Standards Update, IEEE 1003.5: Ada-language Binding

[Jeffrey S. Haemer]

From: Jeffrey S. Haemer <jsh at usenix.org>


            An Update on UNIX* and C Standards Activities

                            December 1989

                 USENIX Standards Watchdog Committee

                   Jeffrey S. Haemer, Report Editor

IEEE 1003.5: Ada-language Binding Update

Ted Baker <tbaker at ajpo.sei.cmu.edu> reports on the October 16-20, 1989
meeting in Brussels, Belgium:

The P1003.5 group is producing an Ada-language binding for 1003.1.
The Brussels meeting had two objectives: to reach consensus on a draft
document to be distributed for mock ballot, and to solicit input from
the European community.  We achieved the first but not the second;
only one of the ten attendees was European (Olle Wikstrom, from
Ericsson).

The technical editor (David Emery) and the chapter authors had worked
very hard between meetings to produce version 3.2 of the document, and
Dave brought copies to the meeting.  The working group reviewed it to
try to correct any serious errors or omissions before mock ballot.

There was a lengthy discussion about schedule and logistics for the
mock ballot.  The present plan is to send out copies of the next
draft, in ISO format, to both the ISO and the entire 1003.5 mock-
ballot mailing list.  [Editor's note: All committees are re-formatting
their documents in ISO format to smooth the way for ISO acceptance
(see Dominic Dunlop's report on WG15 for more details), and an IEEE
copy editor appeared on the scene in Brussels to give P1003.5 guidance
and help in this.] Since there is no way that enough input can be
received before the next POSIX meeting, in January, the group has
scheduled a special meeting for mock ballot resolution, between the
January and April POSIX meetings, to be held in Tallahassee.  The
objective will be to produce a proposed standard to be reviewed at the
April meeting.

Most technical issues discussed were minor, compared with previous
meetings.  The most significant, and complicated, was the treatment of
system configuration limits.  Here are three problem areas:

__________

  * UNIX is a registered trademark of AT&T in the U.S. and other
    countries.

December 1989 Standards Update       IEEE 1003.5: Ada-language Binding


                                - 2 -

  1.  Tri-state configuration parameters (true, false, undefined) in
      the POSIX C binding need to be treated differently in the Ada
      binding, because Ada prohibits references to undefined symbols.
      (I.e., Ada lacks an "#ifdef" facility.)

  2.  For the same reason, it isn't clear how an Ada binding can
      accommodate future POSIX extensions.  Suppose, for example, a
      future extension adds a new configuration constant.  How does
      one write an Ada program that takes advantage of the new feature
      on implementations where it's available without preventing the
      same program from compiling on older implementations, where it's
      not?

  3.  Because Ada compilers can do optimizations, such as dead code
      elimination, based on static expressions (the nearest analog to
      some C preprocessor capabilities), it is important to provide
      compile-time constants, where safe.  At the same time, to
      support "bubble pack" software that is usable on different
      system configurations, programs should also be able to defer
      binding such values until run time.

The group did achieve consensus on a treatment of configuration limits
for the mock ballot.  It includes a combination of functions, to allow
software to defer resolution of system limits and characteristics
until runtime, and implementation-defined constants and numeric
ranges, to allow optimizers to take advantage of information available
at compile time.  This does not fully solve all the problems mentioned
above.  Perhaps the mock ballot process will turn up some suggestions
for improvements.

The treatment of process arguments and environment variables, which
must be provided as parameters when starting a new process or calling
Exec produced another controversy.

Unlike C, Ada does not allow pointers to stack or statically allocated
objects.  An Ada POSIX interface implemented over a C-language binding
must bridge this gap somehow.  For example, an implementation might
use a C-compatible data structure and hide the non-Ada details, or use
an Ada data structure and translate between the two forms.  Everyone
agreed that the interface should avoid constraining the
implementation, but the first interface solutions appeared to rule out
desirable implementations.  The present solution permits an
application to insure that if the Ada POSIX interface machinery
allocates any "heap" storage this storage is be recovered, while
allowing an implementation to impose restrictions that would permit
stack allocation.  A price paid for this compromise is that writing
portable applications takes more care: an application that works OK
with one implementation may lose storage or exceed size limits with
another.

At the previous two meetings, we had substantial interaction both with

December 1989 Standards Update       IEEE 1003.5: Ada-language Binding


                                - 3 -

other groups working on language-independence and with P1003.4 (real-
time).  There was much less this time, partly because the group was
concentrating so hard on getting ready for mock ballot, partly because
meetings were spread over several buildings, and partly because
P1003.4 mostly skipped Brussels.

On the administrative side, Steve Deller was promoted from Vice
Chairman to Chairman (in charge of external affairs and running
meetings) and Jim Lonjers was chosen as Vice Chairman (in charge of
administering ballot resolution).  This change was required because
the ex-Chairman (Maj. Terry Fong) has been unable to participate
regularly in the working group recently, owing to conflicts with his
professional duties.

Another issue that came up was whether working group members are at
liberty to publish papers or present talks on the 1003.5 work.  The
answer is, "Yes." Until now, some members have been exercising self-
censorship, based on an earlier agreement designed to discourage
anyone (e.g., defense department personnel) from making commitments
(e.g., requiring use of the POSIX Ada binding in contracts) based on
erroneous (e.g., overly optimistic) progress reports.  It did not take
much discussion to agree that such censorship is now
counterproductive, and may never have been wise.  At this point,
P1003.5 certainly wants public exposure of its draft document, and
hopes that such exposure will generate more reviewers and active
working group members.

December 1989 Standards Update       IEEE 1003.5: Ada-language Binding


Volume-Number: Volume 18, Number 2