Re: Adding streams for standard out and standard err

[phillip.lord@russet.org.uk (Phillip Lord)]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: phillip.lord@russet.org.uk (Phillip Lord)
>> Date: Wed, 20 Jul 2016 23:48:43 +0100
>> 
>> For a while I've wanted Emacs to have the ability to write to standard
>> out, and/or standard err, when not running in batch. Mostly, I've wanted
>> for debugging, as it involves touching no buffers at all.
>
> Can we take a step back and talk about the need and the use cases?

Mostly debugging, I think. For example, I find debugging the undo code
very taxing. Using GDB is hard because the debug code gets called so
often (I admit to being barely competent at using GDB). Logging to a
buffer touches other buffers. Logging to a variable works, but you can
only see the value of the variable when all is done.

Dumping to stdout, while interacting with a live emacs would give me an
independent channel to getting output, as well as post-hoc debugging.


> Here're some thoughts related to this:
>
>  . We already have (append-to-file START END FILENAME), which can be
>    used to write a buffer or a string to a file.  FILENAME can be
>    "/dev/stderr" on Posix platforms, for example, or it can be a real
>    file name.

We do, and I am grateful that you pointed this out to me. But
append-to-file also writes to the mini-buffer. This makes interactive
use difficult, or painful. Of course, this could be fixed.

Still,

(append-to-file (print buffer-undo-list) nil "/dev/stderr")

seems clunky compared to:

(print buffer-undo-list 'stdout)


>  . On TTY frames, writing to standard streams can end up on the
>    screen, in which case it will mess up the display.

Yep. I would imagine that debugging to standard out will not become
popular as a tool for debugging TTY display.

>  . On GUI frames, writing to standard streams can end up in some
>    unpredictable place on the system, or even in the bitbucket,
>    depending on what window system, desktop, and window manager are
>    installed.  On some systems, standard streams have invalid file
>    descriptors in GUI sessions.

It can also end up in some entirely predictable place, which is enough.


> So this proposed functionality sounds (a) not really necessary, and
> (b) somewhat unreliable/dangerous.  If you have specific use cases
> where the existing functionality doesn't fit the bill, and the issues
> mentioned above are non-issues, please describe those use cases.


I think it has no problems that append-to-file does not not have, and is
neater, more concise and it's use is clearer.

Probably adding a stdin stream would be good also; still, that's a
separate issue.

Phil