An interesting digression....  A few years ago, the architects of MIT's 6.828 course ("Operating Systems Engineering") were unsatisfied with the current stable of systems for teaching, so they did a re-implementation of 6th Edition in modern ANSI C (with a couple of GNU extensions for things like assigning names to registers) targeting a multiprocessor x86.  It's an interesting, accessible piece of work; a modern take on a classic.

        - Dan C.

On Mon, Oct 27, 2014 at 10:48 AM, Noel Chiappa <> wrote:
    > From: Jason Stevens

    > has anyone ever tried to compile any of the old C compilers with a
    > 'modern' C compiler?
    > ...
    > How did any of this compile? How did this stuff run without clobbering
    > each-other?

As Ron Natalie said, the early kernels are absolutely littered with all sorts
of stuff that, by today's standards, are totally unacceptable. Using a
variable declared as an int as a pointer, using a variable declared as a
'foo' pointer as a 'bar' pointer, yadda-yadda.

I ran (tripped, actually :-) across several of these while trying to get my
pipe-splicing code to work. (I used Version 6 since i) I am _totally_
familiar with it, and ii) it's what I had running.)

For example, I tried to be all nice and modern and declared my pointer
variables to be the correct type. The problem is that Unix generated unique
ID's to sleep on with code like "sleep(p+1, PPIPE)", and the value generated
by "p+1" depends on what type "p" is declared as - and if you look in pipe.c,
you'll see it's often declared as an int pointer. So when _I_ wrote
"sleep((p + 1), PPIPE)", with "p" declared as a "stuct file pointer", I got
the wrong number.

I can only speculate as to why they wrote code like this. I think part of it
is, as Brantley Coile points out, historical artifacts due to the evolution
of C from (originally) BCPL. That may have gotten them used to writing code
in a certain way - I don't know. I also expect the modern mindset (of being
really strict about types, and formal about coverting data from one to
another) was still evolving back then - partly because they often didn't
_have_ the tools (e.g. casts) to do it right. Another possibility is that
they were using line editors, and maintaining more extensive source is a pain
with an editor like that. Why write "struct file *p" wnen you can just write
"*p"? And of course everyone was so space-concious back then, with those tiny
disks (an RK05 pack is, after all, only 2.5MB - only slightly larger than a
3.5" floppy!) every byte counted.

I have to say, though, that it's really kind of jarring to read this stuff.

I have so much respect for their overall structure (the way the kernel is
broken down into sub-systems, and the sub-systems into routines), how they
managed to get a very powerful (by anyone's standards, even today's) OS into
such a small amount of code... And the _logic_ of any given routine is
usually quite nice, too: clear and efficient. And I love their commenting
style - no cluttering up the code with comments unless there's something that
really needs elucidation, just a short header to say, at a high level, what
the routine does (and sometimes how and why).

So when I see these funky declarations (e.g. "int *p" for something that's
_only_ going to be used to point to a "struct file"), I just cringe - even
though I sort of understand (see above) why it's like that. It's probably the
thing I would most change, if I could.

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