I worked around it. I put in some code that actually should have been there but never mattered. Now it fixed the problem. That still doesn't explain what happened to Terminal or some of the low level C-commands in OS 10.15. Thanks for your comments.

openssl works fine; just hitting return once works. Evidently something changed in one of the utilities I use. As I wrote above: fgetc, ungetc, getc, foef. This is old code that still works fine in Linux.

Only in terminal. But that's very important! In the procedure that handles terminal I/O I use fgetc, ungetc, getc, foef. Maybe something changed in their implementation?