Link 2 C++ programs in Linux during runtime with variable exchange

Question

I have two indepentend programs written in C++ that now have to be coupled during the runtiime via some kind of an interface. I tried to sketch it below with a pseudo-code:

program1.cpp:

program1(){
   initializeProgram1();

   for (i = 0; i < nIterations; i ++){
        A = veryComplexParallelCalculation();
        postProcessAndOutput(A);
   }

   finishProgram1();
}

---

program2.cpp:

program2(){
   prepareProgram2();
   for (i = 0; i < nIterations; i ++){
        B = evenMoreComplexParallelCalculation();
        OutputAndPostProcess(B);
   }

   finishProgram2();

}

We need to add another function into the loop in program1.cpp that uses the result of the program2.cpp during the runtime at the same value of an iteration i. For example, the modified loops of two programs would look like this:

for (i = 0; i < nIterations; i ++){
        A = veryComplexParallelCalculation();
        postProcessAndOutput(A);

        //iB is the iteration number from program2
        getBfromProgram2(B, iB);
        if (iB == i )
            C = foo(A ,B);

}

And in program2.cpp it will be something like

   for (i = 0; i < nIterations; i ++){
        B = evenMoreComplexParallelCalculation();
        OutputAndPostProcess(B);

        //send data and iteration number to program1
        sendBtoProgram1(B, i);

        doSomeOtherStuff();
   }

So I am wondering if it is possible to catch the variable B during the runtime of the program2 and send it to program1. Note, that two programs are two huge projects that cannot be merged!

Or maybe some kind of an interface or a wrapper could be used (some kind of inter-program MPI_Send/Recv:)? I was also thinking about i/o to/from file. I would appritiate any ideas about it.