Re: Ada and Unix--Blocked Tasks

[haven.umd.edu!uvaarpa!software.org!stluka@purdue.edu (Fred Stluka)]

In article <1991Jul30.193558.9118@beach.csulb.edu> lewicki@beach.csulb.edu (Dav
e Lewicki) writes:
> 
>   I have run up against a problem:      
> 
>   I am working on a program with multiple tasks, one of which makes
>   a call on the Unix "accept()" routine for sockets.  As expected,
>   the task blocks and waits for a connection.  
> 
>   But, what I didn't expect is that the other tasks all block too and 
>   the Ada kernel then goes on to complain about deadlock.  It seems that
>   all the tasks are of the same process, so when one blocks, they all
>   do.  The only workaround that I can figure is to poll a non-blocking
>   "accept()" every 5 seconds or so (yecch).
> 
>   Is this a vendor specific problem, or do all Ada/Unix kernals work
>   this way?  I am using the Verdix compiler running on Ultrix.

Just as an additional point of data...

VAX Ada on VMS has a similar problem.  All tasks of an Ada program
run in the same process.  When a call to a system service causes the 
process to be blocked, then all tasks are blocked.  For this reason,
DEC provides a package called TASKING_SERVICES which provides access
to the same system services in a mode where only the calling task is
blocked.  This was easy for them to do because the VMS system services 
have 2 modes, blocking and non-blocking, and the non-blocking mode can 
be directed to generate an interrupt when it completes.  No polling is
required.

Non-portable, but it works.
--Fred
-- 
Fred Stluka                               Internet: stluka@software.org
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