Re: Asynchronous IO

[nasser@apldbio.com (Nasser Abbasi)]

   From: ohk@edeber.nta.no (Ole-Hjalmar Kristensen FOU.TD/DELAB)

   For obvious resons, it is desirable to be able to do output of data to
   different devices in parallel. Does the Ada95 standard define what
   happens if two tasks try to do IO to two different files at the same
   time? I have tried to find out by reading both the RM and the
   Rationale, but is none the wiser.

   Does one become blocked, waiting for the operation to complete, then
   the other becomes blocked, waiting for the operation to complete
   (which is what i fear)?
   Do both tasks block, then wake up as soon as their respective IO
   operations are finished? This is what I would like.
   Or, is it simply undefined and left to the implementation?

   Ole-Hj. Kristensen


It seems to me that neither Ada nor any other langauge for that matter 
can specify what happens when 2 or more tasks/threads attempt 
access to a shared external system resource such as a file since this 
by its nature, is the resposibility of the operating system (in
particular that of the file system layer) that the program run on top, 
and not the ada-rtl or the ada language. If an ada task issues an 
IO operation, and an other task does the same at about the same time, 
then the file system is resposible for managing concurrent access to 
the file internal data structures and to interact with the device driver
in consistant manner. All what Ada-rtl will do is make calls to the
system lower level calls to do the actual IO, I don't see it doing more than
that in this area. (other than offcourse taking care of
task scheduling after the IO is complete, such as making the task
ready, etc..but that is beside the point).

The behviour should be no different from if 2 or more processes
on the system try to access the same file. The IO operations
will be queued to the file in the order they isseued (in general). If 
it is a blocked/IO operation, the task (or a process) will block untill 
the IO is complete, etc.. even if the 2 tasks each ran on different
cpus in a  multi-cpu system, where an IO operation request can happen 
at exactly the same time, the system is respsible for synchronizing 
access to the device driver.

If the program is an embeded program, where normally there is no
physical files on the board to start with or an operating system
in the normal sense, then access to external devices/ports etc.. 
should be coordinated by the programmer code, either by using protected 
variables or other similar techniques .

I can be competely wrong here becuase I have not also check the RM on
this, but I can bet my house that I have not bought yet that this is
how things would work, and if they don't work like this, then they are
broke :)

nasser

-- 
Nasser Abbasi. C/C++/Ada Solaris. GeneAssist - A client/server 
application for Nucleic acid and protein sequence search and analysis.
PE-Applied BioSystem division. email:  nasser@apldbio.com   
MSEE, MSCS, MSCE, FM (and Karpov is my chess hero! ..).