How to resolve funky sync issues with fork here.

  • Thread starter grocery_stocker
  • Start date
G

grocery_stocker

Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().

#!/usr/bin/perl -w

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "10";

for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";
}

print "AFTER FOR BRACKET\n";

foreach (@childs) {
waitpid($_, 0);
}

And here is what I get when I run it a few times...

[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
AFTER FOR BRACKET
[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
AFTER FOR BRACKET
[cdalten@localhost perl]$

The lines:
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

Sometmes don't appear in sync. How would I get them in sync? Would I
have to insert another wait() into the child? Just need some ideas
here.

Chad
 
G

grocery_stocker

["Followup-To:" header set to comp.lang.perl.misc.]

grocery_stocker said:
Sometmes don't appear in sync. How would I get them in sync?
Click to expand...

$| = 1;
Just need some ideas
here.
Click to expand...

Suffering from Buffering?

http://perl.plover.com/FAQs/Buffering.html
Click to expand...

I tried several variations on $|=1, however, it's still not in sync.

#!/usr/bin/perl -w

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "10";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);

} elsif ($pid == 0) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

print "AFTER FOR BRACKET\n";
#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);


foreach (@childs) {
waitpid($_, 0);
}

However, I still get the following:

[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
AFTER FOR BRACKET
ugh geeze blah test smith bob homes point
[cdalten@localhost perl]$ ./par.pl
 
A

anno4000

grocery_stocker said:
["Followup-To:" header set to comp.lang.perl.misc.]

grocery_stocker said:
Sometmes don't appear in sync. How would I get them in sync?
Click to expand...

$| = 1;
Just need some ideas
here.
Click to expand...

Suffering from Buffering?

http://perl.plover.com/FAQs/Buffering.html
Click to expand...

I tried several variations on $|=1, however, it's still not in sync.
Click to expand...

I don't see how you could expect them to be in sync. The "BEFORE FOR
BRACKET" lines come from the parent, the others from the kids.
Different processes run independent from one another, so each time
around the loop it's up to the OS whether the kid or the parent gets
to execute first.

If you move the "sleep 5" in the kid code to before the "print" line,
you'll see *all* "BEFORE FOR BRACKET" lines before all of the kids'
output. No surprise there.

Anno

[code left in for place reference, also note comment]
#!/usr/bin/perl -w

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "10";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);

} elsif ($pid == 0) {
Click to expand...

You should test for defined-ness here, not for being zero.

} elsif ( defined $pid ) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

print "AFTER FOR BRACKET\n";
#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);


foreach (@childs) {
waitpid($_, 0);
}

However, I still get the following:

[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
BEFORE FOR BRACKET
AFTER FOR BRACKET
ugh geeze blah test smith bob homes point
[cdalten@localhost perl]$ ./par.pl
Click to expand...
 
L

Lew Pitcher

Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().

#!/usr/bin/perl -w

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "10";

for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";

}

print "AFTER FOR BRACKET\n";

foreach (@childs) {
waitpid($_, 0);

}

And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

ugh geeze blah test smith bob homes point
Click to expand...

I presume that the above apparent out-of-order results is what you are
concerned about?
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

BEFORE FOR BRACKET
AFTER FOR BRACKET
[cdalten@localhost perl]$

The lines:
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

Sometmes don't appear in sync.
Click to expand...

Scheduling of ready processes is rarely so simple as to natively
produce an "in sync" result unless you've coded a tight coupling
between the processes. I'd say that there is no real problem here; you
just caught a couple of processes executing in an order that, while
legal and proper, you didn't expect.
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...

Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line. In this way, you maintain a tight coupling between
parent process and child process.
 
G

grocery_stocker

Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().
Click to expand...
#!/usr/bin/perl -w
Click to expand...
my @array = qw(ugh geeze blah test smith bob homes point);
Click to expand...
my $num = "10";
Click to expand...
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";

print "AFTER FOR BRACKET\n";
Click to expand...
foreach (@childs) {
waitpid($_, 0);
}
Click to expand...

And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
ugh geeze blah test smith bob homes point
Click to expand...

I presume that the above apparent out-of-order results is what you are
concerned about?
Click to expand...

Yes, I'm concerned about the out-of-order-results.
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
BEFORE FOR BRACKET
AFTER FOR BRACKET
[cdalten@localhost perl]$
Click to expand...
The lines:
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
Click to expand...
Sometmes don't appear in sync.
Click to expand...

Scheduling of ready processes is rarely so simple as to natively
produce an "in sync" result unless you've coded a tight coupling
between the processes. I'd say that there is no real problem here; you
just caught a couple of processes executing in an order that, while
legal and proper, you didn't expect.
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...

Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line. In this way, you maintain a tight coupling between
parent process and child process.
Click to expand...

I moved the waipid() right before:
print "BEFORE FOR BRACKET\n";

And when I ran the program a a few hundred times. I got the tight
coupling. Now, for whatever reasons, I thought the sleep() in the
child process would somehow produce the tight coupling.

One last question. How can I tell if this program is running
sequentially or in parallel? I tried tracing the code via the
debugger, but all I got where like 10 screens when I ran the code.

Thanks,
Chad
 
X

xhoster

grocery_stocker said:
The lines:
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

Sometmes don't appear in sync. How would I get them in sync?
Click to expand...

If you want to run things in sync, then don't fork. You seem to have a
X-Y problem here.
Would I
have to insert another wait() into the child? Just need some ideas
here.
Click to expand...

The child has nothing to "wait" for.

Xho
 
P

Peter J. Holzer

["Followup-To:" header set to comp.lang.perl.misc.]
Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().

#!/usr/bin/perl -w

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "10";

for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";

}

print "AFTER FOR BRACKET\n";

foreach (@childs) {
waitpid($_, 0);

}

And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl [...]
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point

ugh geeze blah test smith bob homes point
Click to expand...

I presume that the above apparent out-of-order results is what you are
concerned about? [...]
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...

Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line.
Click to expand...

If you do that, what's the point of forking in the first place?

hp
 
G

grocery_stocker

["Followup-To:" header set to comp.lang.perl.misc.]
Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().
#!/usr/bin/perl -w
my @array = qw(ugh geeze blah test smith bob homes point);
my $num = "10";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";
}
print "AFTER FOR BRACKET\n";
foreach (@childs) {
waitpid($_, 0);
}
And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl [...]
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
ugh geeze blah test smith bob homes point
Click to expand...
Click to expand...
I presume that the above apparent out-of-order results is what you are
concerned about? [...]
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...
Click to expand...
Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line.
Click to expand...

If you do that, what's the point of forking in the first place?
Click to expand...

Maybe I'm really not understanding what you are saying. The script is
a stripped down version of a wrapper program. Anyhow, I think I got it
running parallel. Here is the output.

#!/usr/bin/perl -w

use File::stat;
use Time::localtime;

my $file="/home/cdalten/perl/out";


my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "50";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);

} elsif ($pid == 0) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

#sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
foreach (@childs) {
waitpid($_, 0);
}

$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";

print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

print "AFTER FOR BRACKET\n";

#$date_string = ctime(stat($file)->mtime);
#print "file $file updated at $date_string\n";


#print " $atime, $mtime \n";

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

#foreach (@childs) {
# waitpid($_, 0);
#}

Yes, I know that when I redirect a fork to a output file, I need to
probably flush the buffer. However, I was just testing to see if I
would get the same timestamp on the print statements for all the
forks. If the timestamps where the same, then the program in running
in parallel.

[cdalten@localhost perl]$ ./par.pl > out
[cdalten@localhost perl]$ more out
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
 
G

grocery_stocker

["Followup-To:" header set to comp.lang.perl.misc.]
Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().
#!/usr/bin/perl -w
my @array = qw(ugh geeze blah test smith bob homes point);
my $num = "10";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";
}
print "AFTER FOR BRACKET\n";
foreach (@childs) {
waitpid($_, 0);
}
And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl [...]
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
ugh geeze blah test smith bob homes point
Click to expand...
Click to expand...
I presume that the above apparent out-of-order results is what you are
concerned about? [...]
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...
Click to expand...
Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line.
Click to expand...

If you do that, what's the point of forking in the first place?
Click to expand...


Maybe I'm really not understanding what you are saying. The script is
a stripped down version of a wrapper program. Anyhow, I think I got it
running parallel. Here is the output.

#!/usr/bin/perl -w

use File::stat;
use Time::localtime;

my $file="/home/cdalten/perl/out";

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "50";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);

} elsif ($pid == 0) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

#sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
foreach (@childs) {
waitpid($_, 0);
}

$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";

print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

print "AFTER FOR BRACKET\n";

#$date_string = ctime(stat($file)->mtime);
#print "file $file updated at $date_string\n";

#print " $atime, $mtime \n";

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

#foreach (@childs) {
# waitpid($_, 0);
#}

Yes, I know that when I redirect a fork to a output file, I need to
probably flush the buffer. However, I was just testing to see if I
would get the same timestamp on the print statements for all the
forks. If the timestamps are the same, then the program is running
in parallel. Here is part of the ouput.

[cdalten@localhost perl]$ ./par.pl > out
[cdalten@localhost perl]$ more out
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
 
G

grocery_stocker

["Followup-To:" header set to comp.lang.perl.misc.]
Here is a copy a persons code I saw on a blog. The only difference
here is that I enable warnings use my().
#!/usr/bin/perl -w
my @array = qw(ugh geeze blah test smith bob homes point);
my $num = "10";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
} elsif ($pid == 0) {
# child
print "@array\n\n";
sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}
print "BEFORE FOR BRACKET\n";
}
print "AFTER FOR BRACKET\n";
foreach (@childs) {
waitpid($_, 0);
}
And here is what I get when I run it a few times... [snip]
[cdalten@localhost perl]$ ./par.pl [...]
BEFORE FOR BRACKET
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
ugh geeze blah test smith bob homes point
Click to expand...
Click to expand...
I presume that the above apparent out-of-order results is what you are
concerned about? [...]
How would I get them in sync?
Would I have to insert another wait() into the child? Just need some ideas
here.
Click to expand...
Click to expand...
Move your waitpid() call so that it executes immediately prior to
your
print "BEFORE FOR BRACKET\n";
statement. This way, the parent will wait for the child to complete
(and thus write the array) before it proceeds to generate the BEFORE
FOR BRACKET line.
Click to expand...

If you do that, what's the point of forking in the first place?

hp

--
_ | Peter J. Holzer | Blaming Perl for the inability of programmers
|_|_) | Sysadmin WSR | to write clearly is like blaming English for
| | | (e-mail address removed) | the circumlocutions of bureaucrats.
__/ |http://www.hjp.at/| -- Charlton Wilbur in clpm
Click to expand...

Maybe I'm really not understanding what you are saying. The script is
a stripped down version of a wrapper program. Anyhow, I think I got it
running parallel. Here is the output.

#!/usr/bin/perl -w

use File::stat;
use Time::localtime;

my $file="/home/cdalten/perl/out";

my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "50";
for(1..$num) {
my $pid = fork();
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);

} elsif ($pid == 0) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

#sleep 5;
exit(0);
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
foreach (@childs) {
waitpid($_, 0);
}

$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";

print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

print "AFTER FOR BRACKET\n";

#$date_string = ctime(stat($file)->mtime);
#print "file $file updated at $date_string\n";

#print " $atime, $mtime \n";

#$oldestfh = select(STDOUT); $| = 1; select($oldestfh);

#foreach (@childs) {
# waitpid($_, 0);
#}

Yes, I know that when I redirect a fork to a output file, I need to
probably flush the buffer. However, I was just testing to see if I
would get the same timestamp on the print statements for all the
forks. If the timestamps is the same, then the program is running
in parallel. Here is part of the output.

[cdalten@localhost perl]$ ./par.pl > out
[cdalten@localhost perl]$ more out
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
ugh geeze blah test smith bob homes point
file /home/cdalten/perl/out updated at Thu Apr 5 06:28:54 2007
BEFORE FOR BRACKET
 
P

Peter J. Holzer

["Followup-To:" header set to comp.lang.perl.misc.]
Maybe I'm really not understanding what you are saying. The script is
a stripped down version of a wrapper program. Anyhow, I think I got it
running parallel. Here is the output.
Click to expand...

Your original script was running in parallel. But with the change
suggested by Lew, it isn't running in parallel any more:

#!/usr/bin/perl -w

use File::stat;
use Time::localtime;

my $file="/home/cdalten/perl/out";


my @array = qw(ugh geeze blah test smith bob homes point);

my $num = "50";
for(1..$num) {
my $pid = fork();
Click to expand...

here you fork exaclty one child
if ($pid) {
# parent
push(@childs, $pid);
my $oldfh = select(STDOUT); $| = 1; select ($oldfh);
Click to expand...

The parent does nothing except remember the pid of the child
} elsif ($pid == 0) {
# child
#my $oldfh = select(STDOUT); $| = 1;

print "@array\n";
#$oldfh = select(STDOUT); $| = 1; select($oldfh);

#sleep 5;
exit(0);
Click to expand...

The child does its work (put the sleep 5 in again to see that it takes
some time)
} else {
die "couldn't fork: $!\n";
}

#my $olderfh = select(STDOUT); $| = 1;
foreach (@childs) {
waitpid($_, 0);
Click to expand...

here you wait for the child. After that, the child is finished.
}

$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string\n";
Click to expand...

Here the parent does some more work.
print "BEFORE FOR BRACKET\n";
#$olderfh = select(STDOUT); $| = 1; select($olderfh);

}
Click to expand...

So at any point you have either a single child running (and the parent
waiting) or no child at all. There is never more than one process which
does work, so you can just forget about the fork and do everything in
the parent.

Now I realize that you probably need several processes running in
parallel, that's why I think Lew's suggestion doesn't make sense.
Yes, I know that when I redirect a fork to a output file, I need to
probably flush the buffer. However, I was just testing to see if I
would get the same timestamp on the print statements for all the
forks. If the timestamps where the same, then the program in running
in parallel.
Click to expand...

You never change the file, so why should its timestamp change?


To return to your original question: First decide what you need to
synchronise - i.e., where (and why!) the parent needs to wait for a
child to do something before continuing or a child needs to wait for the
parent or one child needs to wait for another. Once you know that you
can decide what the appropriate synchronization mechanism is (waitpid
probably isn't - it can only wait for a child process to die), for
example semaphores, file locks, messages written to pipes, etc.

hp
 

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