c - Hayes AT Commands: Detect Remote Hangup?

Question

How are you supposed to programatically detect when the remote modem on your call hangs up? I am writing a C program which interfaces with a SoftModem device /dev/ttySL0 in Ubuntu linux. I am able to configure the modem using Hayes AT commands and communicate with the remote modem. However, I haven't been able to determine how I'm supposed to detect that the other end has hung up the line.

I have the modem configured so that when the other end hangs up, the device prints NO CARRIER and switches to command mode. However, I can't use the NO CARRIER string because I can't guarantee that the modem won't receive that string while in data mode.

How do you "listen" for remote hang up?

Answer 1

This is a hardware signal on modems, the Carrier Detect (CD) line. You'll need to monitor it to know that the connection was lost. Basics in linux are described in this how-to, you obtain the signal state with ioctl() using the TIOCM_CAR command.