Calling function from another process on Windows

I have two Windows applications (EXEs). I would like to do this:

A:

void foo(int a)
{
    MessageBox(a);
}

B:

p = CreateProcess("A.exe");
CallFunction(p, "foo", 15); 

How this "CallFunction" can be done?

You will need to use an IPC (Inter-Process Communication) method. There are a number of options for this, including DCOM, named pipes, and Windows messages.