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Asked: 2026-06-03T19:56:02+00:00

How are you supposed to programatically detect when the remote modem on your call

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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?

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1 Answer

  1. Editorial Team

    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.

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