From: Lloyd Zusman <ljz@...>
Date: 2007-02-15T19:38:34+09:00
Subject: Re: Another Programming Ruby Question

Bharat Ruparel <bruparel@mercury.com> writes:

> That set me thinking what does the line:
> $stderr.sync = $stdout.sync = true
> actually do?

(from the "sync= (IO)" section of http://www.ruby-doc.org/core/):

  ios.sync = boolean => boolean

  Sets the ``sync mode'' to true or false. When sync mode is true, all
  output is immediately flushed to the underlying operating system and
  is not buffered internally. Returns the new state. See also IO#fsync.

   f = File.new("testfile")
   f.sync = true

  (produces no output)

When data is output through an IO handle, it might be buffered.  What
this means is that the data is held until a certain number of bytes are
received or until certain characters are fed to the stream (such as
newline), and only then are they sent to the output destination.

The "sync mode" determines whether this buffering takes place ... if
it's set to true, then there is no such buffering, and the data is
"synchronized" with the output destination immediately upon being sent
through the IO handle.

The statement you refer to above makes sure that $stdout and $stderr are
both synchronized.  Therefore, in the thread, data written to these
handles will be output immediately.  Without your statement, there could
be a delay between the moment that the data is sent to $stdout or
$stderr and it shows up on the terminal.  That's why you are seeing the
output in a different sequence, depending on whether or not the sync
mode is set.

As you can see, this has nothing to do with setting $stdout and $stderr
to be in sync with each other.


-- 
 Lloyd Zusman
 ljz@asfast.com
 God bless you.