The purpose of this repository is to allow broadcast communication from the GPS serial port to different applications simultaneously.
When the GPS is connected it's seen as /dev/ttyACM0, and only allows for one connection at a time.
ttybus it's a tool from danielinux that allows sharing of tty devices between different processes, and therefore satisfies our needs. The explanation of all of its functions is left to the repository's documentation, this readme will show how it's implemented and used in our context.
Once the car and it's software components boot up, fakebus.service has to be launched to create a fake bus, after that attachbus.service can run to attach the bus to the GPS port /dev/ttyACM0.
This has to be done only once, the bus will be created in /tmp and will expire only when the tty_bus process is killed.
In the same way the bus will remain attached to the bus as long as the tty_attach process is running.
These two services are placed in /etc/systemd/system to automatically start at start up.
The bus is now broadcasting the signal coming from /dev/ttyACM0 and it's ready to accept connections from fake devices created from now on.
Every time a new application needs to read GPS data, a new fake device need to be created and attached to the existing bus, to do that, use the tty_fake C program in the ttybus repository, indicating the bus to attach to, and the position of the new device:
./tty_fake -s /tmp/fakebus /dev/fakedevice1
this command will output the pseudo-terminal just created to use as interface in the application, for example /dev/pts/#.
As for the fake bus, this new interface, let's say/dev/pts/4, will expire when the tty_fake process is killed.
Detaching the application from the interface won't cause its closing.