I think the interactive client does what you want, but I have not tried it.
when terminal() does not work in OpenModelica so it can't be used either.

However, you can terminate the simulation and print any result you want:

$ omc +s a.mo

class A

  Real a = time;

equation

  when time >= 0.9999 then

  terminate("a");

  end when;

end A;

trunk marsj@marsj-laptop:~/tmp

$ make -f A.makefile 

g++ -I"/home/marsj/dev/trunk/build//include/omc" -march=native -mfpmath=sse  -I. -o A A.cpp A_functions.cpp   -lsim -L"/home/marsj/dev/trunk/build//lib/omc" -lc_runtime -lf2c -linteractive -lsendDataHumbug -lpthread 

trunk marsj@marsj-laptop:~/tmp

$ ./A

Simulation call terminate() at time 0.9999

Message : a



Simulation terminated at time 0.9999

This might suit your needs if you just want a quick and dirty solution

I've tried to use it, but it has not worked.
I'm trying to put an external code in C to do what I need.
I'm running outside openModelica (in an exe application)- so I need to get the variable outside the environment.

My collegue developepd a simple opensource tool to extract data from big plt files generated from OpenModelica for complex models. You can select which variable to plot or to export its data and you can also sample your data results also to generate a less big plt file. If you are interested he can give you the tool or post a direct link to download it in the forum.

Best Regards,
Hassen

I don't think that suits his requirements either (creating a 3+ GB file just to get the last 2 results is crazy anyway).

The method I listed does work from another application though. You just need to redirect stdout to a file and parse it. It's quite fast but might be difficult to maintain if OpenModelica output changes or if your simulation usually outputs dassl warnings, etc.