string manipulation

[Brian Oleksa <oleksab () darkcornersoftware com>]

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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