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User Questions
fork(): terminating the child without killing the parent
No replies — Read more | Post response
by Bloehdian
on Oct 25, 2016 at 20:25

    Hello Monks,

    I wrote a test application in which the process is forked. The resulting parent process sends a message continuously via pipe using syswrite to the child.

    I installed a signal handler in the parent for signal SIGHUP, so that I can "advice" the parent from the command line to terminate the child (on sending SIGHUP to the parent process). At least I try to do this.

    Here is the code:

    use strict; use warnings; use Errno qw( EWOULDBLOCK ) ; my $from_parent; my $to_child; my $pid; my $mesg_parent = "Hello Child!"; my $mesg_child; my $test; pipe( $from_parent, $to_child ); if ( $pid = fork() ) { # Parent $SIG{ HUP } = sub{ kill( 15 , $pid ); print( "KILLED CHILD!!!\n" ) +}; close( $from_parent ); while ( 1 ) { eval{ $test = syswrite( $to_child, $mesg_parent, length( $mesg_p +arent ) ); }; select( undef, undef, undef, .5); if ( $@ ) { print( "trapped error when writing to pipe \$to_child_ssl\n" +); next; } if ( defined $test ) { print( "Sent message \"$mesg_parent\" to child\n" ); } } } else { # Child close( $to_child ); while ( 1 ) { $test = sysread( $from_parent, $mesg_child, 13 ); select( undef, undef, undef, .5 ); if ( ! defined $test ) { if ( $! == EWOULDBLOCK ) { print( "sysread would be blocked by \$from_parent: $!\n" ) +; next; } else { die ( "Error when trying to read from \$from_parent: $!\n" + ); } } print( "In child\n" ); print( $mesg_child . "\n" ); $mesg_child =~ s/Child/Parent/; print $mesg_child . "\n"; } }

    When I run the script and execute

    kill -1 <pid>

    in a different terminal window with <pid> being the parent process's PID than the final output is as follows:

    Sent message "Hello Child!" to child In child Hello Child! Hello Parent! Sent message "Hello Child!" to child In child Hello Child! Hello Parent! KILLED CHILD!!! Sent message "Hello Child!" to child Sent message "Hello Child!" to child xyz@v32470:~/projekte/$

    i.e., not only the child┤, but the parent as well is terminated.

    If I use

    kill 1 <pid>

    then obviously only the parent process is killed, the child continues to run:

    In child Hello Child! Hello Parent! Terminated xyz@v32470:~/projekte$ In child Hello Child! Hello Parent! In child In child In child In child

    The kill command comes back with an error message btw:

    xyz@v32470:~$ kill 1 31055 -bash: kill: (1) - Operation not permitted

    How can this behaviour explained and how could I reach that only the child is killed by the parent and the parent continues to work?

    The backgroud: I want to use this code as the basis for a script in which the parent checks whether the child is still running by sending messages via pipe to the child and checks the response (i.e., in the final version the communication would be bidirectional).

    Is this feasible using this approach at all?

    From my ┤point of view it boils down to the qestion whether I could trap an error as indicated in the script when trying to syswrite() to the Řipe after the child passed away. Is this possible?

    Cheers Bloehdian
Syntax Question Related to "join" on a Complex Data Structure
2 direct replies — Read more / Contribute
by perldigious
on Oct 25, 2016 at 17:12

    Due to my own programming vocabulary deficiencies, poor Google foo, or both I'm unable to search effectively for an answer to this syntax question. Sparing what I hope is unnecessary detail, I have in the past done the following and was amazed (not for the first time) that Perl let me do this and furthermore did exactly what I wanted.

    my $sales_line = join "\t", @data[@sales_indexes{@sales_columns}];

    Where I use the @sales_columns array to give me a hash slice of %sales_indexes (using @ instead of % for a list) which I use to take an array slice of @data, join it with tabs, and then store the result as the $sales_line string. Yeah... I think I said that all correctly. When this worked it was one of those, "damn I love Perl" moments for me. I often physically walk over to the only other Perl user I know and say exactly that whenever they happen.

    Which brings me to my question, because it's somewhat similar, and it's a rare case of Perl not understanding what I meant and/or doing what I wanted, and I'm hoping the error is mine and I just have the exact syntax wrong. I tried the following which gives a syntax error.

    my $transcripts_line = join "\t", @transcripts{$transcript_ip}{$transcript_id}{@transcripts_columns};

    Where %transcripts is a 3D hash that I'm trying to use the same up front @ instead of % trick to get the value list I want from the hash slice that I was hoping would be provided by the @transcripts_columns array ($transcript_ip and $transcript_id are simple scalars). I attempted the below as well, which doesn't give a syntax error, but doesn't work as expected either.

    my $transcripts_line = join "\t", ($transcripts{$transcript_ip}{$transcript_id}{@transcripts_columns});

    The way I did solve my problem was as follows.

    my $transcripts_line = join "\t", map {$transcripts{$transcript_ip}{$transcript_id}{$_}} @transcripts_columns;

    Which is fine, but... the first line of code I gave that worked was just so, well... succinct and pretty. :-)

    As I said, I'm hoping I've just messed up the syntax with what I originally tried and I don't actually have to resort to the map inside of a join to make this work.

    Any thoughts from the monastery?

    I love it when things get difficult; after all, difficult pays the mortgage. - Dr. Keith Whites
    I hate it when things get difficult, so I'll just sell my house and rent cheap instead. - perldigious
Devel::Size reports different size after hash access
4 direct replies — Read more / Contribute
by Cristoforo
on Oct 25, 2016 at 15:40
    While working on a problem, I came across something I couldn't explain.

    When asking for the size of a newly created hash, it gave me one size. But after accessing the hash, it gave a size almost 50% larger.

    #!/usr/bin/perl use strict; use warnings; use Devel::Size 'total_size'; my $s = 'AAAAAAAAAAAAAAA'; my %hash = map {$s++ => 1} 1 .. 1000; print total_size(\%hash). ' ' . keys(%hash) . "\n"; open my $fh, '>', 'j1.txt' or die $!; for my $key (keys %hash) { print $fh "$key $hash{$key}\n"; } print total_size(\%hash). ' ' . keys(%hash) . "\n";
    The results of running this code was:
    105248 1000 145288 1000

    I'm using perl version 5.014 and the version of Devel::Size is .08.

If line matches, print column, else print file name
2 direct replies — Read more / Contribute
by Yakup
on Oct 25, 2016 at 14:05

    Hello everyone. I had used Perl a little bit long time ago and now I'm trying to write a little program, but I have got stuck. I would like to check bunch of kickstart files for a '--hostname=' pattern. If it matches I want to get the collumn right after it (actual hostname). Else it should take the name of the file (stripped of '.ks' suffix) and save all of them to the array. Commented lines should not be taken. This is how I have done it in bash + awk

    #!/bin/env bash declare -a hostlist=$(grep -P '(?<!#)--hostname' *.ks | awk -F'=' '{pr +int $2}') declare -a not_predefined_hosts=$(grep -L "\-\-hostname" *.ks | cut -d +\. -f1 ) declare -a commented=$(grep -l ".*#.*\-\-hostname" *.ks | cut -d\. -f1 + ) for host in "${not_predefined_hosts[@]}" do hostlist+="$host" done for host in "${commented[@]}" do hostlist+="$host" done

    This works, but when I want to do it with perl, I get multiple issues.

    #!/usr/bin/env perl use strict; use warnings; use File::Basename; my $lab_root = dirname $0; opendir( DH, $lab_root) or die "Cannot open $lab_root: $!\n"; my @kickstarts = grep ( /\.ks$/, readdir(DH)); my @bsname ; my $hostname; for my $kickstart (@kickstarts) { my $name = (split /\./)[0], $kickstart; open my $fh, $kickstart or die "Cannot open $kickstart: $!"; while (<$fh>) { chomp; my @fields = split /=/; if ( $fields[0] eq '--hostname') { $hostname = $fields[1]; print "$hostname\n"; push @bsname , $hostname; }else { $hostname = $name; print "$hostname\n"; push @bsname , $hostname; } close $fh; } }

    First, I get the warnings

    Provisioner]$ ./ Useless use of private variable in void context at ./ line + 13. Use of uninitialized value $_ in split at ./ line 13. Use of uninitialized value $hostname in concatenation (.) or string at + ./ line 24, <$fh> line 1. readline() on closed filehandle $fh at ./ line 27. Use of uninitialized value $_ in split at ./ line 13. readline() on closed filehandle $fh at ./ line 27. Use of uninitialized value $_ in split at ./ line 13. Use of uninitialized value $hostname in concatenation (.) or string at + ./ line 24, <$fh> line 1. readline() on closed filehandle $fh at ./ line 27. Use of uninitialized value $_ in split at ./ line 13. Use of uninitialized value $fields[0] in string eq at ./ l +ine 18, <$fh> line 1. Use of uninitialized value $hostname in concatenation (.) or string at + ./ line 24, <$fh> line 1. readline() on closed filehandle $fh at ./ line 27.

    I don't understand why I'm getting "Use of uninitialized value" warnings, when I initialize all variables with "my" beforehand. Also, why does the filehandle "$fh" close before the close statement? And what the "Useless use of private variable in void context" mean? All examples I was able to google on it were very different from my code and didn't help me to understand that.

    Second, when the code runs,( with added 'print @bsname;' ) it matches only once (but strangely prints twice). There are multiple kickstart files with "--hostname" directive in it, but it ignores the rest. Also the "else" part doesn't work (none of the file names are matched).

    [################### Provisioner]$ ./[################### Provisioner]$

    Can somebody please point out mistake in my code? I'm sure it will be something trivial, but I can't wrap my head around it. Thanks!

IPC::Run command is correctly constructed but does not have effect
1 direct reply — Read more / Contribute
by byrnejb
on Oct 25, 2016 at 12:55

    I am hacking at an old perl scrip that is used to generate and maintain a private CA. The script contains this code:

    . . . use IPC::Run qw( start pump finish timeout new_appender new_chunker); . . . sub cmd { my $self = shift; my $cmd = shift; my $cmdline = shift; my $args = shift; my $conf; my $cfgcmd; if ( (grep $_ eq $cmd,qw(req ca)) && !$args->{noconfig}) { $conf = $self->{csp}->writeConfig($cmd,$args); $self->{csp}->die("Unable to write configuration file") unless -f $c +onf; $cfgcmd = " -config $conf "; } elsif ($cmd eq 'x509' && !$args->{noconfig}) { $conf = $self->{csp}->writeConfig($cmd,$args); $self->{csp}->die("Unable to write configuration file") unless -f $c +onf; $cfgcmd = " -extfile $conf -extensions extensions "; } $cmd = '' if $cmd eq 'dummy'; ${$self->{_in}} = "$cmd $cfgcmd $cmdline"; if ($ENV{CSPDEBUG}) { $self->warn("Here I am"); $self->warn("# openssl $cmd $cfgcmd $cmdline\n"); } $self->{_handle}->pump while length ${$self->{_in}}; $self->{_handle}->finish; . . .
    When I run this command
    csp CA_HLL_ROOT_2016 init --verbose --type=root --keysize=4096 --days= +7317 --dige +st=sha512 "CN=CA_HLL_ROOT_2016,OU=Networked Data Services,O=Harte & L +yne Limited,L=Hamilton,ST=Ontario,C=CA,DC=harte-lyne,DC=ca"
    then I get this result:
    [CSP][ ] Here I am [CSP][ ] # openssl genrsa -des3 -passout pass:'test me' -out / +home/byrnejb/Projects/Software/rcsp/ca_test_a/csp/CA_HLL_ROOT_2016/pr +ivate/ca.key 4096 [CSP][ ] Here I am [CSP][ ] # openssl req -config /home/byrnejb/Projects/Software +/rcsp/ca_test_a/csp/CA_HLL_ROOT_2016/tmp/csp-32489.conf -x509 -sha51 +2 -days 7317 -key /home/byrnejb/Projects/Software/rcsp/ca_test_a/csp +/CA_HLL_ROOT_2016/private/ca.key -passin pass:'test me' -new -out /ho +me/byrnejb/Projects/Software/rcsp/ca_test_a/csp/CA_HLL_ROOT_2016/ca.c +rt

    There are no errors but the files specified in the command lines are not created. If I copy and paste the command lines that are output from the warn statements then the files are created without problem.

    I have zero experience with Perl. Well, not quite zero any more but not much. Is there something obvious that I am missing here? The entire project is available on github at if more context is desired.

Efficient way to print 2 columns from 2 Array of Arrays next to each other
4 direct replies — Read more / Contribute
by Anonymous Monk
on Oct 25, 2016 at 12:34
    Dear Monks,
    I have 2 arrays of arrays (let's call them @AoA_first and @AoA_second that have same amount of lines and columns. What I want to do is read through each of them, and print in a file, each column of each array of arrays, separated by tabs.
    I managed to do so with the following code, but the thing is that it is very slow and there has to be a faster way that I am not aware of...
    Please advise!
    for $i ( 0 .. $#AoA_first ) { $row_first = $AoA_first[$i]; $row_second = $AoA_second[$i]; open TEMP, ">TMPFILES/$i.tmpfile"; for $j ( 0 .. $#{$row_first} ) { print TEMP "$row_first->[$j]\t$row_second->[$j]\n"; } close TEMP; } }

    I think it is slow for me because it reads through every element of every column of each of the two array of arrays, but I can't think of another way to "dump" the two columns side by side into a new file...
Packaging Libraries before deploying my Scripts.
1 direct reply — Read more / Contribute
by ArunMKumar
on Oct 25, 2016 at 07:02
    Namaste Monks.. I have written a set of scripts, on a local system (my laptop) The work fine here.
    Now My scripts are required to be deployed in remote system(s) and the thing I am worried about is the availability and the installation of the Libraries that I have used.
    The 2 Libraries that are currently in use are "XML::LibXML" and "Spreadsheet::ParseExcel".
    I have this idea where I will put them in a "lib" directory in the same project Directory, and the scripts will refer from them. My questions are as follows.
    I have installed them in a directory, using the commands as follows from the extracted tar files.

    perl Makefile.Pl
    make test
    make install PREFIX=<path_to_lib> LIB=<path_to_lib>

    I also see the .pm files being populated in various tree structures in that lib directory.
    My question now is , How do i force my scripts to reference this lib directory when It wants to look for the modules?
    I am new to perl, from what I searched online I think it has to do with modifying @INC variable, while some solutions say about explicitly including these versions of the library (which I have no clue, as those post included their own perl modules).
    so.. How do i force my scripts to use these libraries exclusively, and not throw an error when i deploy them to different systems (all Linux systems with perl 5 or above for sure).
heredoc and Carriage return
4 direct replies — Read more / Contribute
by gabrielsousa
on Oct 25, 2016 at 05:18

    heredoc read the LF ( \n ) , but dont parse / stores the Carriage return ( \r )

    how can i force heredoc to read and store carriers return ( \r ) ?

    =====================added before======================

    i have a pdf file / binary inside my perl script using heredoc
    has 40 Carriage returns, i can search on VI, i see the ^M
    but when i print to a file i lose all Carriage returns..
    i'm assuming is a problem of heredoc
perl and Docker
3 direct replies — Read more / Contribute
by IAperl
on Oct 24, 2016 at 22:44

    Hi Everyone

    I am working on a Docker project which implements the following script (

    This script is written in python and contains a section of code which converts a certain conf_file written in perl syntax.

    This conf_file is used by an external function 'Nlpfit' which reads perl syntax (

    while running this code in docker environment, i keep getting error:

    String found where operator expected at (eval 8) line 6, near "u'step + ' => 8, u'" and Error reading config from /scratch/volgenmodel-fast-example/workfl +ow/writeconf_01_/fit_stage_01.conf (fix it!)_

    I have checked the conf_file for syntax error, but there doesnt seem to be any string error. My default conf_file looks like this

    @conf = ( {'step' => 8, 'blur_fwhm' => 4, 'iterations' => 20}, {'step' => 6, 'blur_fwhm' => 3, 'iterations' => 20}, {'step' => 4, 'blur_fwhm' => 2, 'iterations' => 10}, {'step' => 2, 'blur_fwhm' => 1, 'iterations' => 10}, );

    Note that this external function (Nlpfit) file uses version 1.2.0 but the docker environment uses Perl5Lib

    Do you think this is due to compatibility issue between Perl version 1.2.0 and Perl version 5.0

    I am new to perl and need guidance on what is causing this error and how can I fix it

    Also note that this algorithm runs fine outside Docker environment


blockchain Someone ?
3 direct replies — Read more / Contribute
by dominix
on Oct 24, 2016 at 21:52
    Is there any API/Module for Perl to deal with blockchain ?

    I've searched on metacpan for blockchain but found nothing, I also searched Github an only find this project

    which is centered on Ethereum but I am looking for something more generic on blockchain.

    I guess Perl desserve an entry point for that technology, I'm confortable enought with Perl5 and I am willing to learn Perl6, so any of these Perl will fit, but I'm not experienced enought on that matter to build such a solution by myself. If you have heard of any project of interest in that matter thank for your inputs.

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