FileOutputStream: write to serial port

Question

I'm trying to write single bytes to a serial port in Vala using a FileOutputStream:

var dev = File.new_for_path("/dev/ttyACM0");
var dev_io = dev.open_readwrite();
var dev_o = dev_io.output_stream as FileOutputStream;
dev_o.write({0x13});
dev_o.flush();

My aim is to do this similar to echo -en '\x13' > /dev/ttyACM0 but it just behaves weirdly. The Byte 0x13 seems to be written multiple times, sometimes /dev/ttyACM0 is blocked for a few seconds, sometimes it's even blocked after the Vala program exited and sometimes it's not blocked at all. If i write my FileOutputStream to a file and send this to the serial port via cat byte_file > /dev/ttyACM0 everything is fine.

It seems to me that GIO struggles with the fact that the file is a device. My problem is that I need GIO to monitor /dev/ttyACM0 if it's plugged in and for asynchronous reading.

Answer 1

The problem is most likely that you have to configure the serial port to set things like baud rate, flow control, and parity. If you don't get all those options right there is a good chance that you'll end up with garbage data like you describe.

Basically, you first need an integer descriptor for the file; the easiest way to get one is probably to just open the file using Posix.open, but you can also use GLib.FileStream.fileno to get the integer descriptor of a GLib.FileStream, etc. Next, use Posix.cfmakeraw and Posix.cfsetspeed to configure it. Then, to get your nice GIO streams, just pass the integer descriptor to the default GLib.UnixInputStream/GLib.UnixOutputStream constructors.

I wrote a class to handle serial communication in Vala many years ago. As an example it is a bit horrible—it's convoluted (I had plans to use it as an abstraction layer), doesn't use GIO or async (Vala didn't have the async keyword), uses char[] instead of uint8[] (we hadn't yet standardized on uint8[]), etc., but it should help you understand what you need to do. Between that example and what I wrote above, you should be able to get it working, but if you are still having trouble after you've played with it let me know and I can throw together a quick example.