
contiguous.



1.5 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.

A value for a column should only be added if the user specified that it
be displayed; to check whether a given column is to be displayed, call
'check_col' with the COL_ value for that field as an argument - it will
return TRUE if the column is to be displayed and FALSE if it is not to
be displayed.

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.

1.5.1 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 "PROTOABBREV":

        if (check_col(pinfo->cinfo, COL_PROTOCOL))
                col_set_str(pinfo->cinfo, COL_PROTOCOL, "PROTOABBREV");


1.5.2 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.


1.5.3 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:

        if (check_col(pinfo->cinfo, COL_INFO))
                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.


1.5.4 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.


1.5.5 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.)


1.5.6 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.

1.5.7 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.

1.5.8 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 prended data if the
fence does not already exist.


1.5.9 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:

if (check_col(pinfo->cinfo, COL_REL_CONV_TIME)) {
  nstime_delta(&ts, &pinfo->fd->abs_ts, &tcpd->ts_first);
  col_set_time(pinfo->cinfo, COL_REL_CONV_TIME, &ts, "tcp.time_relative");
}


1.6 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 a GNode, the N-way tree structure
available within GLIB. 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.

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/Makefile.common";

        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;
        guint32                         bitmask;
        const char                      *blurb;
        .....
};

name
----
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
------
A string with an abbreviation of the field. We concatenate the
abbreviation of the parent protocol with an abbreviation for the field,
using a period as a separator. For example, the "src" field in an IP packet
would have "ip.src" as an abbreviation. 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 abbreviations follow this pattern:
"trmac.errors.iso", "trmac.errors.noniso", etc.

The abbreviation is the identifier used in a display filter.  If it is
an empty string then the field will not be filterable.

type
----
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_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_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_INT64                A 64-bit signed integer.
        FT_FLOAT                A single-precision floating point number.
        FT_DOUBLE               A double-precision floating point number.
        FT_ABSOLUTE_TIME        Seconds (4 bytes) and nanoseconds (4 bytes)
                                of time displayed as month name, month day,
                                year, hours, minutes, and seconds with 9
                                digits after the decimal point.
        FT_RELATIVE_TIME        Seconds (4 bytes) and nanoseconds (4 bytes)
                                of time displayed as seconds and 9 digits
                                after the decimal point.
        FT_STRING               A string of characters, not necessarily
                                NUL-terminated, but possibly NUL-padded.
                                This, and the other string-of-characters
                                types, are to be used for text strings,
                                not raw binary data.
        FT_STRINGZ              A NUL-terminated string of characters.
        FT_EBCDIC               A string of characters, not necessarily
                                NUL-terminated, but possibly NUL-padded.
                                The data from the packet is converted from
                                EBCDIC to ASCII before displaying to the user.
        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_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

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.

display
-------
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. The function pointer of the same type as defined by
custom_fmt_func_t in epan/proto.h, specifically:

  void func(gchar *, guint32);

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.

For FT_BOOLEAN fields that are also bitfields, 'display' is used to tell
the proto_tree how wide the parent bitfield is.  With integers this is
not needed since the type of integer itself (FT_UINT8, FT_UINT16,
FT_UINT24, FT_UINT32, etc.) tells the proto_tree how wide the parent
bitfield is.

Additionally, BASE_NONE is used for 'display' as a NULL-value. That is,
for non-integers 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.

strings
-------
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 {
                guint32  value;
                gchar   *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 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 {
                guint32        value_min;
                guint32        value_max;
                const gchar   *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.
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).

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.

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
-----
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.
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.


1.6.1 Field Registration.

Protocol registration is handled by creating an instance of the
header_field_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 = -1;
        static int hf_field_a = -1;
        static int hf_field_b = -1;

        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.

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

1.6.2 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.

Remember that it only makes sense to add items to a protocol tree if its
proto_tree pointer is not null. Should you add an item to a NULL tree, then
the proto_XXX_DO_YYY() function will immediately return. The cost of this
function call can be avoided by checking for the tree pointer.

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 "gint" variables to hold the subtree type values to
"proto_register_subtree_array()":

        static gint ett_eg = -1;
        static gint ett_field_a = -1;

        static gint *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.

There are several functions that the programmer can use to add either
protocol or field labels to the proto_tree:

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

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

        proto_item *
        proto_tree_add_bits_ret_val(tree, id, tvb, bit_offset, no_of_bits, return_value, little_endian);

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

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

        proto_item *
        proto_tree_add_bytes(tree, id, tvb, start, length, start_ptr);

        proto_item *
        proto_tree_add_bytes_format(tree, id, tvb, start, length, start_ptr,
            format, ...);

        proto_item *
        proto_tree_add_bytes_format_value(tree, id, tvb, start, length,
            start_ptr, format, ...);

        proto_item *
        proto_tree_add_time(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_time_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_time_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

        proto_item *
        proto_tree_add_ipxnet(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_ipv4(tree, id, tvb, start, length, value);

        proto_item *
        proto_tree_add_ipv4_format(tree, id, tvb, start, length, value,
            format, ...);

        proto_item *
        proto_tree_add_ipv4_format_value(tree, id, tvb, start, length,
            value, format, ...);

        proto_item *
        proto_tree_add_ipv6(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_ipv6_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_ipv6_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

        proto_item *
        proto_tree_add_ether(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_ether_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_ether_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

        proto_item *
        proto_tree_add_string(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_string_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_string_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

        proto_item *
        proto_tree_add_boolean(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_float(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_double(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_uint(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_uint64(tree, id, tvb, start, length, value);

        proto_item *
        proto_tree_add_uint64_format(tree, id, tvb, start, length, value,
            format, ...);

        proto_item *
        proto_tree_add_uint64_format_value(tree, id, tvb, start, length,
            value, format, ...);

        proto_item *
        proto_tree_add_int(tree, id, tvb, start, length, value);

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

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

        proto_item *
        proto_tree_add_int64(tree, id, tvb, start, length, value);

        proto_item *
        proto_tree_add_int64_format(tree, id, tvb, start, length, value,
            format, ...);

        proto_item *
        proto_tree_add_int64_format_value(tree, id, tvb, start, length,
            value, format, ...);

        proto_item*
        proto_tree_add_text(tree, tvb, start, length, format, ...);

        proto_item*
        proto_tree_add_text_valist(tree, tvb, start, length, format, ap);

        proto_item *
        proto_tree_add_guid(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_guid_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_guid_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

        proto_item *
        proto_tree_add_oid(tree, id, tvb, start, length, value_ptr);

        proto_item *
        proto_tree_add_oid_format(tree, id, tvb, start, length, value_ptr,
            format, ...);

        proto_item *
        proto_tree_add_oid_format_value(tree, id, tvb, start, length,
            value_ptr, format, ...);

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

        proto_item *
        proto_tree_add_bitmask_text(proto_tree *tree, tvbuff_t *tvb,
            guint offset, guint len, const char *name, const char *fallback,
            gint ett, const int **fields, gboolean little_endian, int flags);

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 lenght 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.

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, for integral and Boolean fields, the byte order of the value; the
byte order is specified by the 'little_endian' argument, which is TRUE
if the value is little-endian and FALSE if it is big-endian.

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, TRUE);

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,  "Adjaced 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_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" (formated as FT_ indicates).

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

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()
------------------------
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.

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.

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.

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()
----------------------------
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()
------------------------------------

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.

proto_tree_add_text()
---------------------
proto_tree_add_text() is used to add a label to the GUI tree.  It will
contain no value, so it is not searchable in the display filter process.
This function was needed in the transition from the old-style proto_tree
to this new-style proto_tree so that Wireshark would still decode all
protocols w/o being able to filter on all protocols and fields.
Otherwise we would have had to cripple Wireshark's functionality while we
converted all the old-style proto_tree calls to the new-style proto_tree
calls.  In other words, you should not use this in new code unless you've got
a specific reason (see below).

This can also 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 value returned by
'proto_tree_add_text()', 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_text()' 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:

        ti = proto_tree_add_text(tree, tvb, offset, length, <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_text_valist()
----------------------------
This is like proto_tree_add_text(), but takes, as the last argument, a
'va_list'; 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_bitmask() and proto_tree_add_bitmask_text()
----------------------------------------------------------
This function provides an easy to use and convenient helper function
to manage many types of common bitmasks that occur in protocols.

This function will dissect a 1/2/3/4 byte large bitmask into its individual
fields.
header is an integer type and must be of type FT_[U]INT{8|16|24|32} and
represents the entire width of the bitmask.

'header' and 'ett' are the hf fields and ett field respectively to create an
expansion that covers the 1-4 bytes of the bitmask.

'**fields' is a NULL terminated a array of pointers to hf fields representing
the individual subfields of the bitmask. These fields must either be integers
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        = -1;
        static int hf_scsi_inq_qualifier         = -1;
        static gint ett_scsi_inq_peripheral = -1;
        ...
        static const int *peripheal_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, peripheal_fields, FALSE);
        offset+=1;
        ...
        { &hf_scsi_inq_peripheral,
          {"Peripheral", "scsi.inquiry.preipheral", 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}},
        ...

Which provides very pretty dissection of this one byte bitmask.

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, ..., FALSE);
                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, ..., FALSE);
                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

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

1.7 Utility routines.

1.7.1 match_strval and val_to_str.

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.

'match_strval()' will do that:

        gchar*
        match_strval(guint32 val, const value_string *vs)

It will look up the value 'val' in the 'value_string' table pointed to
by 'vs', and return either the corresponding string, or NULL if the
value could not be found in the table.  Note that, unless 'val' is
guaranteed to be a value in the 'value_string' table ("guaranteed" as in
"the code has already checked that it's one of those values" or "the
table handles all possible values of the size of 'val'", not "the
protocol spec says it has to be" - protocol specs do not prevent invalid
packets from being put onto a network or into a purported packet capture
file), you must check whether 'match_strval()' returns NULL, and arrange
that its return value not be dereferenced if it's NULL. 'val_to_str()'
can be used to generate a string for values not found in the table:

        gchar*
        val_to_str(guint32 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(val, table, "Unknown %d"));

1.7.2 match_strrval and rval_to_str.

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

'match_strrval()' will do that:

        gchar*
        match_strrval(guint32 val, const range_string *rs)

It will look up the value 'val' in the 'range_string' table pointed to
by 'rs', and return either the corresponding string, or NULL if the
value could not be found in the table. Please note that its base
behavior is inherited from match_strval().

'rval_to_str()' can be used to generate a string for values not found in
the table:

        gchar*
        rval_to_str(guint32 val, const range_string *rs, const char *fmt)

If the value 'val' is found in the 'range_string' table pointed to by
'rs', 'rval_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. Please
note that its base behavior is inherited from match_strval().

1.8 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).

The syntax for creating a new TVBUFF_SUBSET is:

next_tvb = tvb_new_subset(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 0th byte.

        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.


An example from packet-ipx.c -

void
dissect_ipx(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
{
        tvbuff_t        *next_tvb;
        int             reported_length, available_length;


        /* Make the next tvbuff */

/* IPX does have a length value in the header, so calculate report_length */
   Set this to -1 if there isn't any length information in the protocol
*/
        reported_length = ipx_length - IPX_HEADER_LEN;

/* Calculate the available data in the packet,
   set this to -1 to use all the data in the tv_buffer
*/
        available_length = tvb_length(tvb) - IPX_HEADER_LEN;

/* Create the tvbuffer for the next dissector */
        next_tvb = tvb_new_subset(tvb, IPX_HEADER_LEN,
                        MIN(available_length, reported_length),
                        reported_length);

/* call the next dissector */
        dissector_next( next_tvb, pinfo, tree);


1.9 Editing Makefile.common 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' macro in the 'Makefile.common' file in the 'epan/dissectors'
directory.  (Note that this is for modern versions of UNIX, so there
is no 14-character limitation on file names, and for modern versions of
Windows, so there is no 8.3-character limitation on file names.)

If your dissector also has its own header file or files, you must add
them to the 'DISSECTOR_INCLUDES' macro in the 'Makefile.common' file in
the 'epan/dissectors' directory, so that it's included when release source
tarballs are built (otherwise, the source in the release tarballs won't
compile).

1.10 Using the SVN source code tree.

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

1.11 Submitting code for your new dissector.

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

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

  - 'svn add' all the files of your new dissector.

  - 'svn diff' the workspace and save the result to a file.

  - Edit the diff file - remove any changes unrelated to your new dissector,
    e.g. changes in config.nmake

  - Submit a bug report to the Wireshark bug database, found at
    <http://bugs.wireshark.org>, qualified as an enhancement and attach your
    diff file there. Set the review request flag to '?' so it will pop up in
    the patch review list.

  - Create a Wiki page on the protocol at <http://wiki.wireshark.org>.
    A template is provided so it is easy to setup in a consistent style.

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

  - If you find that you are contributing a lot to wireshark on an ongoing
    basis you can request to become a committer which will allow you to
    commit files to subversion directly.

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.

There are five routines that you will use to work with a conversation:
conversation_new, find_conversation, conversation_add_proto_data,
conversation_get_proto_data, and conversation_delete_proto_data.


2.2.1 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_new 'data' variable.  You are responsible for
this clean up if you pass a malloc'ed pointer in this variable.

See item 2.2.8 for more information about the 'data' pointer.


2.2.2 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 ptype 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 know 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(guint32 setup_frame, address *addr1,
            address *addr2, port_type ptype, guint32 port1, guint32 port2,
            guint options);

Where:
        guint32 setup_frame = The lowest numbered frame for this conversation
        address* addr1      = first data packet address
        address* addr2      = second data packet address
        port_type ptype     = port type, this is defined in packet.h
        guint32 port1       = first data packet port
        guint32 port2       = second data packet port
        guint 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.

2.2.3 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(guint32 frame_num, address *addr_a,
            address *addr_b, port_type ptype, guint32 port_a, guint32 port_b,
            guint options);

Where:
        guint32 frame_num = a frame number to match
        address* addr_a = first address
        address* addr_b = second address
        port_type ptype = port type
        guint32 port_a  = first data packet port
        guint32 port_b  = second data packet port
        guint 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.


2.2.4 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.  Using the protocol number allows several dissectors to
associate data with a given conversation.


2.2.5 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.


2.2.6 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, 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.


2.2.7 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->flags.visited 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). There are two columns for conversation timestamps:

COL_REL_CONV_TIME,  /* Relative time to beginning of conversation */
COL_DELTA_CONV_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 lowel 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.


2.2.8 The example conversation code with GMemChunk's.

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

NOTE: Remember to register the init routine (my_dissector_init) in the
protocol_register routine.


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

/* the number of entries in the memory chunk array */
#define my_init_count 10

/* define your structure here */
typedef struct {

} my_entry_t;

/* the GMemChunk base structure */
static GMemChunk *my_vals = NULL;

/* Registered protocol number */
static int my_proto = -1;


/********************* 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->fd->num, &pinfo->src, &pinfo->dst,
        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 = g_mem_chunk_alloc(my_vals);

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

    /* create the conversation with your data pointer  */

    conversation = conversation_new(pinfo->fd->num,  &pinfo->src, &pinfo->dst, 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 dissector init routine *******************/

#define my_init_count 20

static void
my_dissector_init(void)
{

    /* destroy memory chunks if needed */

    if (my_vals)
        g_mem_chunk_destroy(my_vals);

    /* now create memory chunks */

    my_vals = g_mem_chunk_new("my_proto_vals",
            sizeof(my_entry_t),
            my_init_count * sizeof(my_entry_t),
            G_ALLOC_AND_FREE);
}

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

/* register re-init routine */

register_init_routine(&my_dissector_init);

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


2.2.9 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->fd->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->fd->num, &pinfo->src, &pinfo->dst,
            pinfo->ptype, pinfo->srcport, pinfo->destport, 0);
        if (conversation == NULL || (conversation->setup_frame != pinfo->fd->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->fd->num, &pinfo->src,
                    &pinfo->dst, pinfo->ptype, pinfo->srcport, pinfo->destport,
                    NULL, 0);
        }


2.2.10 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_conversation(pinfo->fd->num, &pinfo->src, &pinfo->dst,
            pinfo->ptype, pinfo->srcport, pinfo->destport, 0);
        if (conversation == NULL) {
                /* It's not part of any conversation - create a new one. */
                conversation = conversation_new(pinfo->fd->num, &pinfo->src,
                    &pinfo->dst, pinfo->ptype, pinfo->srcport, pinfo->destport,
                    NULL, 0);
        }

        request_key.conversation = conversation->index;
        request_key.service = pntohs(&rxh->serviceId);
        request_key.callnumber = pntohl(&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 = g_mem_chunk_alloc(afs_request_keys);
                *new_request_key = request_key;

                request_val = g_mem_chunk_alloc(afs_request_vals);
                request_val -> opcode = pntohl(&afsh->opcode);
                opcode = request_val->opcode;

                g_hash_table_insert(afs_request_hash, new_request_key,
                        request_val);
        }



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 know, 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 or
register_dissector.

create_dissector_handle takes as arguments 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.

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->fd->num,
                                &pinfo->src, &pinfo->dst, protocol,
                                src_port, dst_port, new_conv_info, 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 = g_mem_chunk_alloc(new_conv_vals);
            new_conv_info->data1 = value1;

/* create the conversation for the dynamic port */
            conversation = conversation_new(pinfo->fd->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 = create_dissector_handle(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, guint32 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 = g_mem_chunk_alloc(new_conv_vals);
        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->fd->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->fd->num,
            &server_src_addr, 0, protocol,
            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. If you use
malloc or any other dynamic memory allocation scheme, you must release the
data when it isn't required.

void
p_add_proto_data(frame_data *fd, int proto, void *proto_data)
void *
p_get_proto_data(frame_data *fd, int proto)

Where:
        fd         - The fd pointer in the pinfo structure, pinfo->fd
        proto      - Protocol id returned by the proto_register_protocol call
                     during initialization
        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))

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


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, guint base, guint *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, gboolean *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, gint *var,
            const enum_val_t *enumvals, gboolean 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 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,
            guint32 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
         title    - Field title in the preferences dialog
         description - Comments added to the preference file above the
                       preference value
         var      - pointer to the storage location that is updated when the
                    field is changed in the preference dialog box
         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).

An example from packet-beep.c -

  proto_beep = proto_register_protocol("Blocks Extensible Exchange Protocol",
                                       "BEEP", "beep");

        ...

  /* Register our configuration options for BEEP, particularly our port */

  beep_module = prefs_register_protocol(proto_beep, proto_reg_handoff_beep);

  prefs_register_uint_preference(beep_module, "tcp.port", "BEEP TCP Port",
                                 "Set the port for BEEP messages (if other"
                                 " than the default of 10288)",
                                 10, &global_beep_tcp_port);

  prefs_register_bool_preference(beep_module, "strict_header_terminator",
                                 "BEEP Header Requires CRLF",
                                 "Specifies that BEEP requires CRLF as a "
                                 "terminator, and not just CR or LF",
                                 &global_beep_strict_term);

This will create preferences "beep.tcp.port" and
"beep.strict_header_terminator", the first of which is an unsigned
integer and the second of which is a Boolean.

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-dns.c:

        dissector_handle_t dns_udp_handle;
        dissector_handle_t dns_tcp_handle;
        dissector_handle_t mdns_udp_handle;

        dns_udp_handle = create_dissector_handle(dissect_dns_udp,
            proto_dns);
        dns_tcp_handle = create_dissector_handle(dissect_dns_tcp,
            proto_dns);
        mdns_udp_handle = create_dissector_handle(dissect_mdns_udp,
            proto_dns);

        dissector_add("udp.port", UDP_PORT_DNS, dns_udp_handle);
        dissector_add("tcp.port", TCP_PORT_DNS, dns_tcp_handle);
        dissector_add("udp.port", UDP_PORT_MDNS, mdns_udp_handle);
        dissector_add("tcp.port", TCP_PORT_MDNS, dns_tcp_handle);

The dissect_dns_udp function does very little work and calls
dissect_dns_common, while dissect_dns_tcp calls tcp_dissect_pdus with a
reference to a callback which will be called with reassembled data:

        static void
        dissect_dns_tcp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
        {
                tcp_dissect_pdus(tvb, pinfo, tree, dns_desegment, 2,
                    get_dns_pdu_len, dissect_dns_tcp_pdu);
        }

(The dissect_dns_tcp_pdu function acts similarly to dissect_dns_udp.)
The arguments to tcp_dissect_pdus are:

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

        a gboolean 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 should return - *without* throwing an
        exception (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) - the
        total length of the PDU, in bytes;

        a routine that's passed a tvbuff pointer, packet_info pointer,
        and proto_tree pointer, with the tvbuff containing a
        possibly-reassembled PDU, and that should dissect that PDU.

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,
      "C String", HFILL}
     }
   };

/**
*   Dissect a buffer containing C strings.
*
*   @param  tvb     The buffer to dissect.
*   @param  pinfo   Packet Info.
*   @param  tree    The protocol tree.
**/
static void dissect_cstr(tvbuff_t * tvb, packet_info * pinfo, proto_tree * tree)
{
    guint offset = 0;
    while(offset < tvb_reported_length(tvb)) {
        gint available = tvb_reported_length_remaining(tvb, offset);
        gint 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;
        }

        if (check_col(pinfo->cinfo, COL_INFO)) {
            col_set_str(pinfo->cinfo, COL_INFO, "C String");
        }

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

        if (tree) {
            proto_tree_add_item(tree, hf_cstring, tvb, offset, len, FALSE);
        }
        offset += (guint)len;
    }

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

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 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.

2.8.1 ptvcursor API.

ptvcursor_t*
ptvcursor_new(proto_tree* tree, tvbuff_t* tvb, gint 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, gint length, gboolean endianness)
    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 'endianness' parameter matters for
FT_UINT* and FT_INT* fields.

proto_item*
ptvcursor_add_no_advance(ptvcursor_t* ptvc, int hf, gint length, gboolean endianness)
    Like ptvcursor_add, but does not advance the internal cursor.

void
ptvcursor_advance(ptvcursor_t* ptvc, gint 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, gint 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, gint length,
                            gboolean little_endian, gint 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, gint length,
                                gint 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.

2.8.2 Miscellaneous functions.

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

gint
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, gint ett_subtree);
    Creates a subtree and adds it to the cursor as the working tree but does
not save the old working tree.