Gregory, I don't know if this helps but I ended creating functions like the ones below (I didn't need to use the C structures because the IP definition is language independent). I'm sure there are better ways and I didn't test the throughput but I was able to develop a Haskell version of ping and traceroute (both multi-threaded). It would be nice if there were a library so that we didn't end up re-inventing the wheel. Dominic. ipHeaderLength :: String -> Int ipHeaderLength s = (fromIntegral (ord (s !! 0)) .&. 0x0f) * 4 ipTTL :: String -> Int ipTTL s = fromIntegral (ord (s !! 8)) ipProtocol :: String -> IPProto ipProtocol s = toEnum (ord (s !! 9)) ipDestAddr :: String -> HostAddress ipDestAddr s = fromIntegral (ord (s !! 16)) `shiftL` 24 + fromIntegral (ord (s !! 17)) `shiftL` 16 + fromIntegral (ord (s !! 18)) `shiftL` 8 + fromIntegral (ord (s !! 19)) ----- Original Message -----

Date: Sun, 25 Jan 2004 22:44:58 -0500 From: Gregory Wright To: haskell-cafe@haskell.org Subject: [Haskell-cafe] Interpreting fields in a buffer Message-ID: <0356DB09-4FB2-11D8-A3D2-00039398F084@antiope.com> Content-Type: text/plain; charset=US-ASCII; format=flowed MIME-Version: 1.0 (Apple Message framework v609) Content-Transfer-Encoding: 7bit Precedence: list Message: 1

Hi,

I have a question related to a program I'm writing. I have to handle IP packets, which will be read into a buffer. What is the best haskell idiom for getting access to the fields in the buffer?

The IP header has a number of fixed format fields. In C, I would define a struct, then cast the pointer to the beginning of the buffer to a pointer to the struct. I would then be able to access each field in the header as -> <field>.

Is there a way in haskell to declare the format of the header, then access the components of the buffer as elements of those types? I'm only going to do read access on the buffer (an unboxed array). Most fields won't be examined but I can't tell in advance which ones will have to be looked at.

I've not seen an example of this kind and was wondering if this was especially awkward.

Thanks.

Best Wishes, Greg

Gregory Wright Antiope Associates LLC 18 Clay Street Fair Haven, New Jersey 07704

gwright@antiope.com