mutiple forks

raflach has asked for the wisdom of the Perl Monks concerning the following question:

Is the following code legal/possible/acceptable?
Assume that all variables are properly inited etc.

if ( $child = fork ) {
     #Do Parent stuff here
} else {
     while ( $child2 = fork ) { # fork 2nd child, wait for 
          wait;                 # it to finish and re-fork
     }
     #  Do second level child stuff here
     #  Every time we finish, child # 1 starts us again
     exit;
}
[download]

If that is legal, then assume that I open a set of paired filehandles using pipe ( PARENTREAD, PARENTWRITE ); from the IO::Handle module.
Can I write from the secondary child and read from the first parent? i.e.

if ( $child = fork ) {
     close PARENTWRITE;  # Not needed in parent
     while ( ! $child2data ) {
          $child2data = <PARENTREAD>;
     }
     #  Do Parent stuff here 
     #  after ensuring that child2 started
} else {
     close PARENTREAD;  # Not needed in child
     #  Do some processing
     while ( $child2 = fork ) { # fork 2nd child, wait for 
          wait;                 # it to finish and re-fork
     }
     #  Do second level child init stuff here

     print PARENTWRITE 1; # Let parent know we started good
     #  Do more second level child processing

     #  Every time we finish, child # 1 starts us again
     exit;
}
[download]

Comment on mutiple forks Select or Download Code

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Re: mutiple forks by Fastolfe (Vicar) on Sep 27, 2000 at 23:08 UTC
I don't see why not, though you're doing your read in the `<STYLE>`, which reads by lines, so your print statement needs to be altered slightly: `print PARENTWRITE "1\n"; # perhaps something useful like $pid?` [download] You may also get bitten by the fact that `PARENTWRITE` is buffered. Don't expect communication between your two processes to be instantaneous. Use `select(PARENTWRITE); $\|=1; select(STDOUT);` or the IO::Handle equivalent to change this behavior. Note: Code samples are for conceptual use only and generally are not meant to be cut/pasted into a production application. Always '`use strict`', have a thorough understanding of the code you use, and check the return values of functions and handle errors according to your needs. - Fastolfe	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re: mutiple forks
by Fastolfe (Vicar) on Sep 27, 2000 at 23:08 UTC

<STYLE>

print PARENTWRITE "1\n";   # perhaps something useful like $pid?
[download]

PARENTWRITE

select(PARENTWRITE); $|=1; select(STDOUT);

IO::Handle

Note: Code samples are for conceptual use only and generally are not meant to be cut/pasted into a production application. Always 'use strict', have a thorough understanding of the code you use, and check the return values of functions and handle errors according to your needs. - Fastolfe

[reply]
[d/l]
[select]