From: bsl04@... (Brian)
Date: 2002-11-09T14:17:08+09:00
Subject: ioctl to detect CTS

I'm trying to convert some working C that watches a serial port for
CTS to go high, at which point it does some stuff.  It watches for CTS
by doing

....
ioctl(fd, TIOCMGET, &s);
if ((s & TIOCM_CTS) == TIOCM_CTS) {
....

I tried to reimplement this in ruby with stuff like

f = File.open '/dev/modem'
p f.ioctl(0x5415)

but it results in errors.
irb(main):006:0> f.ioctl(0x5415)
Errno::EFAULT: Bad address - "/dev/modem"
        from (irb):6:in `ioctl'
        from (irb):6

So I wrote something to brute out the ioctls that didn't raise an
exception, and tried them all, but I never seem to get anything sane. 
Please give me a clue what's wrong.