Mailing List Archive

> The mod_perl website tells us that modules can pre-define package global variables at server startup time and have the data be shared by child processes:
>
> http://perl.apache.org/guide/porting.html#Sharing_variables_between_proces
>
> >* Global variables initialized at the server startup, through
> >the Perl startup file, can be shared between processes, until
> >modified by some of the processes. e.g. when you write:
> > $My::debug = 1;
> >
> >all processes will read the same value. If one of the processes
> >changes that value to 0, it will still be equal to 1 for any other
> >process, but not for the one which actually made the change. When a
> >process modifies a shared variable, it becomes the process' private
> >copy.
>
> Is the same true for file-scoped "my" variables?
>
> I'd like to create an object that stores some big read-only data in a file-scoped "my" variable:
>
> package ThisObject;
>
> my $BigPrivateReadOnlyData;
> BEGIN
> {
> $BigPrivateReadOnlyData = `cat somebigfile.txt`; ## for example.
> }
>
> sub MyMethod
> {
> my $this = shift;
> my $DataCopy = $BigPrivateReadOnlyData;
>
> # etc.
> }
>
> My guess is that "my $Big..." should work just as well as "$My::Big...".
>
> Does it?

Hmm, why not to try before asking? It takes exactly 1 minute.

It doesn't work. Please read about variable scoping in the perl books and
other docs.

> --------
>
> ANOTHER QUESTION ABOUT SHARED VARIABLES:
>
> What happens if I do pattern-matching that might modify the pos() of a
> scalar that's been shared?
>
> For example:
>
> my $FoodCount = ($BigPrivateReadOnlyData =~ m/dog: (food)/g);
>
> Does modifying the pos() of a shared variable make the variable unshared?

this is an offtopic question -- please use the appropriate forum for this
kind of questions. e.g. news:comp.lang.perl.misc


_______________________________________________________________________
Stas Bekman mailto:sbekman@iname.com http://www.stason.org/stas
Perl,CGI,Apache,Linux,Web,Java,PC http://www.stason.org/stas/TULARC
perl.apache.org modperl.sourcegarden.org perlmonth.com perl.org
single o-> + single o-+ = singlesheaven http://www.singlesheaven.com

Hi Stas,

Thanks for taking the time to answer...

>> My guess is that the value of [.a file-scoped "my" variable whose value is set only once in a BEGIN{} block] should [be shared by child processes] just as well as a global.
>
>Hmm, why not to try before asking? It takes exactly 1 minute.
>It doesn't work. Please read about variable scoping in the perl books and
>other docs.


>> Does modifying the pos() of a shared variable make the variable unshared?
>
>this is an offtopic question -- please use the appropriate forum for this
>kind of questions. e.g. news:comp.lang.perl.misc

==> What? Variables shared between parent/children Perl processes is a situation that arises uniquely in a modperl context, as far as I can tell -- it would be off-topic on a general perl list. In any case, the fact that it's something a modperl user could benefit from knowing when writing code for modperl seems to be justification alone for asking it here. So the question stands.

-c

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Hi there,

On Sun, 20 Feb 2000, Chris Thorman wrote:

> This was not a newbie question about scoping.
> It's the sharedness that I am questioning.

At the evident risk of taking some flack here:), have you considered
Doing It Another Way? There are several ways to achieve the objective
of sharing information between processes. I don't think I'd want to
spend much time forcing lexically scoped variables to be one of them
because of the trouble with sharing them in nested subroutines that
arises so commonly; occasionally unexpectedly; and if you're careless,
silently. That topic is well covered in the Guide. It's just going
to give you trouble later - a maintenance nightmare in a large system.

There was for example a thread on Feb 19 and 20 on exactly the right
topic, "Announce: File::Cache 0.04". Gunther Birznieks' analysis on
Feb 20 is well worth reading.

Hope this helps,

73,
Ged.

> >> My guess is that the value of [a file-scoped "my" variable whose
> value is set only once in a BEGIN{} block] should [be shared by child
> processes] just as well as a global.

> >Hmm, why not to try before asking? It takes exactly 1 minute. It
> doesn't work. Please read about variable scoping in the perl books and
> >other docs.
>
> ==> This was not a newbie question about scoping. In fact, it does seem
> to work just fine. Maybe it's not getting shared. Will test to see if
> BEGIN block is re-executing in child processes. BEGIN blocks are
> supposed to execute at compile time. Since there's only one compile
> time (in the parent process), it seems like the "my" variables are
> getting shared. But frankly, the question is not DOES it work (in
> whatever build I happen to be using) so much as, what's it SUPPOSED to
> do, in theory, and why. The scope of the variables is obvious. It's
> the sharedness that I am questioning.

> >> Does modifying the pos() of a shared variable make the variable unshared?
> >
> >this is an offtopic question -- please use the appropriate forum for this
> >kind of questions. e.g. news:comp.lang.perl.misc
>
> ==> What? Variables shared between parent/children Perl processes is a
> situation that arises uniquely in a modperl context, as far as I can
> tell -- it would be off-topic on a general perl list. In any case, the
> fact that it's something a modperl user could benefit from knowing when
> writing code for modperl seems to be justification alone for asking it
> here. So the question stands.

Please accept my apology, you are very on-topic :)

Here is the example of how to test your considerations:

Shared.pm (preloaded at the server startup)
---------
package Shared;
use Apache::Peek;
my $lexical = "Chris";
sub match{ $lexical =~ /\w/g; }
sub print_pos{ print "pos: ", pos($lexical), "\n"; }
sub dump { Dump($lexical); }
1;
--------

test.pl
-------
use Shared;
print "Content-type: text/plain\n\n";
print "PID: $$\n";
Shared::match();
Shared::print_pos();
Shared::dump();
--------------

Set :
MaxClients 2
for easier tracking.

after several execution, where pos() returns different values for the 2
processes we see:

PID: 27040
pos: 1
SV = PVMG(0x853db20) at 0x8250e8c
REFCNT = 3
FLAGS = (PADBUSY,PADMY,SMG,POK,pPOK)
IV = 0
NV = 0
PV = 0x8271af0 "Chris"\0
CUR = 5
LEN = 6
MAGIC = 0x853dd80
MG_VIRTUAL = &vtbl_mglob
MG_TYPE = 'g'
MG_LEN = 1

PID: 27041
pos: 2
SV = PVMG(0x853db20) at 0x8250e8c
REFCNT = 3
FLAGS = (PADBUSY,PADMY,SMG,POK,pPOK)
IV = 0
NV = 0
PV = 0x8271af0 "Chris"\0
CUR = 5
LEN = 6
MAGIC = 0x853dd80
MG_VIRTUAL = &vtbl_mglob
MG_TYPE = 'g'
MG_LEN = 2

All the addresses are the same, therefore the variable is still shared.
The only difference is in SV.MAGIC.MG_LEN, which is not shared. I don't
know how properly to call the state of the variable, but at large it's
non-shared, while the biggest part of it is shared.

Regarding the my() scoped variable, I meant that you cannot access it
outside the package, of course it can be shared like a global variable if
you use package's internal subs to access it. And using the above example
you can test your variables for being shared.


_______________________________________________________________________
Stas Bekman mailto:sbekman@iname.com http://www.stason.org/stas
Perl,CGI,Apache,Linux,Web,Java,PC http://www.stason.org/stas/TULARC
perl.apache.org modperl.sourcegarden.org perlmonth.com perl.org
single o-> + single o-+ = singlesheaven http://www.singlesheaven.com