When you retrieve a variable with engGetVariable you are getting a deep copy of the variable from the engine into your program. You can manipulate it all you want and it will not affect the variable in the engine workspace. In the External Interfaces section of the doc, look at mxIsStruct, mxGetField, mxGetFieldByNumber, mxSetField, mxSetFieldByNumber and friends. If you are changing the contents of a currently existing field, be sure to use mxDestroyArray on the current field first or you will get a memory leak. e.g., do something like this:
mx = engGetVariable(etc);
field = mxGetField(etc);
mxDestroyArray(field);
mxSetField(etc);
Also, be careful not to reuse the same variable to set multiple fields or you will risk a program bomb when the variable gets destroyed. There is a way to do this correctly by bumping up the reference count of the variable, but this technique is not officially supported.
James Tursa
