Re: User vs. Kernel (was: To be smug, or not to be smug, that is , the question)

Richard Gooch (rgooch@atnf.csiro.au)
Wed, 27 Jan 1999 09:52:23 +1100

doctor@fruitbat.org writes:
> Richard Gooch said ...
> >
> > Alan Cox writes:
> > > > >> entirely from userland.
> > > > > sure it was implemented - i've used it in (apparently small) project.
> > > > In userland, you can't support message queues in select() like AIX can.
> > > Im amazed people stil use them, they are a classic example of retrofitting
> > > somethng un-needed onto unix without the people at the time thinking what
> > > they were doing.
> > Agreed! What the hell is wrong with a pipe/socketpair anyway? It fits
> > right in there in the everything-is-a-file model. Message queues don't.
>
> So? What's wrong with that? Shared memory isn't a file.
> Semaphores aren't files? Neither are message queues and a whole
> slew of other things that aren't files. They all have their uses.
> Not everthing has to be a file. I've done a number of small
> projects where we used message queues specifically because of the
> message typing attribute. What's your problem?

Smeaphores and shared memory provide very different conceptually
different services from pipes. Message queues, however, provide the
same conceptual service as pipes. So now we have duplication of
functionality, with the disadvantage that sending a message via a
message queue is limited to the same machine (yes, I'm ignoring DIPC
and such: they open up another can of worms). Sending a message via a
pipe (i.e. write()ing to a FD) is not limited to the same machine
(just call connect() instead of socketpair()).

Anyway, I don't have a problem with message queues in the sense that I
think they should be removed or anything like that. I just think that
supporting them via select()/poll() is pointless. If you want
multiplexing of message queues, use pipes instead.

Frankly, I'd like to see SysV message queues implemented in libc on
top of pipes. Once shmfs is in the kernel, I'd like to see the SysV
shared memory support ripped out of the kernel and put into user space
too. It's good enough for POSIX SHM.

Regards,

Richard....

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