===================================================================
RCS file: /cvs/mandoc/TODO,v
retrieving revision 1.302
retrieving revision 1.304
diff -u -p -r1.302 -r1.304
--- mandoc/TODO	2020/04/26 21:41:07	1.302
+++ mandoc/TODO	2020/06/24 12:09:29	1.304
@@ -1,6 +1,6 @@
 ************************************************************************
 * Official mandoc TODO.
-* $Id: TODO,v 1.302 2020/04/26 21:41:07 schwarze Exp $
+* $Id: TODO,v 1.304 2020/06/24 12:09:29 schwarze Exp $
 ************************************************************************
 
 Many issues are annotated for difficulty as follows:
@@ -82,6 +82,24 @@ are mere guesses, and some may be wrong.
   Found in groffer(1) version 1.19
   Jan Stary 20 Apr 2019 20:16:54 +0200
   loc *  exist ***  algo ***  size **  imp *
+
+- mandoc replaces all ASCII control characters except tab and line feed
+  with '?' during input.  It would be better to replace them with
+  Unicode escapes in preconv_encode() or somewhere in the vicinity,
+  such that the already existing better replacement strings show
+  up in the output.  Emulating groff is not desirable: groff replaces
+  0x00, 0x0b, and 0x0d to 0x1f with the empty string (bad because
+  that's easy to overlook for the document author), 0x01 with '.'
+  (very confusing), and passes through 0x02 to 0x08, 0x0c, and 0x7f
+  raw (bad because that is insecure output).  Remember that 0x07 may
+  need special handling because it is sometimes used for certain
+  delimiters, so it may need handling *after* roff.c rather than before.
+  reminded by John Gardner 16 Jun 2020 14:26:28 +1000
+  Actually, more ASCII control characters than just 0x07 may need
+  later handling because they can for example be used in macro names.
+  So they may need handling after roff(7) processing.
+  pointed out by John Gardner 23 Jun 2020 18:28:08 +1000
+  loc **  exist **  algo **  size **  imp *
 
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