From: Ken Brown <kbrown@cornell.edu>
Date: Sun, 11 Jul 2021 11:24:58 -0400
Fmake_serial_process calls Fserial_process_configure, which calls
serial_configure, which calls cfsetspeed with the speed argument equal to the
numerical baud rate (e.g., 9600). But the documentation of cfsetspeed says that
the speed argument must be one of the Bnnn constants defined in termios.h (e.g.,
B9600). See, for example,
https://man7.org/linux/man-pages/man3/termios.3.html
This incorrect call of cfsetspeed happens to succeed on GNU/Linux because
glibc's cfsetspeed allows the argument to be the numerical baud rate, which it
converts to the appropriate Bnnn constant. But I don't think emacs should be
relying on this undocumented behavior. In particular, this doesn't work on
Cygwin. And it wouldn't even work on GNU/Linux if emacs used the cfsetspeed
replacement defined in sysdep.c instead of glibc's cfsetspeed.
I think the way to fix this is to imitate the glibc code that converts the baud
rate to a Bnnn constant, but maybe someone has a better idea.