I am working on a robot automation project and I have run into a road block. To control the robot, one needs to connect with it wirelessly via telnet and send commands through the tcp/ip protocol. (ex. The 'Mabc' command moves it forward based on the left wheel speed (a), the right wheel speed (b) and time (c)). What I am trying to do is do some calculations in a C program and then send a message to the robot based on the value of the calculation.
How can send commands via tcp/ip protocol in a C program?
Thanks!
Erik
You are looking for sockets. This is a comprehensive guide to socket programming in C. Telnet is also a well defined protocol, although I don't know if this robot would use telnet or not (it's extra processing overhead for a protocol that wouldn't have much added benefit for a robot control program). Telnet is covered in detail by RFC 854
Expect would allow you to interact with external programs, but I am not aware of a C port of expect. Otherwise you would find a telnet library in C or write your own using socket programming.
I would use libcurl: http://curl.haxx.se/libcurl/. It'll do what you want, and handle all the telnet goo that you really don't want to handle.
Expect was designed to do exactly this - hold conversations with interactive programs. It's written in Tcl, extending the Tcl interpreter with various commands. Tcl is very easy to extend; it was designed to be an embedded scripting language right from the word go. The main C API uses argv-style constructs to pass parameters to Tcl commands and is very easy to use. The best guide to the C API is Ousterhout's original book and it took me one two-hour lab session to get my first embedded Tcl interpreter up and running.
As a bonus you also get a built-in Tcl interpereter, which you can use to add a scripting capability to your application. You'll probably find that quite a bit of it can be implemented in Tcl if you feel so inclined, so it will probably save you time overall.
I would be:
writing some simple shell scripts containing the telnet interractions written as here documents.
using a .telnetrc file in your home directory to control aspects of your telnet session, e.g. crmod
calling the script using system calls.
This way your turnaround time to change your interractions with the robot won't involve having to recompile your programm all the time.
BTW This sounds like fun.
HTH.
cheers,
Rob
Related
I am thinking of creating a C program that will use picocom to read and write serial streams to ports (GPS module, NTPD). The c program will run on a Debian OS.
Is it possible to do this using Picocom? Is it bad design to interact with Picocom through c and system commands?
While that's technically possible, it's a bad idea; picocom is a somewhat uncommon tool, and it's primarily designed to be used interactively by a user. Trying to use it from within another application will be pretty weird.
It'll be much easier to interact with the serial port directly from your application. There's a nice introduction to doing that in the answer to How to open, read, and write from serial port in C.
This question might sound fool, because I know there are bunch of frameworks that does it for you. What I want is actually get in touch with low level C API deeply and able to write a program that sits on computer and intercepts packets between local machine and outer spaces. I tried to figure it out by looking at open source code (i.e. tcpdump) but it's quite difficult for me to find out which file actually performs network sniffing. Any suggestions would be appreciated !
You have to use raw socket. Here's an example.
At least for what concern Linux and Unix like operating systems. I don't know about Windows.
If you're using a UNIX based system[*] then the simplest mechanism is libpcap, which is part of the tcpdump project.
Your process will need root privileges to be able to access the network interface (as would also be the case with raw sockets).
Usually you'll end up having to decode ethernet frames, IP headers, etc yourself, although for most protocols this isn't that hard.
[*] It is actually available for Win32 as well, but I've not used it under Windows myself.
I am developing some experimental setup in C.
I am exploring a scenario as follows and I need help to understand it.
I have a system A which has a lot of Applications using cryptographic algorithms.
But these crypto calls(openssl calls) should be sent to another System B which takes care of cryptography.
Therefore, I have to send any calls to cryptographic (openssl) engines via socket to a remote system(B) which has openssl support.
My plan is to have a small socket prog on System A which forwards these calls to system B.
What I'm still unclear at this moment is how I handle the received commands at System B.
Do I actually get these commands and translate them into corresponding calls to openssl locally in my system? This means I have to program whatever is done on System A right?
Or is there a way to tunnel/send these raw lines of code to the openssl libs directly and just received the result and then resend to System A
How do you think I should go about the problem?
PS: Oh by the way, the calls to cryptography(like EngineUpdate, VerifyFinal etc or Digest on System A can be either on Java or C.. I already wrote a Java/C program to send these commands to System B via sockets...
The problem is only on System B and how I have to handle..
You could use sockets on B, but that means you need to define a protocol for that. Or you use RPC (remote procedure calls).
Examples for socket programming can be found here.
RPC is explained here.
The easiest (not to say "the easy", but still) way I can imagine would be to:
Write wrapper (proxy) versions of the libraries you want to make remote.
Write a server program that listens to calls, performs them using the real local libraries, and sends the result back.
Preload the proxy library before running any application where you want to do this.
Of course, there are many many problems with this approach:
It's not exactly trivial to define a serializing protocol for generic C function calls.
It's not exactly trivial to write the server, either.
Applications will slow a lot, since the proxy call needs to be synchronous.
What about security of the data on the network?
UPDATE:
As requested in a comment, I'll try to expand a bit. By "wrapper" I mean a new library, that has the same API as another one, but does not in fact contain the same code. Instead, the wrapper library will contain code to serialize the arguments, call the server, wait for a response, de-serialize the result(s), and present them to the calling program as if nothing happened.
Since this involves a lot of tedious, repetitive and error-prone code, it's probably best to abstract it by making it code-driven. The best would be to use the original library's header file to define the serialization needed, but that (of course) requires quite heavy C parsing. Failing that, you might start bottom-up and make a custom language to describe the calls, and then use that to generate the serialization, de-serialization, and proxy code.
On Linux systems, you can control the dynamic linker so that it loads your proxy library instead of the "real" library. You could of course also replace (on disk) the real library with the proxy, but that will break all applications that use it if the server is not working, which seems very risky.
So you basically have two choices, each outlined by unwind and ammoQ respectively:
(1) Write a server and do the socket/protocol work etc., yourself. You can minimize some of the pain by using solutions like Google's protocol buffers.
(2) use an existing middleware solution like (a) message queues or (b) an RPC mechanism like CORBA and its many alternatives
Either is probably more work than you anticipated. So really you have to answer this yourself. How serious is your project? How varied is your hardware? How likely is the hardware and software configuration to change in the future?
If this is more than a learning or pet project you are going to be bored with in a month or two then an existing middleware solution is probably the way to go. The downside is there is a somewhat intimidating learning curve.
You can go the RPC route with CORBA, ICE, or whatever the Java solutions are these days (RMI? EJB?), and a bunch of others. This is an elegant solution since your calls to the remote encryption machine appear to your SystemA as simple function calls and the middleware handles the data issues and sockets. But you aren't going to learn them in a weekend.
Personally I would look to see if a message queue solution like AMQP would work for you first. There is less of a learning curve than RPC.
hello and good day to you guys. I am running Windows XP which I am given to understand is a 32 bit windows system.
I have a microcontroller that continuously sends data serially through a COM port. I want to process data in a C program. The options I'm looking at so far are:
get serial data via python and pass to C
read data serially and use in C
The first option seems too hard for me. I was trying to use swig and am stuck.
any other suggestions?
You'll find it much easier just to receive the data straight via C, if you're going to process it there in the end anyway. Here's a quick overview of how to set things up. Essentially you call CreateFile on, eg, "COM1", then use GetCommState and SetCommState on the resulting handle to configure the port. If you need to do GUI interaction as well, have the reading code run on a different thread, and communicate the data it reads back to the GUI thread by posting custom (WM_USER, etc) messages to one of your windows.
I need to send a signal via my remote PC to the Internet that let me know if this pc is connected.
I could send a link with GET values to my page and then from that php page make a query to the database.
How do I send this value through a C program that runs on this remote PC?
thanks!
(it's a windows pc)
For making HTTP requests I recommend libcurl, which is the library that almost everybody seems to be using.
http://curl.haxx.se/libcurl/
What operating system? Linux? Windows? Does the program need to be cross-platform? The reason I ask is that it influences whether you should use a library, or TCP/IP sockets, given that the request will be very simple.
Also, why not use Perl, or better yet, wget? You could schedule a task in windows, or a cronjob in unix, to wget http://yoururl/path?pcname=`uname` or similar..
What about using a client like dyndns? I'm not sure using a C program would be such a good idea for that purpose; it's a system administration task and using scripting for this would work best, unless you have a specific need in mind.