[tex-k] Potential bug in MF: missing output in transcript file & long diagnostics on terminal

Dominik Leininger dominik.leininger at t-online.de
Thu Aug 17 23:39:39 CEST 2023

Hello everyone,

I've come across the following strange behavior of MF, which doesn't 
seem to be described in The MFbook, and therefore may not be intended 

Running MF with `\ show tracingonline; show origin..right; end' (either 
on the command line after `mf' or immediately after `**') will give the 
following output:

     >> 0
     >> Path at line 0:
     (0,0)..controls (0.33333,0) and (0.66667,0)

     Transcript written on mfput.log.

According to `>> 0', the path should not be displayed on the terminal, 
but it does. The second thing I noticed is that you should see the path 
in the transcript file, but mfput.log only contains the following line 
after the MF's welcoming:

     **\ show tracingonline; show origin..right; end

If you just type \ after **, hit enter and then type the show command 
after the single `*', everything works as expected.

Note that the transcript file also lacks the output of message commands 
when used immediately after `**':


     **\ message "A";
     *message "B";


     **\ message "A";

     *message "B";

I also checked picture variables (`\ show unitpixel;') and pen variables 
(`\ show pensquare;') for tracingonline:=0 (which is default).
I also checked for tracingonline:=-1.
I tested on Windows 10 (METAFONT, Version 2.71828182 (TeX Live 2023)) 
and Ubuntu 22.04 (also METAFONT, Version 2.71828182 (TeX Live 2023)).

What am I missing here? Is this behavior intended? If so, I cannot find 
a description of it in The MFbook. The behavior feels strange, 
especially that the transcript file does not include the output of 
show/message commands when used immediately after `**'. My understanding 
is that everything after `**\' should behave the same as after `*'.

Summary of unexpected behaviors:
- output of message and show commands immediately after `**' (or after 
`mf \' in the command line) don't get logged in the transcript file.
- paths, pens and pictures are shown on the terminal even though 
tracingonline<=0 when the show command is used immediately after `**' 
(or after `mf \' in the command line)

Has anyone experienced this behavior or can anyone reproduce it?

Thanks and best regards,


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