Re: My pipe flushes late

[Eric Blake <ebb9 at byu dot net>]

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Let's hope your plumbing doesn't overflow.  Although the hippos might like
the smell...

According to Robert Schmidt on 8/20/2008 5:54 AM:
> I basically need to pass some output through d2u, then add a prefix to
> each line.  Most importantly, I need the line to flush through the pipe
> immediately.  However, d2u (or the pipe itself) caches/flushes late.

What you want is to convert d2u's stdout from fully buffered (default, as
per POSIX, for all non-interactive file descriptors) to line-buffered or
unbuffered.  On Linux, this can be done with an LD_PRELOAD of a library
that calls setvbuf prior to letting main() run, but I'm not sure cygwin
quite has those hooks.  The coreutils list has even seen a proposal for
adding an app just for this purpose, although it is yet to materialize:
http://lists.gnu.org/archive/html/bug-coreutils/2006-03/msg00123.html

At this point, your best bet is to use something that presents d2u with a
pty as stdout (thus output would be line-buffered), rather than a pipe,
then consumes the pty output and throws it over the wall to the original
pipe to sed with flushing.  Have you tried expect - according to the link
above, expect provides a script unbuffer that does just what I described?

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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