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From: gisle@kondor.ii.uib.no (Gisle =?iso-8859-1?Q?S=E6lensminde?=)
Subject: Re: First time Ada has let me down
Date: 2000/10/25
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Date: 2000-10-25T11:36:59+00:00
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In article <8t60eo$m0m$1@nnrp1.deja.com>, wv12@my-deja.com wrote:
>In article <8FD7DEBEEsynoptikdamudderfuck@news>,
>  avlogue@home.com (Shifty) wrote:
>> OK, here's the story:
>>
>> I had a problem at work where I needed to calculate the offset of a
>> "magic number" within a buffer containing an IP datagram.
>>
>> The magic number is 3 protocol headers deep within the datagram, and
>> all three headers are variable length.  I needed to find the HLEN
>> fields of the IP and TCP headers in memory.  Turns out both of these
>> are stored in 4 bits.  The data in the other 4 bits of the octet
>> belong to another field in the header.
>>
>the IP header length occupies the 4 LSB. Assuming p pointing to the
>byte offset,
>the length is just len = (*p)&0xf;

Or len = P and 16#0F# in Ada95

>the TCP header length occupies the 4 MSB. the length in this case
>is then  ((*p)&0xf0)>>4;

Or Shift_Left(P and #16#F0#, 4) in Ada 95.

Or if P is represented as a packed boolean array, i.e. it's a bit-array,
you can  index the bits directly:

P(5..8)        

What you writes is simply wrong. Ada95 is IMHO better than C on
bit manipulation. There are more examples then those above.
 

--
Gisle S�lensminde ( gisle@ii.uib.no )   

With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going
to land, and it could be dangerous sitting under them as they fly
overhead. (from RFC 1925)