in reply to Re^2: How can I read DATA in parent and child?
in thread How can I read DATA in parent and child?

The docs on fork perlfork say that parent and child share the same seek pointer:

Any filehandles open at the time of the fork() will be dup()-ed. Thus, the files can be closed independently in the parent and child, but beware that the dup()-ed handles will still share the same seek pointer. Changing the seek position in the parent will change it in the child and vice-versa. One can avoid this by opening files that need distinct seek pointers separately in the child.

So I find it surprising that this version works.

  • Comment on Re^3: How can I read DATA in parent and child?

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Re^4: How can I read DATA in parent and child?
by choroba (Cardinal) on Feb 18, 2019 at 18:18 UTC
    > So I find it surprising that this version works.

    It works sometimes . I've run it 10 times and got the following output twice: 

    Start p
p: a
Start c
stop c
p: b
p: c
p: d
p: e
stop p

    [download]

    map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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      It works sometimes.

      As I do an fdopen, I think it should work each time. Infact: I'm just running below code and it's giving me the expected result now the 20th time.

      Here is the (slightly modified) code, which I think should work. 

      use IO::Handle;
use strict;
use warnings;

my $pos= tell *DATA;

my $pid= fork();
die unless defined $pid;
if ($pid == 0) {
  my $x= IO::Handle->new();
  $x->fdopen(fileno(DATA), "r");
  seek $x, $pos, 0;
  print "Start c\n";
  sleep 4;
  while (<$x>) {
    print "c: $_"; sleep 1;
  }
  print "stop c\n";
}
else {
  my $y= IO::Handle->new();
  $y->fdopen(fileno(DATA), "r");
  seek $y, $pos, 0;
  print "Start p\n";
  while (<$y>) {
    print "p: $_"; sleep 1;
  }
  print "waiting for child\n";
  wait;
  print "stop p\n";
}
__DATA__
a
b
c
d
e

      [download]
      s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
      +.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e
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        I've run it 20 times, it returned 15 lines 18 times, but only 10 lines 2 times. 
        #!/bin/bash
i=1
while : ; do
    n=$( 1230099.pl | wc -l )
    (( n == 15 )) || break
    printf '%d\r' $((i++))
done
echo "$i:$n"

        [download]

        It stopped after four another iterations with only 10 lines of output.

        map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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