Threads with subroutines returns nothing - regex

Question

I'm new to threads. I written a script to check the threads module in which i have some modules which do some pattern matching job and returns to two values 1st the input and second match found instance along with its line number. But using the threads it returns nothing.
here is the code
use threads;
sub pattern_finder($){
my $filebuf = shift;
my #threads;
my $pattern_found;
my $thr1 = threads->create(\&sub_pattern1,$filebuf);
push(#threads, $thr1);
my $thr2 = threads->create(\&sub_pattern2,$filebuf);
push(#threads, $thr2);
my $thr3 = threads->create(\&sub_pattern3,$filebuf);
push(#threads, $thr3);
for my $t(#threads){
($filebuf, $tmp)= $t->join();
$pattern_found .= $tmp;
}
return $filebuf, $pattern_found;
}
sub sub_pattern1($)
{
my ($filebuf) = shift;
my $found;
while($filebuf =~ /<LINE(\d+)>Pattern1/gsi)
{
$found .= "Pattern1 found at line $1\n";
}
return $filebuf, $found;
}
sub sub_pattern2($)
{
my ($filebuf) = shift;
my $found;
while($filebuf =~ /<LINE(\d+)>Pattern2/gsi)
{
$found .= "Pattern2 found at line $1\n";
}
return $filebuf, $found;
}
sub sub_pattern3($)
{
my ($filebuf) = shift;
my $found;
while($filebuf =~ /<LINE(\d+)>Pattern3/gsi)
{
$found .= "Pattern3 found at line $1\n";
}
$found = "$pre_checks"."$found";
return $filebuf, $found;
}
Can anyone suggest me what is wrong in the code?

Answer 1

Ah yes, a common problem: The code reference you pass to threads->create is called in the context that the create method was called. Here, this is scalar context. Thus return $filebuf, $found is equivalent to return $found.
As you use that single element in list context when joining the thread, you do something equivalent to $filebuf = $t->join, $tmp = undef which is not what you're expecting.
This can be fixed in two ways:
create the thread in list context:
my ($thr1) = threads->create(...)
explicitly specify the context:
my $thr1 = threads->create({context => 'list'}, \&sub_pattern1, $filebuf);
See also the section on context in the threads documentation.
Note also that there is no reason to return $filebuf at all as you don't really use that value – you just return the $filebuf of the last thread which was joined.
Tip: If you're using multithreading for performance, you should benchmark your code both with and without threads – in your case the non-threaded (sequential) solution is likely to perform better.