opensees runs a TCL interpreter.
The primary access method is to write the desired commands to a file, and then system() to run the opensys executable redirecting from the file you just created
In Windows, there would be an additional more interactive method available, by using .NET to create a process and using .NET functions to write to the process. If I recall correctly you use System.Diagnostics.Process
In Mac or Linux, you could open a named pipe to talk to the program.
