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bug#49524: 28.0.50; make-serial-process is not portable

From: Ken Brown
Subject: bug#49524: 28.0.50; make-serial-process is not portable
Date: Sun, 11 Jul 2021 11:24:58 -0400
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

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,


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.

By the way, I came across this issue while investigating the failure of process-tests/fd-setsize-no-crash/make-serial-process on Cygwin.

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