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

This works: 

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

my $x= IO::Handle->new();
$x->fdopen(fileno(DATA), "r");

my $y= IO::Handle->new();
$y->fdopen(fileno(DATA), "r");

my $pos= tell *DATA;

my $pid= fork();
die unless defined $pid;
if ($pid == 0) {
  seek $x, $pos, 0;
  print "Start c\n";
  while (<$x>) {
    print "c: $_"; sleep 1;
  }
  print "stop c\n";
}
else {
  seek $y, $pos, 0;
  print "Start p\n";
  while (<$y>) {
    print "p: $_"; sleep 1;
  }
  print "stop p\n";
}
__DATA__
a
b
c
d
e

[download]

Simply reposition the filepointer…

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

Replies are listed 'Best First'.
Re^3: How can I read DATA in parent and child?
by hdb (Monsignor) on Feb 18, 2019 at 15:04 UTC

    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.

[reply]
      > 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]
[reply]
[d/l]
[select]
        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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[d/l]
[select]

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