Making a serial port notify on change of handshaking lines

Question

Is it possible to open a serial device (such as /dev/ttyS0) and be informed via select/poll/etc... or a signal when the handshaking lines (such as CTS/RTS or DSR/DTR) change? I know at the hardware level there's an interrupt from the UART to tell the kernel it has changed, but can I be informed of that up in userland?

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Edit: I am aware of TIOCMIWAIT, but that ioctl call blocks until the status lines change. I would like instead to keep processing generally and have a poll or similar be informed on change, as well as other events.

Answer 1

There's no way to wait for the DTR/RTS lines to change in userland. The only way to do this on Linux is to constantly poll the device, checking to see if the status of the RTS/DTR lines have changed. I generally steal my serial port code from gtkerm, and it polls.

You can try using TIOCMIWAIT, but if I remember correctly that's going to be very tied to the driver for the serial port that Linux is using, and so may not work from driver to driver.