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Re^7: How can I read DATA in parent and child?

by Skeeve (Parson)
on Feb 19, 2019 at 10:25 UTC ( #1230156=note: print w/replies, xml ) Need Help??


in reply to Re^6: How can I read DATA in parent and child?
in thread How can I read DATA in parent and child?

This is really weird…

I removed the sleep commands and printed the $pos in the start message.

Both processes tell the same startposition. Nevertheless I also get non-15 lines of output every now and then. 1536 loops were okay, then the parent didn't read any lines.

My modified 123099.pl code:

#!/usr/bin/perl 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 $pos\n"; while (<$x>) { print "c: $_"; } print "stop c\n"; } else { my $y= IO::Handle->new(); $y->fdopen(fileno(DATA), "r"); seek $y, $pos, 0; print "Start p $pos\n"; while (<$y>) { print "p: $_"; } print "waiting for child\n"; wait; print "stop p\n"; } __DATA__ a b c d e

And my modified version of your test-bash:

#!/bin/bash i=1 while : ; do o=$( ./1230099.pl ) n=$( echo "$o" | wc -l ) (( n == 15 )) || break printf '%d\r' $((i++)) done echo "$i:$n" echo "$o"

Update I've run it several times now and this is the result:

2182: 10 1504: 10 560: 10 14: 10 919: 10 360: 10 908: 10 200: 10 65: 10 108: 10 43: 10 105: 10 296: 10 345: 10 100: 10 1084: 10 1278: 10 224: 10 6: 10 8: 10 125: 10 684: 10 96: 10 8: 10 579: 10

So after an unpredictable amount of iterations (6 to 2182) the scripts prints 10 instead of 15 lines…


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

Replies are listed 'Best First'.
Re^8: How can I read DATA in parent and child?
by tybalt89 (Prior) on Feb 19, 2019 at 16:41 UTC

    Your race condition is: one side of fork gets to the (buffered) read before the other side gets to its seek().

    I find it easier to see what is going on by using non buffered reads (sysread). Half-offsetting (timewise) the two forks leads to consistent results.

    Removing the (unnecessary) seeks()'s should also lead to consistent output size (although it could be from either the parent or child).

    #!/usr/bin/perl # https://perlmonks.org/?node_id=1230099 use strict; use warnings; use IO::Handle; $| = 1; my $pos = tell DATA; my $pid = fork(); my $x= IO::Handle->new(); $x->fdopen(fileno(DATA), "r"); $pid and select undef, undef, undef, 0.05; seek $x, $pos, 0; while( sysread $x, $_, 1 ) { print $pid ? uc : lc; select undef, undef, undef, 0.1; } 1 while wait > 0; __DATA__ aa bb cc dd ee

    Outputs:

    aAa bB Cc dD Ee

    Note the extra 'a' as a result of the seek().

      Sorry to reply this late, but your result is different from what I wanted to achieve.

      Your parent and child read the same DATA once meaning: Every entry is read by either parent or child.

      I want to read DATA independently. Parent and child in my real world application need to have the same set of information to do different things with it.


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

        See Re^3: How can I read DATA in parent and child? fdopen doesn't help, it's still the same shared seek pointer. *nix doesn't do what you want. If you want to do reads on handles, try this instead.

        #!/usr/bin/perl # https://perlmonks.org/?node_id=1230099 use strict; use warnings; open my $x, '<', \do { local $/; <DATA> }; my $who = fork() ? 'p' : 'c'; while( <$x> ) { print "$who $_"; } 1 while wait > 0; __DATA__ a b c d e
        Parent and child in my real world application need to have the same set of information to do different things with it.

        Is this an XY Problem? The order of operations should then be:

        1. Read and store all the data
        2. Fork
        3. Process all the already-read data in different ways

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