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Re: Writing to bash's tty too fast: workarounds?


From: George Nachman
Subject: Re: Writing to bash's tty too fast: workarounds?
Date: Tue, 10 Apr 2012 10:22:19 -0700

Thanks for getting back to me, Chet. The theory about the kernel's handling
of a full input queue seems likely. In the case of bash + readline, would
it be feasible to avoid readline changing the terminal settings while
reading a here document? Seems like the only remedy if the kernel is
misbehaving in this case.

On Tue, Apr 10, 2012 at 9:20 AM, Chet Ramey <address@hidden> wrote:

> On 6/13/11 1:45 AM, George wrote:
> > I'm the maintainer of a terminal emulator on Mac OS and a user
> > reported that if he pastes a large string of the form:
> >
> > cat << EOF > whatever.txt
> > blah blah blah
> > blah blah blah
> > ... etc for 100k ...
> > EOF
> >
> > That the output is badly mangled. I can reproduce this consistently
> > with a smallish test program. Having traced into readline, the culprit
> > seems to be that readline fiddles with the terminal between lines of
> > input (I'm pretty sure the things done in set_tty_settings() are
> > responsible, but I haven't proved it yet). This seems to cause it to
> > drop some or all of the input that's in the input buffer.
>
> Since readline is intended to read a single line of input at a time, it's
> natural that it modifies the terminal settings before it reads any data
> and after it's read a complete line.  The code is careful to use flags to
> tcsetattr(3) that force the system to wait until all output has drained
> and does not throw away input characters.
>
> I suspect that the cause is the kernel's behavior when the input queue
> is full.  Readline doesn't use IXON/IXOFF, so there won't be any
> stopping when the queue is full (see the description of IXOFF in
> termios(4)).
>
> If you want to paste large amounts of data, you're not interested in any
> input line editing, so it makes sense to turn off editing as a workaround
> (set +o emacs +o vi) before running the command using the here document.
>
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                 ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, ITS, CWRU    address@hidden
> http://cnswww.cns.cwru.edu/~chet/
>


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