Concurrency & I/O

Concurrency & I/O

[Ed Colbert]

Hi All,

Can someone confirm that it's still implementation defined whether I/O
operations (Put, Get, 'Write, 'Read) are atomic, blocking, or subject to
Priority_Ceiling.  In quick review of the manual & rational, I found the
following:

1) File_Type's and Root_Stream_Type are not publically defined using pragma
Atomic.
2) Get_Immediate is non-blocking according Rationale section A.1.
3) A.10.7(24) Says Get_Immediate doesn't wait for end of line.
4) Annex H provides NO_IO pragma to disallow use of standard IO packages
(and Ravenscare requires this pragma).
5) Annex D doesn't mention I/O.

I don't see a specific statement about the semanics of I/O operations in a
concurrent environment.

Take Care,
Ed

Re: Concurrency & I/O

[Robert A Duff]

"Ed Colbert" <colbert@abssw.com> writes:

> Can someone confirm that it's still implementation defined whether I/O
> operations (Put, Get, 'Write, 'Read) are atomic, blocking, or subject to
> Priority_Ceiling.  In quick review of the manual & rational, I found the
> following:

I don't fully understand the question.
I/O operations are "potentially blocking".
I/O operations that can't be done immediately may block the whole
process, or just the current task (this is implementation dependent).

'Write and 'Read are not I/O operations -- they just call the underlying
stream operations, and the underlying stream might or might not do I/O,
and might or might not block.

None of these things are atomic.  If you have two tasks simultaneously
writing to standard output, that's erroneous (although you might
get away with it, with interspersed output).

- Bob