Dynamically sized IO ports in simulink

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Benjamin Wang
Benjamin Wang on 14 Apr 2011
I am working on a C-MEX s-function, which has 2 inputs and 1 output. All of the i/o ports are dynamically sized. The output port should always have the same size with the second input port.
Here are some fragments of my code.
/* ... */
static void mdlInitializeSizes(SimStruct *S)
... // Set two input and one output ports to dynamically sized
#if defined(MATLAB_MEX_FILE)
static void mdlSetInputPortWidth(SimStruct *S, int_T port, int_T inputPortWidth)
ssSetInputPortWidth(S, port, inputPortWidth);
/* This is very likely wrong */
if (1 == port) {
ssSetOutputPortWidth(S, port, inputPortWidth);
static void mdlSetOutputPortWidth(/* ... */)
/* I don't know how to set here */
static void mdlSetDefaultPortDimensionInfo(SimStruct *S)
ssSetInputPortWidth(S, 0, 10);
ssSetInputPortWidth(S, 1, 3);
ssSetOutputPortWidth(S, 0, 3);
/* ... */
It could be compiled with no error, but everytime I run the test simulink model with this s-function in it, MATLAB crashes.
Could someone tell me how to solve this, please?
Thanks in advance.

Accepted Answer

MarkB on 14 Apr 2011
The code above is mostly right, but there are some minor changes that you will need to make:
  • The calls to "mdlSetOutputPortWidth" and "mdlSetInputPortWidth" can occur for any port that was originally set as dynamically typed, even if was assigned a dimension later on. As a result, you should confirm that the output port is still dynamically typed before you assign the output port dimension in "mdlSetInputPortWidth". If it's still dynamically typed, assign it. If it isn't check to see if the output port dimensions match what you would have assigned. If not, throw an error.
  • The "mdlSetOutputPortWidth" should be a mirror-image/reverse of your "mdlSetInputPortWidth" function. People often mistakenly assume that dimensions only propagate "downstream", but it is entirely possible that your block's output port dimension gets assigned first. In this case, you should assign that dimension to the second input port as well.
  • "mdlSetDefaultPortDimensionInfo" gets called if any of the ports are still dynamically typed, so it is incorrect to assume that it means that all of the ports are still unspecified. Therefore, instead of just assigning dimensions to all of the ports, you should check each one to confirm that it is still dynamically typed, and only assign dimensions in that case.
  1 Comment
Benjamin Wang
Benjamin Wang on 14 Apr 2011
Thank you Mark. The information you posted is very helpful. I've changed my code according to your suggestion.
The reason why my code got crashed was that I accidently initialized an 'int_T' type pointer with the return value of ssGetInputPortSignal, which actually is a 'real_T' type pointer. Then I used it to index some array and got pointer overflow. Did not see this until just now...

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