mkfifo/mknode

azaria has asked for the wisdom of the Perl Monks concerning the following question:

I am looking for a perl code that reads/writes to a data file, every time any other program tries to read/write from that file the reading/writing program will block until the first will complete

To make things simpler I am currently trying to have a program which reads a file , sort the file and pop the first line.

For e.g , input file is: (called "a_file")

8

7

6

5

4

3

2

1
[download]

If 3 processes are running in parallel or serially, the final file will be:

1

2

3

4

5
[download]

I wrote:

 

#! /usr/local/bin/perl

 

chomp($pid  = $$);

$LOCK_EXCLUSIVE = 2;

$UNLOCK         = 8;

$fifo             = "ENV{HOME}/a_file";

 

popup($fifo);

 

sub popup

{

        my($fifo) = @_;

        while (! -e "$fifo.lock")

        {

                open (LOCK, "> $fifo.lock") || die "problem opening $f
+ifo\n";

                flock LOCK , $LOCK_EXCLUSIVE;

                print LOCK $pid;

 

                open (FILE, "< $fifo") || die "problem opening $fifo\n
+";

                @lines = <FILE>;

                close(FILE) || warn "update queue file exited $?\n";;

 

                open (FILE, "> $fifo") || die "problem opening $fifo\n
+";

 

                @sorted = sort @lines;

                $pop = shift @sorted;

                print "$hostname is poping $pop\n";

                print FILE "@sorted\n";

 

                close(FILE) || warn "update queue file exited $?\n";;

                close(LOCK) || warn "lock file exited $?\n";;

                $str=`cat $fifo.lock`;

 

                flock LOCK, $UNLOCK;

        }

        if ($pid eq $str)

        {

                print "PID lock file is going to be removed\n" ;

                system("rm $fifo.lock");

 

        }

}
[download]

But it looks that while I running in parallel on 3 different hosts it does not work as I expect , I get:

linux24 is poping


PID lock file is going to be removed

linux28 is poping

 

PID lock file is going to be removed
[download]

Also I found this code over the web but I fail to run it , it get into infinite loop –

   chdir; # go home

    $FIFO = '.signature';

    $ENV{PATH} .= ":/etc:/usr/games";

 

 

    while (1) {

        unless (-p $FIFO) {

            unlink $FIFO;

            system('mknod', $FIFO, 'p')

                && die "can't mknod $FIFO: $!";

        }

 

 

        # next line blocks until there's a reader

        open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";

        print FIFO "John Smith (smith\@host.org\n";

        close FIFO;

        sleep 2;    # to avoid dup signals

    }
[download]

Any idea? I will highly appreciate if you can advise.

Janitored by Corion: Added formatting, code tags, as per Writeup Formatting Tips

GrandFather added readmore tags

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Re: mkfifo/mknode by jwkrahn (Abbot) on Jun 14, 2006 at 10:35 UTC
You should use the warnings and strict pragmas so the second and third lines of your program should be: `use warnings; use strict;` [download] `chomp($pid = $$);` [download] $$ does not contain a newline so there is no point in using chomp(). `$LOCK_EXCLUSIVE = 2; $UNLOCK = 8;` [download] You should use the constants from the Fcntl module: `use Fcntl ':flock';` [download] `$fifo = "ENV{HOME}/a_file";` [download] You probably meant to use the %ENV hash there: `$fifo = "$ENV{HOME}/a_file";` [download] `close(FILE) \|\| warn "update queue file exited $?\n";` [download] The $? variable will only contain useful information if you are running an external program. which you are not. However, you should include the $! variable in any system error messages. close(LOCK) \|\| warn "lock file exited $?\n";; $str=`cat $fifo.lock`; flock LOCK, $UNLOCK; [download] close(LOCK) will also unlock the file so the subsequent attempt to unlock it is superfluous. `system("rm $fifo.lock");` [download] Is there a reason that you couldn't use the built-in unlink function? Have you read the entries in perlfaq5 on file locking? You are locking the PID file but not the data file. This may work better: #!/usr/local/bin/perl use warnings; use strict; use Fcntl ':flock'; use Tie::File; my $fifo = "$ENV{HOME}/a_file"; my $lock = "$fifo.lock"; tie( my @pid, 'Tie::File', $lock )->flock( LOCK_EX ) or die "Cannot open '$lock' $!"; @pid = $$; tie( my @lines, 'Tie::File', $fifo )->flock( LOCK_EX ) or die "Cannot open '$fifo' $!"; ( my $pop, @lines ) = sort @lines; print "$hostname is poping $pop\n"; untie @lines; print "PID lock file is going to be removed\n"; untie @pid; unlink $lock or die "Cannot unlink '$lock' $!"; __END__ [download]	[reply] [d/l] [select]
Re^2: mkfifo/mknode by azaria (Beadle) on Jun 14, 2006 at 12:15 UTC
First thanks for your detailed reply. I used the following code but there is a problem with the flock package - `Usage: Fcntl::constant(name, arg) at Fcntl.pm line 225.` [download] Can you please help ? azaria ------------------------------------------------------- #! /usr/local/bin/perl chomp($hostname=`hostname`); chomp($date = `date \'\+\%y\%m\%d\%H\%M\%S\'`); use warnings; use strict; use Fcntl ':flock'; use Tie::File; my $fifo = "$ENV{HOME}/a_file"; my $lock = "$fifo.lock"; tie( my @pid, 'Tie::File', $lock )->flock( LOCK_EX ) or die "Cannot open '$lock' $!"; @pid = $$; tie( my @lines, 'Tie::File', $fifo )->flock( LOCK_EX ) or die "Cannot open '$fifo' $!"; ( my $pop, @lines ) = sort @lines; print "$hostname is poping $pop\n"; untie @lines; print "PID lock file is going to be removed\n"; untie @pid; unlink $lock or die "Cannot unlink '$lock' $!"; __END__ [download] Janitored by Corion: Added formatting, code tags, as per Writeup Formatting Tips	[reply] [d/l] [select]
Re^3: mkfifo/mknode by jwkrahn (Abbot) on Jun 15, 2006 at 01:16 UTC
I used the following code but there is a problem with the flock package - `Usage: Fcntl::constant(name, arg) at Fcntl.pm line 225.` [download] The Fcntl module that I am using does not have any "usage" messages in it however the Tie::File module does. Which version of the Tie::File module are you using? You can find out like this: `$ perl -MTie::File -le'print $Tie::File::VERSION' 0.97` [download] If the Fcntl module does not work you could go back to using: `my $LOCK_EX = 2;` [download] And change all occurances of LOCK_EX to $LOCK_EX. Also, to get the hostname you could use the Sys::Hostname module: `use Sys::Hostname; my $hostname = hostname;` [download] And to get a formatted date you could use the POSIX module: `use POSIX 'strftime'; my $date = strftime '%y%m%d%H%M%S', localtime;` [download]	[reply] [d/l] [select]
Re: mkfifo/mknode by Moron (Curate) on Jun 14, 2006 at 09:51 UTC
I use flock for this type of locking and it always worked like a dream for me. Update: What I meant to say is that I use flock on the file itself to be locked, not on a separate lock file - it doesn't quite work like that. I have done what you are doing on occasion, but in that case I opened the lock file '+<', did the flock and then a seek 0 and (over-) wrote the pid for testing, though I left that in there for support purposes. -M Free your mind	[reply]
Re^2: mkfifo/mknode by azaria (Beadle) on Jun 14, 2006 at 13:08 UTC
Hello Moron, I tried to use the flock function but i got the error - Usage: Fcntl::constant(name, arg) at Fcntl.pm line 225. Do you have any simple example ? Thanks	[reply]
Re^3: mkfifo/mknode by Moron (Curate) on Jun 14, 2006 at 14:02 UTC
Here is a simple example of using a dedicated lock file `my $LOCK_EX = 2; # exclusive lock on file my $LOCK_UN = 8; # unlock file my $lock_file = $ENV{ XYZ_DIR } . "/$0.lock"; open my $lh, ">$lock_file" \|\| die "Can't open lock file $lock_file: $! +"; flock $lh, $LOCK_EX; # will wait for exclusive lock # on lock file # do the exclusive actions flock $lh, $LOCK_UN; close $lh;` [download] -M Free your mind	[reply] [d/l]
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