$Id: README.thread,v 1.1 1999/05/14 17:15:21 mpeppler Exp $

Threaded Perl and sybperl
=========================

Perl 5.005 and later have the ability to run in threaded mode. That
means that you can have multiple threads of execution in a single
executable, which is a lot less processor intensive than having to
fork() to run parallel operations.

However, enabling threading in a program is far from trivial. All the
code you write (both at the perl level and in the C libraries that are 
called) must be re-entrant, and thread-safe.

If you have Sybase OpenClient 10.x or earlier you can stop right now - 
those libraries are NOT thread safe.

Even if you have the 11.x release of OpenClient there are still quite
a few problems.

First, DB-Library isn't thread-safe in any incarnation. So don't even
attempt to write programs with the threaded version of perl using
Sybase::DBlib or Sybase::BCP. It will not work.

Sybase's Client Library on the other hand exists in two versions for
OpenClient 11.x and later. The normal libraries are not thread-safe
(and are probably a little bit faster), but there are _r versions of
the libraries that *are* thread safe. Sybase::CTlib's Makefile.PL will
attempt to use these libraries if it detects that threading is enabled 
in the version of perl that is being used to build it.

Unfortunately threaded perl is still quite buggy.

In my (limited) tests I have found that you can run multiple parallel
queries (as long as you open one connection in each thread!).

But accessing a handle attribute (eg $dbh->{UseDateTime}) causes a
SIGBUS, at least here on Solaris 2.5.1.

The following little script illustrates the problem:

#!/u4/builds/perl-thread/bin/perl -w

use strict;

use blib;

use Thread;
use Sybase::CTlib;

for (1..1) {
    my $thread = new Thread \&runit, $_;
#    runit($_);
}

sub runit {
    my $id = shift;
    my $dbh = new Sybase::CTlib 'sa';
    print "$dbh->{UseDateTime}\n";   # dumps core here.
#    $dbh->{UseDateTime} = 0;
}

__END__


If you have some time and are interested in this issue you are welcome 
to go spelunking in the perl internals to figure out what the problem
might be...

Michael