Re: Thread-safe & vars scope clarification

Hi Perrin,

Thank you for your answer.

Well, for the moment I only use the prefork (non-threaded) MPM.
But I would have liked to test performance of the threaded MPM, so I would have liked my code to be thread-ready.

According to this :
https://perl.apache.org/docs/2.0/user/performance/mpm.html#toc_Memory_Requirements_in_Threaded_MPM
Sounds like threads could have an interest, but according to your message, should not be a general case.

Thank you,

Ben

> Le 4 avr. 2016 à 22:49, Perrin Harkins <[email protected]> a écrit :
> 
> Hi Ben.
> 
> Before you get too far into the details of using threads, can I ask why you're considering it? The memory footprint and performance of using forked processes with Perl is generally going to be better than that of threads, due to copy-on-write.
> 
> - Perrin
> 
> On Mon, Apr 4, 2016 at 4:44 PM, Ben RUBSON <[email protected] <mailto:[email protected]>> wrote:
> Hello,
> 
> I have some questions regarding mod_perl, threads, thread-safe functions, special vars scope...
> 
> From https://perl.apache.org/docs/2.0/user/coding/coding.html#toc_Thread_environment_Issues <https://perl.apache.org/docs/2.0/user/coding/coding.html#toc_Thread_environment_Issues> :
> "if you chdir() in one thread, all other thread now see the current working directory of that thread that chdir()'ed to that directory"
> Do we speak about threads from threaded MPM ?
> (so the specific Apache process which owns the thread running this Perl script, and all its (the process) threads, will chdir()).
> 
> From the same page :
> "Other usually problematic (non thread-safe) functions include readdir(), srand(), etc."
> Why readdir() would not be thread-safe ? I can't see why. Perhaps readdir() chdir() to read a directory content ?
> How could we then read dir content being thread-safe ?
> 
> From https://perl.apache.org/docs/general/perl_reference/perl_reference.html#toc_The_Scope_of_the_Special_Perl_Variables <https://perl.apache.org/docs/general/perl_reference/perl_reference.html#toc_The_Scope_of_the_Special_Perl_Variables> :
> "In the mod_perl situation where the child server doesn't usually exit, if in one of your scripts you modify a global variable it will be changed for the rest of the process' life and will affect all the scripts executed by the same process."
> So in a threaded MPM, if one of the threads modifies one of the special Perl variables, it will instantly affect all other threads of this process ?
> 
> What about variables like $? ? (return code of system() calls)
> And ${^CHILD_ERROR_NATIVE} ? (return code too)
> Does their value stay local to the thread ? / Does each thread have these variables with their own proper value ?
> Or is it unsafe to read them because they could be modified by another thread ?
> 
> Thank you very much for your clarification,
> 
> Best regards,
> 
> Ben
> 
>