On Wed, 3 Mar 2010, Stéphane Ducasse wrote:
On Mar 2, 2010, at 10:33 PM, Levente Uzonyi wrote:
On Tue, 2 Mar 2010, Henrik Sperre Johansen wrote:
PS. For the not-so-faint-of-heart, open a Transcript and try evaluating:
|proc|
proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show:
'Finish!' Processor yield.]] newProcess.
proc resume.
Processor yield.
proc suspend.
proc terminate.
Why don't we get it printing?
Forgot a . there, supposed to be
|proc|
proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show:
'Finish!'. Processor yield.]] newProcess.
proc resume.
Processor yield.
proc suspend.
proc terminate.
on my machine it prints:
*Start!Start!Finish!
The problem occurs because Transcript >> #endEntry (sent from #show:)
takes a while to execute, so the process (proc) prints 'Start!', but
it gets terminated before execution reaches #resetContents (#reset in
Squeak). So 'Start!' is still in the stream. Then our process executes the
#ensure: block and it prints 'Start!' and 'Finish!' too.
Thanks for the explanation levente!
But I'm still dizzy :) why Transcript show: 'Finish' would also print 'start' ?
Transcript is just a stream. #show: is just a combination of #nextPutAll:
and #endEntry. #endEntry displays the contents of the stream and when
that's done it removes everything from the stream with #resetContents.
#resetContents is never executed, because the process is terminated before
that. So the contents of the stream are kept and displayed again later.
Levente
There's worse problem with #ensure: and #terminate is that, a process executing
an #ensure: block can be terminated. If you evaluate this code:
| process stage1 stage2 stage3 counter |
stage1 := stage2 := stage3 := false.
counter := 0.
process := [
[ stage1 := true ] ensure: [
stage2 := true.
1000000 timesRepeat: [ counter := counter + 1 ].
stage3 := true ] ] newProcess.
process resume.
Processor yield.
process suspend.
process terminate.
1000 milliSeconds asDelay wait.
{ stage1. stage2. stage3. counter } explore
you will find that stage1 and stage2 is reached as expected, but stage3 is not
and counter is less than 1000000. That's because the forked process started
evaluating the #ensure: block, but it was terminated by our process.
Is this the expected behavior when sending #terminate to a process which is
evaluating an #ensure: block?
Cheers,
Balázs & Levente
*yay
Henry*
*
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