example level 2 matlab code for an Euler integrator s-function

Hi all.

I'm trying to understand how a "level 2 Matlab s-function" works, and so I want to try the following simple example.

I would like to construct an example of the Matlab code behind a "level 2 Matlab s-function" designed to integrate using Euler's method. The resulting block should behave pretty much just like the standard integrator (1/s) block in the simulink library, except without any bells and whistles (e.g. no saturation limits or zero crossing detection---I want to keep it as simple as possible), and also that it will always use Euler with fixed step size instead of the global simulink solver setting.

I need it to take one input du/dt and spit back one output u=u(t). It should have at least two parameters u_0 and dt, where u_0 is the value of u at the simulation start time, and dt is the fixed step size. I'm not sure if it needs other parameters also.

Here's what I've got so far:

function dvolume(block)
setup(block);

function setup(block)

% Register the number of ports.
block.NumInputPorts  = 1;
block.NumOutputPorts = 1;

% Set up the port properties to be inherited or dynamic.
block.SetPreCompInpPortInfoToDynamic;
block.SetPreCompOutPortInfoToDynamic;

% Override the input port properties.
block.InputPort(1).DatatypeID  = 0;  % double
block.InputPort(1).Complexity  = 'Real';

% Override the output port properties.
block.OutputPort(1).DatatypeID  = 0; % double
block.OutputPort(1).Complexity  = 'Real';

% Register the parameters.
block.NumDialogPrms     = 1;
block.DialogPrmsTunable = { 'u_0', 'dt' };

% Register the sample times.
block.SampleTimes = [dt offset];

% Specify if Accelerator should use TLC or call back to the 
% MATLAB file
block.SetAccelRunOnTLC(false);

% Specify the block simStateCompliance.
block.SimStateCompliance = 'DefaultSimState';

    %%Register methods
  block.RegBlockMethod('euler',    @euler);

function euler(block)

block.OutputPort(1).Data = u + dt * block.InputPort(1).Data;

yeah, so as you can see, this thing can't possibly run because it doesn't know what to do about u. I'm sure there are other problems too. This leads me to some questions.

(*) The symbol 'u' is supposed to denote the computed value of u(t) at the beginning of the time step, so that

block.OutputPort(1).Data

gives me the computed value of u(t+dt). How do I write that in code?

() Is the line

block.SampleTimes = [dt offset];

correct?

(*) How do I tell it to use u_0 as the initial value of u ?

(**) What other mistakes have I made?

Any help would be much appreciated. Thanks in advance guys!