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/ttyS

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.