Bugs fixed in the 2021 TeX tune-up

Don Knuth's report on the TeX tuneup of 2021 was published in TUGboat. More references and info.

This page is a summary of the bugs in TeX fixed in the 2021 tune-up, with a code example that reproduces the bug (or doesn't in the fixed version). The list is sorted by the order in errorlog.tex, plus two entries at the end that are not in that file.

Thanks to Phelype Oleinik for putting this together. Knuth is not responsible for this page. Please send corrections to webmaster@tug.org.

I948. Don't pause on errors when tracing paragraphs (Udo Wermuth). @826

Summary: While writing paragraph tracing info, TeX would be writing to term_only, and if an error occurred there, the user wouldn't see and TeX would be apparently stuck (replying to the error prompt would work normally).

Reproducing: Run TeX on a file (not from the command line; TeX needs an open .log file for this) that contains the line:

    \tracingparagraphs=1 Press\hss return.\end

S949. Don't try to interact when in \batchmode (Xiaosa Zhang). @83

Original report. (By the way, if Xiaosa or anyone who knows him is reading this, please contact Don Knuth; he has checks for you.)

Summary: A certain combination of replies to TeX's prompt would make TeX try to interact while in \batchmode, thus trying to write to a closed IO stream, resulting in a segfault.

Reproducing: Create a file, say, invalid.in with:

    \catcode`\^=7 \catcode`\^^?=15 \s^^?E

then run tex -ini <invalid.in. Or run tex -ini and type each line followed by RET.

S950. Don't try to edit when no file is active (Xiaosa Zhang). @84

Original report.

Summary: If you caused an error, then replied to TeX's prompt by inserting yet another error, and then asked TeX to open the editor with E, TeX would segfault trying to get the file name from the inserted text.

Reproducing: Create a file with an error, say:


then run TeX on that file, and at the error prompt reply:

    I\AGAIN !

and this time reply to the prompt:


and TeX will segfault.

R951. Take date and time sometimes from system, not user (Udo Wermuth). @241,536

Summary: Before setting \jobname (more precisely before starting the .log file) you could change the value of \year, \month, \day and \time, and that would be written in the header line of the .log. \month was particularly bad because it takes three characters from TeX's memory, so you could access arbitrary points of TeX's memory, or with extreme enough values, cause a segfault.

Reproducing: For example, run (not from a file):

    tex '\month=-100000 \end'

If you choose smaller values for \month, or change the other values, the .log will contain a bogus date.

B952. Don't allow implicit left brace after |#| (Udo Wermuth). @476

Summary: TeX would allow an implicit left brace in place of an explicit one in the #{ argument.


    \catcode`\{=1 \catcode`\}=2 \catcode`\#=6

That shows a command delimited by \bgroup, which inserts another \bgroup at the end of the replacement text.

R953. After nine parameters, must delete offending tokens (Bruno Le Floch). @476

Summary: While scanning the parameter text of a definition with 9 parameters already, TeX would leave any token after a # to be part of the parameter text.


    \catcode`\{=1 \catcode`\}=2 \catcode`\#=6 \catcode`\%=14
    \nonstopmode % ignore errors while defining
    \foo........} }#

That shows a command whose ninth parameter is delimited by }#, and then shows that an unbalanced } was grabbed as argument; use \tracingmacros=1 to see the } being grabbed.

D954. Garbage visible in buffer after file ends prematurely (DRF). @486

Summary: After the File ended within \read error message, TeX could be holding garbage tokens in buffer, which could appear in the error context.

Reproducing: Create a file unbal.tex with a single line containing:


then run this:

    \catcode`{=1 \catcode`}=2 \catcode`#=6
    \openin1 unbal
    \def\A#1#2#3#4#5#6#7#8#9{\read1to \x}
    \def\B#1#2#3#4#5#6#7#8#9{\A#1#2#3#4#5#6#7#8#9 \relax}
    \def\C#1#2#3#4#5#6#7#8#9{\B#1#2#3#4#5#6#7#8#9 \relax}
    \def\D#1#2#3#4#5#6#7#8#9{\C#1#2#3#4#5#6#7#8#9 \relax}
    \def\E#1#2#3#4#5#6#7#8#9{\D#1#2#3#4#5#6#7#8#9 \relax}

and the error would start with:

    Runaway definition?
    ! File ended within \read.
    <read 1> {^^M#5#6#7#8#9{\D

The <read 1> context shown after the { is junk.

R955. Force nonexistent characters to have null specs (DRF). @722

Summary: In a TFM file, a non-existent character is marked by its width index being zero, and TeX assumes that if that is true, all other metrics of said character are zero as well, but nothing was enforced.

If that weren't the case, though, when reading a character from a font, TeX would only look at its width, and assume everything else is zero, without enforcing. But if a TFM was made so that the width was zero, but for example the italic correction were not, that index would not be zeroed and the wrong italic correction would be used.

To reproduce this you need a bad TFM file with a character's width zero and other indexes non-zero (which I failed to produce).

C956. Don't mark fraction noads as temporarily Inner (DRF). @761

Summary: A fraction (like {1\over2}) was written to produce an Inner math atom, but the fact that fractions are almost always enclosed in braces to delimit their scope makes the fraction an Ord atom. The only two cases where a fraction would remain an Inner atom were:

  1. If the equation contains only the fraction: $1\over2$
  2. If the fraction is surrounded by delimiters: \left 1\over2 \right
but in the first case there is no spacing applied, so the noad type doesn't matter, and in the second case the subformula would be an Inner atom anyway because of \left...\right, so there was no use in making a fraction an Inner atom.

This was largely a change in The TeXbook, correcting the statements that said that fractions were Inner math atoms, but TeX was changed to not assign t:=inner_noad.

Q957. Reset \newlinechar before logging the stats (Udo Wermuth). @1333,1335

Summary: The user's choice of \newlinechar would remain when TeX was printing the final statistics, so characters that matched the value of \newlinechar would cause a newline to be printed instead.

Reproducing: For example, run:

    tex '\newlinechar=32 \end'

Missing (\tabskip) indication in Underfull message; not in errorlog.tex (Igor Liferenko)

Original report.

Summary: TeX would omit a (\tabskip) glue indication when showing an underfull alignment.

Reproducing: Run this:

    \catcode`\{=1 \catcode`\}=2 \catcode`\&=4 \catcode`\#=6
    \showboxdepth=1 \tracingonline=1
    \tabskip=0pt plus10pt \halign to200pt{&#\hfil\cr
      \hbox to50pt{}&\hbox to60pt{}\cr}

and you will see the last line of the box typeout says

    .\glue 0.0 plus 10.0

whereas it should say

    .\glue(\tabskip) 0.0 plus 10.0

Internal variable overflow in \hyphenation; not in errorlog.tex (DRF)

Summary: The code tries to store a value that's not in the declared range of the receiving variable. In particular, in the hyphenate function, when <Look for the word |hc[1..hn]|...> (page B392, module 930) is called, hn can already be 63, but then this module increments it to 64 for a while (to fit the cur_lang byte), which puts hn out of range for a small_number.

Reproducing: This isn't discoverable by running everyday TeX. DRF found this with his modified TeX with memory address checking. The code to trigger the bug is:


For any discussion about these issues, or further reports to be listed here, please use the contact information on the main TeX bugs page here.

$Date: 2021/07/21 21:30:29 $;
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