Version 1.3 of ai12s/ai12-0130-1.txt

Unformatted version of ai12s/ai12-0130-1.txt version 1.3
Other versions for file ai12s/ai12-0130-1.txt

!standard A.8.1(10)          14-10-09 AI12-0130-1/01
!standard A.8.2(28/3)
!standard A.8.4(10)
!class Amendment 14-10-09
!status work item 14-10-09
!status received 14-10-01
!priority Low
!difficulty Easy
!subject All I/O packages should have Flush
!summary
Flush is added to Sequential_IO and Direct_IO.
!problem
Text_IO and Stream_IO have procedure Flush to ensure that internal buffers are "flushed". However, Sequential_IO and Direct_IO do not. This makes it impossible to ensure that a write using one of these packages has completed.
!proposal
(See summary.)
!wording
Add after A.8.1(10) and A.8.4(10):
procedure Flush (File : in File_Type);
Add after A.8.2(28/3):
procedure Flush (File : in File_Type);
The effect of Flush is the same as the corresponding subprogram in Streams.Stream_IO (see A.12.1).
[Editor's note: This is how Text_IO.Flush is defined. We could consider removing A.10.3(20-21) since it would be mostly subsumed by A.8.2 (and rest could be moved to A.10.2), but that seems like work. :-) Note that Flush doesn't make much sense being defined in A.10.3 (which is about Standard_Output and the like).]
!discussion
Consistency suggests that Flush should be in all of the I/O packages, not just two.
It's thought that it wasn't included in Sequential_IO and Direct_IO because these packages don't use any buffering. But of course the target system might use buffering whether or not it makes sense to the Ada program. Most implementations of Flush also call a target OS function to ensure that any pending writes are completed. It's odd that this functionality isn't available in Sequential_IO and Direct_IO.
[Editor's note: I used "It's thought" because I don't know for sure, this is my best guess. Maybe Tucker or Bob could explain this oddity. If not, delete this note!]
It's possible to use Stream_IO (which has Flush) to emulate Sequential_IO and Direct_IO. But such code is necessarily more complicated and error-prone than using the original packages (especially for Direct_IO, which requires calculating a positioning location, an operation that is easy to get wrong).
!ASIS
No impact.
!ACATS test
!appendix

From: Dmitry A. Kazakov
Sent: Wednesday, October  1, 2014  3:00 PM

[Editor's note: just the part that's relevant to this AI. The entire thread
can be found in AC-00264.]

...
If something gets changed then I have two further issues:

1. Missing procedure Flush.

***************************************************************

From: Randy Brukardt
Sent: Thursday, October  2, 2014  2:12 PM

...
> 1. Missing procedure Flush.

Text_IO and Stream_IO have Flush. Sequential_IO and Direct_IO do not. I believe
the presumption is that neither of these other packages will use any buffering,
at least WRT to the underlying target OS. So Flush would have no effect (the
definition in Ada is about flushing *internal* buffers, it doesn't say anything
about the effect on the target OS). [Aside: I believe most implementations of
Flush try to call something in the target OS, if there is such an operation, but
that is clearly not required by the Standard.]

There might be a consistency argument for the other packages, but that clearly
was considered and rejected when Flush was added in Ada 95. Perhaps someone
knows why that was?

***************************************************************

Questions? Ask the ACAA Technical Agent