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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().


In reply to Re^8: How can I read DATA in parent and child? by tybalt89
in thread How can I read DATA in parent and child? by Skeeve

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