Line-by-line processing when stdin is not a tty

Cameron Simpson cs at zip.com.au
Thu Aug 12 00:12:49 CEST 2010


On 11Aug2010 12:35, Tim Harig <usernet at ilthio.net> wrote:
| > The buffering is a performance choice. Every write requires a context
| > switch from userspace to kernel space, and availability of data in the
| > pipe will wake up a downstream process blocked trying to read.
| > It is far more efficient to do as few such copies as possible, [...]
| 
| Right, I don't question the optimization.  I question whether the
| intelligence that performes that optimation should be placed within cat or
| whether it should be placed within the shell.  It seems to me that the
| shell has a better idea of how the command is being used and can therefore
| make a better decision about whether or not buffering is appropriate.

I would argue it's not much better placed, though it would be nice if
the control could be issued from there. But it can't.

Regarding the former, in this pipeline:

  cat some files... | python filter program | something else

how shall the shell know if the python filter (to take the OP's case)
wants its input line buffered (rare) or block buffered (usually ok)?

What might be useful would be a way to attach an attribute to a pipe
or other file descriptor indicating the desired buffering behaviour
that writers to the file descriptor should adopt.

Of course, the ugly sides to that are how many buffering regimes should
it be possible to express and how and when should the upstream (writing)
program decide to check? In a pipeline the pipes are made _before_ any
of the programs commence because the programs need to be attached to the
pipes (this is done before the programs themselves are dispatched). So,
_after_ dispatch the python-wanting-line-buffering issues an ioctl on
the pipe saying "I want line buffering". However, the upstream program
may well already have commenced operation before that happens. It may
even have run to completion before that happens! So, shall all upstream
programs be required to poll? How often? On every write? Shall they
receive a signal? What if they don't catch it? If the downstream
program _requires_ line buffering then the whole situation is racey
and unreliable.

You can see that on reflection this isn't easy to resolve cleanly from
_outside_ the writing program.

To do it from inside requires all programs to sprout an option like
GNU cat's -u option.

Cheers,
-- 
Cameron Simpson <cs at zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

What progress we are making. In the Middle Ages they would have burned
me. Now they are content with burning my books. - Sigmund Freud



More information about the Python-list mailing list