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In winsup/cygwin/fhandler/serial.cc, the function fhandler_serial::switch_modem_lines() is called when TIOCMBIS/TIOCMBIC are used in an ioctl() call. This function uses EscapeCommFunction() for setting and resetting RTS and DTR signals of a serial port. Unfortunately, this function does not work on USB CDC devices. This is not a true bug of CYGWIN but an issue of the usbser.sys driver, from Windows 2000 to the latest Windows 11. Both 32bit and 64bit versions of the operating system are affected. Actually, I tested EscapeCommFunction() also when using a real UART, based on the traditional 16550 driver and it works fine. Using thirdy party CDC drivers, like the one provided by FTDI for their USB bridge chips, probably also works. However, it is also possible to drive the RTS/DTR signals by writing their state with SetCommState(), which proved to be working fine all types of connection. This is also a better solution for handling these signals since RTS/DTR can be set at the same time rather than having two separate calls with a visible delay between them.
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.