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

by Skeeve (Parson)
on Feb 18, 2019 at 14:24 UTC ( #1230107=note: print w/replies, xml ) Need Help??


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

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.

      > 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

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

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

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