Wireshark-dev: Re: [Wireshark-dev] need to read three bytes of offset
From: "Maynard, Chris" <[email protected]>
Date: Thu, 4 Mar 2010 16:11:05 -0500

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: [email protected] [mailto:[email protected]] 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,




On Fri, Mar 5, 2010 at 2:17 AM, Guy Harris <[email protected]> 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):


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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