I am trying to write a C or C++ program to control a telnet session. For the purpose of this question we could simplify the issue by assuming that I am relaying questions and answers to a user at the console. That's a simplification, but close enough.

I started out by opening a socket to the telnet port 23. I soon discovered that the handshake is complicated and there is no good documentation on what I have to do to satisfy the thing. Please don't tell me to read the telnet RFC because I did and it is the furthest thing imaginable from instructions on the negotiation.

Next I checked into the possibility that cURL could handle it, but there is nothing approaching actual instructions on how to write such a program and when I type cURL telnet://xxx.xxx.xxx.xxx to see if at least I can do it interactively, it fails to relay things to and from the screen properly and freezes a lot.

Next, I checked into using popen, but I soon became convinced that I would need two pipes and simply don't know how to do it. I can control programs I write with a pair of pipes, but I am trying to control the telnet app that comes with Linux and this would seem to involve stdin and stdout. There is undoubtedly a way to do it by taking over stdin and stdout, but I sure don't know how.

Next up was IPWorks by nsoftware, and they even have a Telnet class, but it is utterly undocumented except for a class reference, which would be very hard to write an app from.

I would be grateful for any help I could get with this. I have been writing code and Googling for a day. Thanks in advance.

Brandon

Expect ?

Doesn't Expect have some requirements for the users' computers, like Tcl or something? If so, I can't use it because this has to work on many user configurations.

Take a look at the PuTTY sources. It is distributed under the MIT license. (Like the page says, you can use the code however you like, even in proprietary software, as long as you show the copyright notice and the license text in the combined/derivative product.)
At a quick glance, it seems the telnet stuff is pretty well contained in putty-version/telnet.c file, although it is not exactly in library form.

Actually, I finally got the IPWorks Telnet class to work.

Also, I understand that I can run interactive programs from C in general by forking a child, connecting pipes to it, and then using one of the exec commands. That looks like a useful thing to know how to do, although I guess I have what I need in this case. I have seen some of the Putty code and the negotiation with the Telnet server is really ugly. As for simply calling Putty, I cannot ask users to have it on every box running this code. I appreciate the input, though.