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Wireshark-dev: [Wireshark-dev] string manipulation

From: Brian Oleksa <oleksab@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 25 Jan 2010 16:11:16 -0500
Guy

This past e-mail was very helpful. I have made some big leaps fixing some of my code to adhere to the coding standards.

But I do have some more questions:    I am on too strings now.

This is what I am currently doing:

PS. I am still working on fixing the ptr stuff....but wanted to start small and work one problem at a time.

                   char flowname[9];
                   strncpy(flowname, ptr, 8);
                   flowname[8] = '\0';
                   ptr += 8;
proto_tree_add_uint_format(helen_sub_tree, hf_helen_length, tvb, offset, 8, 0,
                           "Flowname: %s", flowname);
                   offset += 8;

In the developers README file... it tells you NOT to use strcpy...but instead use g_snprintf().

I am having a hard time interpreting this into what I already have. Any help (or examples using g_snprintf) is greatly appreciated.

Thanks,
Brian





Guy Harris wrote:
On Jan 21, 2010, at 11:59 AM, Brian Oleksa wrote:

But how I start the initial counting process is I do the following:

guint8 * ptr = (guint8*) tvb->real_data;

Don't do that.

First of all, that isn't guaranteed to work for all tvbuffs; we currently aren't using composite tvbuffs (and there are apparently some bugs in them), but, if we do, there is no guarantee that the "real_data" field of a tvbuff will always be valid.

Second of all, you should not just extract fields from the packet data yourself:

	doing so means that no bounds checking is done, so you might run past the end of the packet data (do *NOT* assume either that all packets for your protocol will be valid or that the capture wasn't cut short by setting the snapshot length when capturing);

	doing so means that if a field isn't aligned on an appropriate boundary in memory, attempting to fetch the data could fail (SPARC processors, for example, do not support unaligned accesses, and we *do* support Wireshark on SPARC);

	doing so means means that you have to do the byte swapping yourself.

Instead, use the tvb_get_ routines to fetch fields individually, using the offset variable.

Speaking of byte swapping, this:

	https://www.darkcornersoftware.com/confluence/display/open/Packet+Structure

says "All values are in network byte order", so if you're running on a machine with the most popular family of desktop and notebook processors - i.e., an x86 or x86-64 processor - you *would* have to byte-swap values if you fetch them yourself.  That also means that the tvb_get_ntoh routines should be used to fetch numerical field values.

Actually..... maybe you can see your answer better in the code. I have attached the packet-helen.c file.

Please don't use hf_helen_length for anything other than an actual length field.  Each field in the packet should have its *own* hf_ value.

Once you've started using the tvb_get_ routines, and the offset variable, to process the fields, and have given each field its own hf_ value, then:

The hf_ item for the time stamp should be something like

	{ &hf_helen_time,
	    { "Time", "helen.time", FT_ABSOLUTE_TIME, BASE_NONE, NULL, 0x0, "Time", HFILL}},

If you want to display the times as UTC, then the way you'd do this depends on the version of Wireshark you're using.

Wireshark 1.0.x or 1.2.x:

You would add that field to the packet by doing something such as

	nstime_t t;
	guint64 msecs_since_the_epoch;
	struct tm *tmp;
	static const char *mon_names[12] = {
		"Jan",
		"Feb",
		"Mar",
		"Apr",
		"May",
		"Jun",
		"Jul",
		"Aug",
		"Sep",
		"Oct",
		"Nov",
		"Dec"
	};

	msecs_since_the_epoch = tvb_get_ntoh64(tvb, offset);
	t.secs = msecs_since_the_epoch/1000;
	t.nsecs = (msecs_since_the_epoch%1000)*1000000;	/* milliseconds to nanoseconds */
	tmp = gmtime(&t.secs);
	proto_tree_add_time_format_value(helen_sub_tree, hf_helen_time, tvb, offset, 8, &t,
	    "%s %2d, %d %02d:%02d:%02d.%09ld UTC",
	    mon_names[tmp->tm_mon],
	    tmp->tm_mday,
	    tmp->tm_year + 1900,
	    tmp->tm_hour,
	    tmp->tm_min,
	    tmp->tm_sec,
	    (long)t.nsecs);

Current top-of-tree Wireshark:

You would have the hf_ item be something like

	{ &hf_helen_time,
	    { "Time", "helen.time", FT_ABSOLUTE_TIME, ABSOLUTE_TIME_UTC, NULL, 0x0, "Time", HFILL}},

and do something like

	nstime_t t;
	guint64 msecs_since_the_epoch;

	msecs_since_the_epoch = tvb_get_ntoh64(tvb, offset);
	t.secs = msecs_since_the_epoch/1000;
	t.nsecs = (msecs_since_the_epoch%1000)*1000000;	/* milliseconds to nanoseconds */
	proto_tree_add_time(helen_sub_tree, hf_helen_time, tvb, offset, 8, &t);

If you want to display the time as local time (in the time zone of the machine running Wireshark), that's a bit easier.
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