Path: utzoo!attcan!ncrcan!scocan!larryp
From: larryp@sco.COM (Larry Philps)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  TCP/IP Source Code Search
Message-ID: <1990Nov16.184809.18378@sco.com>
Date: 16 Nov 90 18:48:09 GMT
References: <9011052355.AA28967@yuba.WRS.COM> <1990Nov8.172640.13894@mdivax1.uucp>
Sender: news@sco.com (News administration)
Organization: SCO Canada, Inc. (formerly HCR Corporation)
Lines: 42

In article <1990Nov8.172640.13894@mdivax1.uucp> mdivax1!robinson (Jim Robinson) writes:
>
>One possible problem is the use of non-int bit fields in the tcp and ip
>header structures. Pre-ANSI C requires only that unsigned int bit fields be
>supported by a compiler and ANSI C requires only (*I believe*) signed or
>unsigned bit fields be supported. Thus, if your compiler does not support,
>say, unsigned char bit fields, which are indeed used in the BSD code, you
>will have to do a bit of work to work around this problem. If you are
>unlucky, as I was, your compiler will *not* complain, but merely generate
>incorrect code.
>
>You will also run into a number of annoying problems if the sizes of ints
>and pointers on your target machine are greater than 32 bits, since there
>are several coding practices employed that assume 32 bit pointers and ints.
>-- 
>Jim Robinson
>{uunet,ubc-cs}!van-bc!mdivax1!robinson

Hi Jim! That was a *fun* project wasn't it!  (The machine did not have a 16
bit data type, and could not do 16 bit arithmetic!)

Originally the compiler only supported int bitfields (8 bytes on that
machine), but the standard IP and TCP header structures are both 20 bytes.
This is not divisble by 8.  Since the machine enforced alignment, it was
impossible to reference both the IP and TCP headers of a packet without
moving things around.  We eventually had to change the compiler to support
short bitfields.

The BSD code also used an incredibly *portable* method of moving 16 bit
quantities around (ie. tcp options) via

	*(u_short *) cp = something.

which unfortunately did not work too well with our 32 bit u_shorts.

---
Larry Philps,	 SCO Canada, Inc (Formerly: HCR Corporation)
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