Hi!

I got the same result with your version. After running it 24 times, I got
the following output (on Emacs 22) -- please note the extra text and
control characters towards the end of the string (which I have changed to
the \xNN syntax, in order not to confuse the mail program).

"\"(\\\\(co\\\\(?:mbine-after-change-calls\\\\|nd\\\\(?:ition-case\\\\(?:-unless-debug\\\\)?\\\\)?\\\\)\\\\|eval-\\\\(?:a\\\\(?:fter-load\\\\|nd-compile\\\\)\\\\|next-after-load\\\\|when\\\\(?:-compile\\\\)?\\\\)\\\\\\\\|p\\\\(?:case\\\\(?:-let\\\\*?\\\\)?\\\\|rog[*12nv]?\\\\)\\\\|save-\\\\(?:current-buffer\\\\|excursion\\\\|match-data\\\\|restriction\\\\|selected-window\\\\|window-excursion\\\\)\\\\|track-mouse\\\\|unwind-protect\\\\|w\\\\(?:hile\\\\(?:-no-input\\\\)?\\\\|ith-\\\\(?:c\\\\(?:a\\\\(?:\\\\(?:se\\\\|tegory\\\\)-table\\\\)\\\\|urrent-buffer\\\\)\\\\|demoted-errors\\\\|electric-help\\\\|local-quit\\\\|no-warnings\\\\|output-to-\\\\(?:string\\\\|temp-buffer\\\\)\\\\|s\\\\(?:elected-\\\\(?:frame\\\\|window\\\\)\\\\|ilent-modifications\\\\|yntax-table\\\\)\\\\|t\\\\(?:emp-\\\\(?:buffer\\\\|\\\\(?:fil\\\\|messag\\\\)e\\\\)\\\\|imeout\\\\(?:-handler\\\\)?\\\\)\\\\|wrapper-hook\\\\)\\\\)\\\\)\\\\>\x00cs/
\xdc\xac\\\"(\\\\\\\\(co\\\\\\\\(?:mbine-after-change-calls\\\\\\\\|n\""

When it comes to the original program, I believe that it's correct. The
function bound to `standard-output' is called from within `prin1', which is
called from inside the scope of `let'. Hence, it will work on the local
version of the variable. (If it wouldn't have been correct -- why would it
return the correct value *most* of the time?)

    -- Anders


On Tue, Jan 28, 2014 at 4:59 PM, Eli Zaretskii <eliz@gnu.org> wrote:

> > Date: Tue, 28 Jan 2014 10:09:08 +0100
> > From: Anders Lindgren <andlind@gmail.com>
> >
> >     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
> >
> "(\\(co\\(?:mbine-after-change-calls\\|nd\\(?:ition-case\\(?:-unless-debug\\)?\\)?\\)\\|eval-\\(?:a\\(?:fter-load\\|nd-compile\\)\\|next-after-load\\|when\\(?:-compile\\)?\\)\\|i\\(?:f\\|nline\\)\\|l\\(?:ambda\\|et\\(?:\\*\\|rec\\)?\\)\\|p\\(?:case\\(?:-let\\*?\\)?\\|rog[*12nv]?\\)\\|save-\\(?:current-buffer\\|excursion\\|match-data\\|restriction\\|selected-window\\|window-excursion\\)\\|track-mouse\\|unwind-protect\\|w\\(?:hile\\(?:-no-input\\)?\\|ith-\\(?:c\\(?:a\\(?:\\(?:se\\|tegory\\)-table\\)\\|urrent-buffer\\)\\|demoted-errors\\|electric-help\\|local-quit\\|no-warnings\\|output-to-\\(?:string\\|temp-buffer\\)\\|s\\(?:elected-\\(?:frame\\|window\\)\\|ilent-modifications\\|yntax-table\\)\\|t\\(?:emp-\\(?:buffer\\|\\(?:fil\\|messag\\)e\\)\\|imeout\\(?:-handler\\)?\\)\\|wrapper-hook\\)\\)\\)\\>")
> >         s0)
> >     (prin1 s)
> >     bug-bind-output-str))
> >
> > Type this a number of times:
> >
> >     M-x (bug-bind-output-test) RET
>
> I think there's a bug in your test program.  You let-bind
> bug-bind-output-str, and print that local binding after you call
> prin1.  But your print function concatenates each character onto the
> _global_ binding of bug-bind-output-str, so the result is not in your
> local binding, it's in the global one.
>
> If you change your program like this:
>
>   (defun bug-bind-output-test ()
>     (interactive)
>     (setq bug-bind-output-str "")
>     (let ((standard-output 'bug-bind-output-function)
>           (s
>
> "(\\(co\\(?:mbine-after-change-calls\\|nd\\(?:ition-case\\(?:-unless-debug\\)?\\)?\\)\\|eval-\\(?:a\\(?:fter-load\\|nd-compile\\)\\|next-after-load\\|when\\(?:-compile\\)?\\)\\|i\\(?:f\\|nline\\)\\|l\\(?:ambda\\|et\\(?:\\*\\|rec\\)?\\)\\|p\\(?:case\\(?:-let\\*?\\)?\\|rog[*12nv]?\\)\\|save-\\(?:current-buffer\\|excursion\\|match-data\\|restriction\\|selected-window\\|window-excursion\\)\\|track-mouse\\|unwind-protect\\|w\\(?:hile\\(?:-no-input\\)?\\|ith-\\(?:c\\(?:a\\(?:\\(?:se\\|tegory\\)-table\\)\\|urrent-buffer\\)\\|demoted-errors\\|electric-help\\|local-quit\\|no-warnings\\|output-to-\\(?:string\\|temp-buffer\\)\\|s\\(?:elected-\\(?:frame\\|window\\)\\|ilent-modifications\\|yntax-table\\)\\|t\\(?:emp-\\(?:buffer\\|\\(?:fil\\|messag\\)e\\)\\|imeout\\(?:-handler\\)?\\)\\|wrapper-hook\\)\\)\\)\\>")
>           s0)
>       (prin1 s)
>       bug-bind-output-str))
>
> i.e., work with the global binding, then the program works as
> expected, AFAICS.
>