Eli Zaretskii <eliz@gnu.org> writes:

>> In any case, supporting stderr could be a starting point, as it seems to
>> be a long-standing request:
>> https://lists.gnu.org/archive/html/emacs-devel/2004-04/msg01051.html
>> and it wouldn't involve a portability issue.
>
> Redirecting the child's stderr is already supported on Windows, so
> this is only a matter of having the higher layers DTRT.

So, let's start with small (but common) things.  The attached patch adds
a new keyword `:stderr' to `make-process'.  The argument can be either a
buffer or a (newly introduced) pipe process.

One could write:

  (make-process :name "test"
                :buffer (current-buffer)
                :command (list (expand-file-name "./test.sh"))
                :stderr "stderr")

to collect the stderr output in the "stderr" buffer, or could write:

  (let ((stderr (make-pipe-process :name "stderr")))
    (make-process :name "test"
                  :buffer (current-buffer)
                  :command (list (expand-file-name "./test.sh"))
                  :stderr stderr)
    (set-process-filter stderr ...))

to collect the stderr output with a process filter.

The patch should apply after the make-process patch:

> From 206196c18652601920017b3a30316ac4205b42dd Mon Sep 17 00:00:00 2001
> From: Daiki Ueno <ueno@gnu.org>
> Date: Mon, 16 Mar 2015 11:38:05 +0900
> Subject: [PATCH] Generalize start-process with keyword args

Comments appreciated.  To be honest, a new process type to collect
stderr might be overkill, but it would make it possible for the current
process I/O functions to support further file descriptors / handles.

Regards,
-- 
Daiki Ueno