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Wireshark-dev: Re: [Wireshark-dev] need to read three bytes of offset

From: prashanth s <prbanglore@xxxxxxxxx>
Date: Fri, 5 Mar 2010 03:20:33 +0530
Hi Chris,

I am reading the 3 bytes directly from the pcap file. I have implemented my parser using the libpcap source code.

I wish I could use a function similar to tvb_get_ntoh24 ( ). The functions provided by wireshark are marvelous.
I have written a couple of dissectors on wireshark earlier. 

Regards,
Prashanth

On Fri, Mar 5, 2010 at 2:41 AM, Maynard, Chris <Christopher.Maynard@xxxxxxxxx> wrote:

Are you really reading those 3 bytes directly from the pcap file, or are you reading them from a tvbuff that wireshark hands your dissector?

 

If from a tvbuff, why not just use tvb_get_ntoh24()?  And if you’re not doing anything with that value except for adding it to your tree, you can just use proto_tree_add_item() with the item specified as either FT_INT24 or FT_UINT24, whichever the case may be.

 

- Chris

 

 

From: wireshark-dev-bounces@xxxxxxxxxxxxx [mailto:wireshark-dev-bounces@xxxxxxxxxxxxx] On Behalf Of prashanth s
Sent: Thursday, March 04, 2010 3:57 PM


To: Developer support list for Wireshark
Subject: Re: [Wireshark-dev] need to read three bytes of offset

 

Oh, then it looks correct to write a function that reads 3 bytes int value from pacp / wire and convert it to an int? This is what I have done: I read the three bytes from the pcap in to an array of three bytes. And then I memcpy these 3 bytes from array to an int variable(which has been memset with 0's) And then I do a ntohl on this int variable and return it as an int.

 

Thanks Guy Harris,

Regards,

Prashanth

 

On Fri, Mar 5, 2010 at 2:17 AM, Guy Harris <guy@xxxxxxxxxxxx> wrote:


On Mar 4, 2010, at 12:42 PM, prashanth joshi wrote:

> thanks for the clarification.
> I was writing a function to read such 3 byte values and convert them to int. Unaware that uint24 field is supported in C (I have seen uint24 field only in wireshark).
> Now I am using the uint24 type.

Standard C doesn't have a uint24 data type (even C99 doesn't have uint24_t).  I suppose if you're using C on the Datacraft/Harris machines (no relation):

       http://bolt.beetlebolt.com/blog/?m=200706

you'd have a 24-bit data type, but most if not all machines capable of running Wireshark have 8-bit bytes and 32-bit or 64-bit registers.

Wireshark supports 24-bit integral-valued fields - but you put the values into 32-bit variables.

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