I always believed that fork() was the very essence of the beauty of UNIX. So simple, yet so powerful.  And it is far simpler than its predecessor on Genie, which looked a lot more like spawn().

With fork, the child inherits properties which may not have existed when the code was written, so it's much easier to reason about the behavior of sub-processes.  Fork made writing the shell and pipelines much more obvious.

Today we know that threads and shared mutable memory are a really bad idea, it's just that the hardware gave us no alternatives. I claim UNIX is directly responsible for the existence of MMUs in microprocessors.  What if CPU designers would add facilities to directly implement inter-process or inter-processor messaging?  Sadly, there has to be a dominant software paradigm for the hardware guys to target, so there's a nasty chicken and egg problem.  Imagine if the Erlang model of distributed systems had taken off.  Go gets us part of the way there, but cross-machine messaging is still a mess.

On Tue, Aug 3, 2021 at 8:02 AM Douglas McIlroy <douglas.mcilroy@dartmouth.edu> wrote:
> fork() is a great model for a single-threaded text processing pipeline to do
> automated typesetting.  (More generally, anything that is a straightforward
>  composition of filter/transform stages.)  Which is, y'know, what Unix is *for*.

> It's not so great for a responsive GUI in front of a multi-function interactive  program.

"Single-threaded" is not a term I would apply to multiple processes in
a  pipeline. If you mean a single track of data flow, fine, but the
fact that that's a prevalent configuration of cooperating processes in
Unix is an artifact of shell syntax, not an inherent property of
pipe-style IPC. The cooperating processes in Rob Pike's 20th century
window systems and screen editors, for example, worked smoothly
without interrupts or events - only stream connections. I see no
abstract distinction between these programs and  "stuff people play
with on their phones."

It bears repeating, too, that stream connections are much easier to
reason about than asynchronous communication. Thus code built on
streams is far less vulnerable to timing bugs.

At last a prince has come to awaken the sleeping beauty of stream
connections. In  Go (Pike again) we have a widely accepted programming
language that can fully exploit them, "[w]hich is, y'know, what Unix
is 'for'."

(If you wish, you may read "process" above to include threads, but
I'll stay out of that.)


- Tom