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.

Answer 2

Testing for NO CARRIER as text will not suffice. This text frequently occurs on sites in the net, even on Q&A sites.

Coming from the modem, it should be enclosed in line breaks.

Besides, after you detect that text, you can try to switch to command mode with +++. If that works, your connection persists and you can reattach it and continue using it. If it doesn't (because you are already there and +++ is an invalid command), the connection has gone.