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From: Ian Lance Taylor
Date: 07 Sep 2003 14:11:22 -0700
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.2

Paul Eggert <address@hidden> writes:

> > I'm thinking of older systems.  As far as I know SA_RESTART was
> > introduced by Solaris.  I doubt it is in SVR4.2, for example, though I
> > admit that I don't have a way to check.
> Rich Stevens wrote in 1991 that SA_RESTART was in SVID Issue 3,
> in SVR4, and in 4.3BSD Reno.  See:
> <>

Ah, OK.  I have SVID Issue 2, which doesn't have it.

> > I checked Ultrix 4.0, and it has sigaction but does not have SA_RESTART.
> That was released in 1990, right?  It was probably based on 4.3BSD
> Tahoe then, and I guess Tahoe didn't have it.  Reno came out in June 1990.

I think it's actually based on 4.2BSD, with a lot of stuff added on
for increased System V compatibility.

> > [AC_DIAGNOSE([obsolete],
> > [$0: System call restartability is now typically set at runtime.
> > and adjust your code to use `sigaction' with `SA_RESTART' instead.])dnl
> Perhaps that's a bit draconian.  Can you suggest a better wording for
> that diagnostic?

How about something like this:
  $0: AC_SYS_RESTARTABLE_SYSCALLS is only useful when supporting old
  systems which do not provide `sigaction'.  Don't use this macro unless
  you intend to support very old systems.

The point is that people shouldn't check AC_SYS_RESTARTABLE_SYSCALLS
because they want restartable system calls.  They should check it to
see if that is all they can expect.

(I think that restartable system calls are a superficially attractive
idea which interacts poorly with the Unix I/O interface, because they
leave you with no good way to do a timed read or write.  If Unix had a
standard interface for a timed read/write of up to N characters, then
restartable system calls make a lot of sense.  Admittedly efficient
Unix serial I/O is a black art anyhow.)

> Wouldn't it be better for uucp to determine the value dynamically?
> That should be fairly easy to do, by running a bit of test code.  Then
> you wouldn't need to guess, and you wouldn't need
> AC_SYS_RESTARTABLE_SYSCALLS either.  (You would need an ancient system
> to test it on, but I guess if you have Ultrix 4.0 then that is ancient
> enough.  :-)

I couldn't think of any dynamic test which ran quickly.  Note that the
autoconf test takes a few seconds.  It's not worth taking a few
seconds to run a dynamic test every time uucico starts up to make a
serial connection.


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