Perl Thread Safe Modules

Question

I am trying to take a Perl program I wrote and thread it. The problem is I read that some modules aren't "thread safe". How do I know if a module is thread safe? I've looked around for a list and cannot locate one.

To test out one module I use frequently (Text::CSV_XS) I tried the following code out:

use strict;
use warnings;
use threads;
use threads::shared;
require Text::CSV_XS;

my $CSV = Text::CSV_XS->new ({ binary => 1, eol => "\n" }) or die("Cannot use CSV: ".Text::CSV->error_diag());
open my $OUTPUT , ">:encoding(utf8)", "test.csv" or die("test.csv: $!");

share($CSV);

my $thr1 = threads->create(\&sayHello('1'));
my $thr2 = threads->create(\&sayHello('2'));
my $thr3 = threads->create(\&sayHello('3'));


sub sayHello
{
 my($num) = @_;

 print("Hello thread number: $num\n");

 my @row = ($num); 
  lock($CSV);{
   $CSV->print($OUTPUT, \@row);
   $OUTPUT->autoflush(1);
  }#lock
}#sayHello

The output I receive is the following:

Hello thread number: 1
Segmentation fault

Does this mean the module is not thread safe, or is it another problem?

Thanks

Answer 1

Generally speaking, core and high-visibility modules are thread-safe unless their documentation says otherwise.

That said, there are a few missteps in your post:

1. share($CSV)
   This clears $CSV (a blessed hashref), just as documented in threads. Generally, you want to share() complex objects prior to initialization or, perhaps in this case, share() some dumb $lock variable between threads.
   Since $CSV holds state for the underlying XS, this might lead to undefined behavior.
   
   But this isn't your segfault.

2. threads->create(\&sayHello('1'));
   You are mistakenly invoking sayHello(1) in the main thread and passing a reference to its return value to threads->create() as a (bogus) start routine. You meant to say:

   threads->create(\&sayHello, '1');
   

   But this isn't your segfault.

   (EDIT Just to clarify -- a bad start routine here doesn't risk a SEGV in any case. threads::create properly complains if an unrecognized subroutine name or non-CODE ref is passed in. In your case, however, you are segfaulting too quickly to reach this error handling.)

3. Encodings are not thread-safe.
   Again as documented in encodings, the encoding module is not thread-safe. Here's the smallest possible code I could get to reproduce your symptoms:

   use threads;
   open my $OUTPUT , ">:encoding(utf8)", "/dev/null" or die $!;
   threads->create( sub {} )->join;
   

   That's perl 5.12.1 with threads-1.77 on i686-linux-thread-multi, if you're interested. Drop the "utf8" magic, and it works just fine.
   

   This is your segfault