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Date:     14 March 91                           Message No:     037

To:       TeX implementors and distributors

From:     Barbara Beeton

Subject:  Messages from DEK, part 2


************************************************************************

>>> inconsistent expansion of \string, etc.; \newlinechar

 [ dek:
        Here he has come across exactly what I think I fixed in the
        _other_ change made to 3.14a.  I am contacting him directly
        about this.
 ]

Date: Mon, 4 Nov 91 10:42:40 +0100
From: Bernd Raichle <raichle@azu.informatik.uni-stuttgart.de>
Subject: TeX.web 3.14:  Bug Report #1 (+ Fix Proposal): \string,
         \special, ...

Dear Ms. Beeton,

I have found two bugs in TeX.Web 3.14:
Bug #1: Incorrect "expansion" of unprintable chars for |selector=new_string|
Bug #2: Incorrect handling of \newlinechar
        (I'll send a second mail with the report of bug #2.)


The first bug looks like an inconsistent behaviour of \string (and
\meaning, \fontname, \jobname, \special) in all TeX versions.

Example:
1.  \catcode`\^^Y=12 \string^^Y
2.  \catcode`\^^Y=13 \string^^Y

TeX 3.14 prints line 1 always as character "19 of the current font.
Line 2 is printed either as char"19 or as three characters ^^Y,
dependent on the TeX implementation!


Other examples:
3. If ^^Y has one of the catcodes 1-4, 6-8, 11 or 12
   |\string ^^Y| is printed like |\char"19|.
4. With \escapechar="19, the output of |\string\relax| is
   either like |^^Yrelax| or |\char"19 relax|.
5. The same is true for  \catcode`\^^Y=11 \string\test^^Ytest
6. \newlinechar=`\^^Y \message{^^Y \string ^^Y} outputs two newlines
   only, if ^^Y is a printable character.
   (Remark: I have discovered the second bug with a variation of
   this example.)
7. The same is true for things like \special{^^Y}.
8. etc. for other characters (e.g. national characters)!


An TeX implementor can (and should) change section 21 and 22
(character set translations) and section 49 (unprintable characters)
to make TeX more user friendly.  The predicate in section 49 specifies
if a characters is printable or unprintable.
Example: \ss (= char "19 in CM fonts) should be output as a single
character instead of ^^Y.  An implementor will change section 49 to
exclude this character (i.e., to make this character printable).

[With TeX 3 this is done for more and more characters on more and more
machines with extended character set (especially if the new DC/EC
fonts with national characters are available).]


This implementation dependency is no problem (*), if the output is
printed on a terminal or written in a file (log file or output
stream).  [(*) see the second bug report for some problems]

But it can be a problem, if it is "printed" for further `internal'
use (i.e., with |selector:=new_string|).

Example:
  \def\a{\expandafter\b\string}
  \def\b#1{{\tt #1}}

The output (in the dvi file) is implementation dependent, if \a is
called with national characters, control sequence names with national
characters, etc.
The dvi file will also be different, if someone uses \special commands
with national characters.  E.g. \special{^^Y} should put one byte
with value "19 into the dvi file, but for most TeX versions the three
characters "^^Y" are written.

Fix proposal:

|print| and |slow_print| (\S 59,60) should print a single character
without using |str_pool| if |selector = new_string|.  (I.e., the
`expansion' of unprintable characters to ^^X or ^^xx is inhibited.)

  procedure print(s : integer);
    label exit;
    var j : poolpointer;
    begin if s >= str_ptr then s:="???"
    else if s < 256 then
         if s < 0 then s:="???"
+        else if selector = new_string then
+                begin print_char(s); return
+                end;
              else if (s = new_line_char) then
                   if selector < pseudo then
                      begin print_ln; return
                      end;
    j:=str_start[s];
    while j < str_start[s+1] do
      begin print_char(so(str_pool[j])); incr(j);
      end;
  exit: end;

... and the same for |slow_print|.


Additional changes:

    |issue_message| (used by \message and \errmessage) calls |print|
and |print_err|.  Because the string argument can contain unprintable
character with the fix above, it has to be replaced by |slow_print|.
Additionally the |print_err| call has to be replaced by a call to a
new procedure |slow_print_err|, which calls |slow_print| instead of
|print|.
    I don't know, if it's better to print strings like |job_name|,
|log_name|, |font_name()|...  with |slow_print| (This could be a third
bug?).  If this is true, the string |formatident| has to be printed
with |slow_print|, because it incorporates |job_name|.
    All other strings `printed' with |selector:=new_string| are either
made out of internal strings and integers or they are used only
temporary (|special_out|, |ship_out|) or printed with |print_esc|
(|font_id_text()|), so they do not need further changes.


Possible problems:

- Perhaps some macro writers assume the current behaviour of \string et.al.
  I.e., they assume that \string expands to a fixed set of "printable"
  characters only (e.g. for verbatim text like {\tt \string\foo}),
  but this set of printable characters changes if section 49 is changed.
- The behaviour of \message and \errmessage will change: \newlinechar
  has an effect to these primitives (the TeXbook describes only the
  effect for \write).
- I have made and tested the changes above...  and this changed TeX
  passes the Trip-Test without problems.  Perhaps it would be a good
  idea to add checks for \special and \string with arguments containing
  unprintable characters to `trip.tex'.
  Additionally there should be a short note, that the Trip-Test can
  produce a different output, if section 49 is changed.  The output
  is different if one of the characters ^^@, ..., ^^82, ... is
  printable.


Yours,
  Bernd Raichle
 __________________________________________________________________________
 Bernd Raichle, Student der Universit"at Stuttgart | "Le langage est source
 home:  Stettener Str. 73, D-W-7300 Esslingen, FRG |  de malentendus"
 email: raichle@azu.informatik.uni-stuttgart.de    |  (A. de Saint-Exupery)
 -------

Date: Mon, 4 Nov 91 10:43:42 +0100
From: Bernd Raichle <raichle@azu.informatik.uni-stuttgart.de>
Subject: TeX.web 3.14:  Bug Report #2 (+ Fix Proposal): \newlinechar

Dear Ms. Beeton,

here is the report of the second bug.  I have found this bug when
experimenting with examples (to test the behaviour of \message and
\write) and possible fixes for the first bug.

The second bug deals with \newlinechar and the inconsistent behaviour
of \message and \write when it is set to some "exotic" values.

Example:

  \catcode`\^^Y=11  \edef\test{\string\t^^Yt}
  % (1)
  \newlinechar=`\^
  \message{^^Y ^ \string\^^Y ^ \test}
  \immediate\write0{^^Y ^ \string\^^Y ^ \test}
  % (2)
  \newlinechar=`\^^Y \escapechar=`\^^Y
  \message{^^Y \string ^^Y \relax}
  \immediate\write0{^^Y \string ^^Y \relax}
  % (3)
  \newlinechar=`\o


The output of this example is implementation dependent!  The output
differs, if section 49 of TeX.web is changed to make the character ^^Y
printable.

1. In part (1) of the example, the character ^^Y is printed as two
   newlines followed by a `Y' (if ^^Y is unprintable).  The same
   aplies for all other ^^Y characters, even for ^^Y characters
   inside control sequences.
2. The same ^^Y character is printed as character |xchr["19]|, if
   section 49 is changed to make it printable.
3. The output of \write and \message in part (2) differs, if ^^Y is
   unprintable.  \message produces three |^^Y|, \write prints
   three newlines.
4. There is no difference between \write and \message in part (1) and
   (2), if character ^^Y is made printable.

(Remark: the TeXbook doesn't mention \newlinechar w.r.t. \message, so
one could assume that changing \newlinechar will not change the output
of \message (and \errmessage).)


What behaviour should be correct for |print|, |slow_print|, ...
w.r.t. \newlinechar, if something is printed to a file or the
terminal?

Here are my wishes:

1) if one of these functions is called with the current \newlinechar,
   it should output a newline.
2) if a tokenlist printed with \write, \message, \errmessage contains
   a \newlinechar, it should be printed as a newline.
3) _all_ other characters should never produce a newline character
   (even if there are unprintable characters and \newlinechar = `\^ !)
4) control sequence names containing a character equal to the current
   \newlinechar should not produce a newline (a user should use \string,
   if the newline character in the csname should produce a newline)
5) all fixed strings printed to the log file should never produce a
   newline.  I think error messages in the log file should be readable
   for all values of \newlinechar!  I have added part (3) in my example
   to show the current (mis-)behaviour.


 [ dek:
        Here we have a disagreement ...
        I don't believe it is necessary to support every possible
        \newlinechar in an optimum way; here the simpler rule without
        exceptions wins
 ]

Fix proposal:

The changes below needs the changes in |issue_message|, I have
described in my first bug report (replace call of |print| in this
function with |slow_print|, introduce new function |slow_print_err|
and replace |print_err| in |issue_message| with a call of this
funtion).  Otherwise \message and \errmessage produce incorrect
output for unprintable characters.

I have made the following changes to |print_char|, |print| and
|slow_print|:

  (* the old |print_char| procedure: *)
  procedure print_the_char ( s : ASCII_code )
    (* special handling of current new_line_char deleted *)
    case selector of
      ... etc, etc. old code of |print_char|...
  end;

  procedure print_char ( s : ASCII_code )
    label exit;
    begin if <Character s is the current new-line character> then
      if selector < pseudo then
        begin print_ln; return;
        end;
    print_the_char(s);
  exit: end;

  procedure print ( s : integer );
    label exit;
    var j : poolpointer;
    begin if s >= str_ptr then s:="???"
    else if s < 256 then
         if s < 0 then s:="???"
         (* bug fix from bug report #1: *)
+        else if selector = new_string then
+                begin print_the_char(s); return
+                end;
              else if (s = new_line_char) then
                   if selector < pseudo then
                      begin print_ln; return
                      end;
    j:=str_start[s];
    while j < str_start[s+1] do
      (* to fulfill wish #4: *)
!     begin print_the_char(so(str_pool[j])); incr(j);
      end;
  exit: end;

  procedure slow_print ( s : integer );
    label exit;
    var j : poolpointer;
    begin if s >= str_ptr then s:="???"
    else if s < 256 then
      (* for wish #3: *)
!     begin print(s); return
      end;
    j:=str_start[s];
    while j < str_start[s+1] do
      begin print(so(str_pool[j])); incr(j);
      end;
  exit: end;


I was very astonished that (after I had applied the changes) the
resulted "TeX" passes the Trip-Test!


Possible Problems(?):

- \newlinechar handling is done in \write, \message or \errmessage.
  \write uses |token_show| to output the tokenlist and this is done
  by calls of |print_char| or |print| with single characters or by
  calls of |print_cs|.
  \message and \errmessage uses |slow_print| or |slow_err_print|,
  which handles newline characters at the `toplevel' of the string.

- There should be additional changes to satisfy my wish #5
  (I don't know, if it is enough to change the |print_char| calls
  in the procedure |show_token_list| to call a |print_char| procedure
  with \newlinechar handling.  And the calls of |print_char| in all
  other places are calls to a |print_char| version without \newlinechar
  handling (e.g. the |print_the_char| above).)


Yours,
  Bernd Raichle

<<< end inconsistent expansion of \string, etc.; \newlinechar

************************************************************************

>>> \halign + box assignment within displays

Date: Thu, 21 Nov 91 16:55:55 GMT
From: Chris Thompson <CET1@phoenix.cambridge.ac.uk>
Cc: Robert Hunt <REH10@phoenix.cambridge.ac.uk>
Subject: TeX bug in math alignments (Robert Hunt)

Barbara,

  I received the following bug report from Robert Hunt

> Accepted:  12:02:31 21 Nov 91
> Submitted: 23:36:12 19 Nov 91
> IPMessageId: A4D2794E8E47F040
> From: Robert Hunt <REH10>
> To: cet1
> Subject: Bug (sigh) in TeX
>
> Here's a bug in TeX: or possibly in the TeXbook, though I'd much
> prefer it to be considered the former! The input
>
> $$\halign{#\cr x\cr}\global\setbox1=\hbox{y}$$
>
> generates a "Missing $$ inserted" error just before the "y" token.
> (Pressing RETURN then generates an "I can't go on meeting you like
> this" error, and TeX bombs out.) This appears to be because TeX
> objects to having horizontal mode material to process after an
> alignment in displayed maths mode. However, page 291 of the TeXbook
> clearly states that optional <assignments> may follow the closing
> brace of an \halign in display maths mode, and \setbox certainly
> counts as an <assignment>!

It is definitely a bug in TeX: here's a log file (duplicated on Unix
TeX, by the way)

 --------------------start log file
This is TeX, Phoenix Version 3.14:0.75 (preloaded format=plain 90.7.18)
  21 NOV 1991 14:17
**sysin
(&TEXR
! Missing $$ inserted.
<to be read again>
                   y
l.1 $$\halign{#\cr x\cr}\global\setbox1=\hbox{y
                                               }$$
? h
Displays can use special alignments (like \eqalignno)
only if nothing but the alignment itself is between $$'s.

?
! I can't go on meeting you like this.
<to be read again>
                   y
l.1 $$\halign{#\cr x\cr}\global\setbox1=\hbox{y
                                               }$$
One of your faux pas seems to have wounded me deeply...
 [ dek:                            ^^^^^^^^^^^^^^^^^      how true ]
in fact, I'm barely conscious. Please fix it and try again.

No pages of output.
 --------------------end log file

The problem is that the state of the world when |do_assignments| in
module 1206 returns can be quite different from when it is called,
in respect of the mode nest and the save stack in particular. It
occurred to me that one should be able to create a simlar effect
with the call in |make_accent| (module 1123), and indeed, one can:

 --------------------start log file
This is TeX, Phoenix Version 3.14:0.75 (preloaded format=plain 90.7.18)
  21 NOV 1991 15:05
**sysin
(&TEXS
! This can't happen (vpack).
<recently read> }

l.1 \accent"7F\global\setbox1=\vbox{\vskip1in}
                                              z
I'm broken. Please show this to someone who can fix can fix

No pages of output.
 --------------------end log file

 [ dek:                                         $327.68 made out to Hunt ]

Can you add this to Don's input queue, please?

 [ dek:
        As Chris undoubtedly knows, this is not at all easy to fix.
        I think it would be far too risky trying to allow arbitrary
        boxes to be assigned after \halign in display or between \accent
        and the accentee ...  making _fin_align_ and _accent_ recursive
        would open up a huge can of worms and would make \afterassignment
        even murkier than it is now.  Therefore I am modifying the
        TeXbook to disallow such things [which nobody has ever used
        because they never have worked] and modifying TeX to detect
        this prohibited activity
 ]

Chris Thompson
Cambridge University Computing Service
JANET:    cet1@uk.ac.cam.phx
Internet: cet1@phx.cam.ac.uk

<<< end \halign + box assignment within displays

************************************************************************

>>> positive/negative xn_over_d, tex.web section 107

Date: Thu, 21 Nov 91 18:38:51 GMT
From: CET1@phoenix.cambridge.ac.uk
Cc: Wayne Sullivan <WSULLIVAN@IRLEARN.UCD.ie>
Subject: Wayne Sullivan's new TeX bug

Barbara,

   Here is the promised bug report from Wayne Sullivan. I don't think
I have anything useful to add to the messages below. I can produce
backtraces very like Wayne's. The calls can come from |math_kern| as
well as from |math_glue|.

> Accepted:  16:47:07 15 Nov 91
> Submitted: 14:42:13 15 Nov 91
> From: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> To: Chris Thompson <CET1@UK.AC.CAMBRIDGE.PHOENIX>
> Subject: possible bug in TeX

In TeX WEB section 107, Knuth claims that for use of the procedure xn_over_d,
n and d are always nonnegative. Whether the actual code makes use of this
in section 107, I am not sure. However, some implementations utilize the
assumed positivity in optimizing this section. In sections 716 and 717
xn_over_d is used in context with cur_mu. Though one would expect cur_mu
to be nonnegative, I see no place where this is assured: I think one could
simply change the corresponding font parameter to a negative value.

                                                    Wayne Sullivan

 [ dek:
        $327.68
        \S716-7
 ]

> Submitted: 17:18:39 15 Nov 91
> IPMessageId: A4CD1EF77B402240
> From: Chris Thompson <CET1>
> To: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> Subject: Re: [possible bug in TeX]

I think you are right. |cur_mu| is set in section 703 and there is
no reason why it has to be positive. Hence |f| in both sections 716
and 717 can be negative (as |x_over_n| gives a negative remainder
when x and n have opposite signs). |xn_over_d| is *not* robust against
negative n, quite apart from the promises in sections 107 and 99,
because it causes u to be negative, and so (a) you can't assume
u=d*(u div d)+(u mod d) in ANSI Pascal any longer, & (b) the test
for (u div d) being too large would need to be supplemented by one
for it being too small.

I would like to construct a hard example of obvious bad results
before passing this to Barbara & Don, though.

Chris Thompson

> Accepted:  14:57:26 18 Nov 91
> Submitted: 09:55:30 18 Nov 91
> From: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> To: Chris Thompson <CET1@UK.AC.CAMBRIDGE.PHOENIX>
> Subject: xn_over_d

I grabbed the pascal code for xn_over_d out of my change file to compare
the result with an optimized version. On tangling I discovered that the
intermediate values t,u,v are declared as nonnegative_integer. Hence a
range checking compiler will choke on a negative cur_mu: Can DEK escape
from that?

> Accepted:  15:38:21 18 Nov 91
> Submitted: 09:32:41 18 Nov 91
> From: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> To: Chris Thompson <CET1@UK.AC.CAMBRIDGE.PHOENIX>
> Subject: xn_over_d

The use of xn_over_d with the possibly negative cur_mu only affects the
the lower precision term in a `split' precision calculation so that
the problem of overflow arises only if the input values are beyond the
allowed values, i.e., with valid input values no overflow arises. Thus
the algorithm will give the correct answer even though cur_mu is negative.
However, this does depend on the div procedure satisfying negation symmetry:
(-n) div d =-(n div d) when n,d>0. At any rate the documentation needs to
be corrected so that optimized versions of the code will also give the
correct result.

> Accepted:  17:40:28 18 Nov 91
> Submitted: 11:05:39 18 Nov 91
> From: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> To: Chris Thompson <CET1@UK.AC.CAMBRIDGE.PHOENIX>
> Subject: xnod

Here is an example of the xn_over_d problem. Sorry about the antique version
of TeX currently on our VM/CMS: I use it only to illustrate the problem.
      The TeX file:
\showboxdepth100
\showboxbreadth100
\scrollmode
\showthe\fontdimen6 \tensy
\fontdimen6 \tensy=-10pt
\showthe\fontdimen6 \tensy
$$a\;a$$
\showlists
\end

 The LOG file (errors were reported on the terminal). Look for negative glue.

This is TeX, VM/CMS Version 2.1 (preloaded format=PLAIN 87.5.19)  18 NOV 1991 11
**xnod
(xnod.tex.*
> 10.00002pt.
l.4 \showthe\fontdimen6 \tensy


> -10.0pt.
l.6 \showthe\fontdimen6 \tensy



### horizontal mode entered at line 7
spacefactor 1000
### vertical mode entered at line 0
### current page:
\glue(\topskip) 10.0
\hbox(0.0+0.0)x469.75499, glue set 449.75488fil
.\hbox(0.0+0.0)x20.0
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\penalty 10000
\glue(\abovedisplayshortskip) 0.0 plus 3.0
\glue(\baselineskip) 7.69446
\hbox(4.30554+0.0)x7.79407, shifted 230.98047
.\teni a
.\glue -2.77771 plus -2.77771
.\teni a
\penalty 0
\glue(\belowdisplayshortskip) 7.0 plus 3.0 minus 4.0
total height 29.0 plus 6.0 minus 4.0
 goal height 643.20255
prevdepth 0.0, prevgraf 4 lines

! OK.
l.8 \showlists


[1]
Output written on xnod.dvi (1 page, 264 bytes).

> Submitted: 17:48:09 18 Nov 91
> IPMessageId: A4D0EAFE6C983690
> From: Chris Thompson <CET1>
> To: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> Subject: Re: [xnod]

Wayne,

  Thanks for the messages. There is no doubt that there is a bug: the
declaration of t,u,v as |nonnegative_integer|, which I had missed, makes
it definite. I have traced through |xn_over_d| using a debugger, and
found it storing negative values, as expected.

  I'll put all this together and send it off to Barbara Beeton, if you
are agreeable.

Chris Thompson

> Accepted:  12:02:21 21 Nov 91
> Submitted: 09:54:31 19 Nov 91
> From: "Wayne G. Sullivan" <WSULIVAN@IE.UCD.IRLEARN>
> To: Chris Thompson <CET1@UK.AC.CAMBRIDGE.PHOENIX>
> Subject: xn_over_d

Thanks in advance for passing the problem on to BNB/DEK. Enclosed below
are screen messages which appear when xnod.tex is processed on our ancient
VM/CMS version of TeX. Other things happen with the PC version I use, but
that uses an assembly language equivalent of the old xn_over_d which will
have to be upgraded to whatever Knuth decides to the procedure. It is unlikely
that he will employ the obvious modification.
 [ dek:                 ^^^^^^^^^^^^^^^^^^^^ which is? ]

Ready; T=0.05/0.11 09:49:43
tex xnod
This is TeX, VM/CMS Version 2.1 (no format preloaded)
**
(xnod.tex.*
> 10.00002pt.
l.4 \showthe\fontdimen6 \tensy

> -10.0pt.
l.6 \showthe\fontdimen6 \tensy


 AMPX031E Low bound checking error
      TRACE BACK OF CALLED ROUTINES
 ROUTINE           STMT AT ADDRESS IN MODULE
 XNOVERD             6     024014     TEX
 MATHGLUE            4     03E1B8     TEX
 MLISTTOHLIST       73     041F54     TEX
 AFTERMATH          68     052EBE     TEX
 MAINCONTROL       128     058098     TEX
 <MAIN-PROGRAM>    110     05B86C     TEX
 PASCAL/VS                 05BDAE


 AMPX031E Low bound checking error
      TRACE BACK OF CALLED ROUTINES
 ROUTINE           STMT AT ADDRESS IN MODULE
 XNOVERD            10     024074     TEX
 MATHGLUE            4     03E1B8     TEX
 MLISTTOHLIST       73     041F54     TEX
 AFTERMATH          68     052EBE     TEX
 MAINCONTROL       128     058098     TEX
 <MAIN-PROGRAM>    110     05B86C     TEX
 PASCAL/VS                 05BDAE


 AMPX031E Low bound checking error
      TRACE BACK OF CALLED ROUTINES
 ROUTINE           STMT AT ADDRESS IN MODULE
 XNOVERD             6     024014     TEX
 MATHGLUE            7     03E236     TEX
 MLISTTOHLIST       73     041F54     TEX
 AFTERMATH          68     052EBE     TEX
 MAINCONTROL       128     058098     TEX
 <MAIN-PROGRAM>    110     05B86C     TEX
 PASCAL/VS                 05BDAE


 AMPX031E Low bound checking error
      TRACE BACK OF CALLED ROUTINES
 ROUTINE           STMT AT ADDRESS IN MODULE
 XNOVERD            10     024074     TEX
 MATHGLUE            7     03E236     TEX
 MLISTTOHLIST       73     041F54     TEX
 AFTERMATH          68     052EBE     TEX
 MAINCONTROL       128     058098     TEX
 <MAIN-PROGRAM>    110     05B86C     TEX
 PASCAL/VS                 05BDAE

! OK (see the transcript file).
l.8 \showlists

[1]
(see the transcript file for additional information)
Output written on xnod.dvi (1 page, 264 bytes).
Transcript written on xnod.texlog.

 ----- End of Included messages -----

Chris Thompson
JANET:    cet1@uk.ac.cam.phx
Internet: cet1@phx.cam.ac.uk

<<< end positive/negative xn_over_d, tex.web section 107


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