Subject: bug#16576: closed (Re: bug#16576: Binding `standard-output' to a function doesn't work -- function sometimes called with garbage)

From: [email-masked] (GNU bug Tracking System)
Date: Wed, 29 Jan 2014 18:03:03 +0000

Your bug report

#16576: Binding `standard-output' to a function doesn't work -- function sometimes called with garbage

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to [email-masked].

-- 
16576: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16576
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#16576: Binding `standard-output' to a function doesn't work -- function sometimes called with garbage
From: Eli Zaretskii <[email-masked]>
To: Anders Lindgren <[email-masked]>
Cc: [email-masked]
Date: Wed, 29 Jan 2014 20:01:57 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

> Date: Tue, 28 Jan 2014 23:00:32 +0100
> From: Anders Lindgren <[email-masked]>
> Cc: [email-masked]
> 
> Well, the main issue here is if this is a real bug or not. You originally
> thought this was simply a mistake in the example I provided -- do you still
> think so?

There was indeed a bug, now fixed on the trunk, which caused the
problem.  It was a very old bug, it went unnoticed until now because,
I guess, no one conses a string one character at a time -- this is a
terrible idea in Emacs Lisp.  Use a temporary buffer instead.

There was nothing wrong with prin1 per se, btw, it's just that its
subroutine which prints a string wasn't safe when GC hit in the middle
of a prin1 call.  The code protected the string from GC, but that
doesn't preclude relocation of string data, so holding to C 'char *'
pointers is not safe in these situations.  And printing a
800-character string like you did triggers GC every time, because it
creates 800 string objects for a grand total of about 320000 bytes.

Anyway, problem solved, and I'm closing the bug.  Thanks.

Subject: Binding `standard-output' to a function doesn't work -- function sometimes called with garbage
From: Anders Lindgren <[email-masked]>
To: [email-masked]
Date: Tue, 28 Jan 2014 10:09:08 +0100
List-Id: <debbugs-submit.debbugs.gnu.org>

Hi!

When binding `standard-output' to a function, is seems as though the
function sometimes is called with garbage, like random control characters
or part of the string in "random" order.

I've seen this problem in Emacs 22 and Emacs 24.3. I haven't been able to
reproduce it on the Emacs trunk, but I've seen some problems (although less
frequent) that could be explained if the problem existed there as well.

Steps to repeat:

    emacs -Q
    Eval the following:

(defvar bug-bind-output-str nil)

(defun bug-bind-output-function (char)
  (setq bug-bind-output-str
        (concat bug-bind-output-str (list char))))

(defun bug-bind-output-test ()
  (interactive)
  (let ((bug-bind-output-str "")
        (standard-output 'bug-bind-output-function)
        (s
        s0)
    (prin1 s)
    bug-bind-output-str))

Type this a number of times:

    M-x (bug-bind-output-test) RET

A correct output looks like:

This is an example of bad output, here, the last couple of lines does not
correspond to the string being printed.

Other examples of bad output is if the string contains random control
characters.

I've noticed that this is intermittent. Sometimes, say, every tenth call
returns a broken string. Sometimes it feels as though you can call the
function any number of times without getting a bad string.

Sincerely,
    Anders Lindgren

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/Applications/Emacs24.3.app/Contents/Resources/etc/DEBUG.

In GNU Emacs 24.3.1 (x86_64-apple-darwin, NS apple-appkit-1038.36)
 of 2013-03-13 on bob.porkrind.org
Windowing system distributor `Apple', version 10.3.1265
Configured using:
 `configure '--host=x86_64-apple-darwin' '--build=i686-apple-darwin'
 '--with-ns' 'build_alias=i686-apple-darwin'
 'host_alias=x86_64-apple-darwin' 'CC=gcc -mmacosx-version-min=10.7
 -isystem
 /Users/david/Xcode-10.7_4.5.2/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk/usr/include/
 -F/Users/david/Xcode-10.7_4.5.2/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.7.sdk/System/Library/Frameworks''

Important settings:
  value of $LC_CTYPE: UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
<down> <right> <right> <escape> C-x <down> <down> <down>
<escape> C-x <down> <down> <down> <down> <down> <down>
<escape> C-x <escape> : ( b i g <backspace> <backspace>
u g - b i <tab> t <tab> ) <return> <escape> : <up>
<return> <escape> : <up> <return> <escape> : <up> <return>
<escape> : <up> <return> <escape> : <up> <return> <escape>
: <up> <return> <escape> : <up> <return> <escape> :
<up> <return> <escape> : <up> <return> <escape> : <up>
<return> <escape> : <up> <return> <escape> x e m a
c s - b u <tab> C-g <escape> x m <backspace> e m a
c s - b i <backspace> u g <tab> <s-backspace> <s-backspace>
<escape> <backspace> <escape> <backspace> <escape>
C-g r e p o r t - e m <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading vc-svn...done
bug-bind-output-str
bug-bind-output-function
bug-bind-output-test
[11 times]
Quit

Load-path shadows:
None found.

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