From: Robert Klemme <shortcutter@...>
Date: 2009-12-12T00:32:44+09:00
Subject: Re: Redirecting standard output

2009/12/11 Robert Gleeson <rob@flowof.info>:
>> The gem was not needed for the redirection functionality but for
>> getting at information about the method.
> Yup, I completely missed that! I was tired, and misread the thread
> completely.
>
>>> $stdout = File.new '/path/to/file', 'w'
>>> puts 'foobar'
>>> $stdout = STDOUT
>>> puts 'foobar'
>>
>> This does only work for redirections of output for the particular
>> process.  It will fail in these cases
>>
>> - sub processes started
>
> Hmm really? I thought a fork() would copy the memory from the parent
> process into the subprocess.
> Example:
>
>  $stdout = File.open 'foobar.txt', 'w'; fork { $stdout.puts 'Hello' }
>
>  $ cat foobar.txt
>    Hello
>    => 4301
>
> Am I misunderstanding you?

I probably wasn't specific enough: sub processes which use exec to
overlay the image will not inherit redirection if you simply assign to
$stdout:

16:24:59 ~$ ruby19 -e '$stdout = File.open("log","w"); puts 123; system("date")'
Fri Dec 11 16:25:05 WEST 2009
16:25:05 ~$ cat -n log
     1  123
16:25:08 ~$ ruby19 -e '$stdout.reopen("log"); puts 456; system("date")'
16:25:14 ~$ cat -n log
     1  456
     2  Fri Dec 11 16:25:13 WEST 2009
16:25:15 ~$

>> - all entities which remembered $stdout instance (e.g. a logger)
>
> I'm not sure I follow you here either.
> A reference to $stdout would persist.
>
>  foo = $stdout
>  $stdout = File.new 'foobar.txt', 'w'
>  # foo still references $stdout
>
> Unless they use #dup, I don't see how the reference to $stdout wouldn't
> persist.

And exactly that is a problem: redirection does not take place for
whoever owns foo => it failed.

Cheers

robert

-- 
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