Simulink: How to use a local variable in a level 2 s-function?

Question

I have written a Simulink S-function (Level 2) in C. The resulting block has one output and one parameter. This parameter is stored in a variable, which is defined at file scope, right after setting up the block: #define NUM_PARAMS 1 #define NUM_INPORTS 0 #define NUM_OUTPORTS 1 unsigned short int MASK_INDEX; I assign it within mdlInitializeSizes, and do some operations on its value: static void mdlInitializeSizes(SimStruct *S) {    // Check Parameters   ssSetNumSFcnParams(S, NUM_PARAMS);   if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {     return;   }   MASK_INDEX = *mxGetPr(ssGetSFcnParam(S, 0)); (...) operations } My problem is, that the variable MASK_INDEX seems to be global, and shared among all blocks of the same type. Therefore, it holds the same value for all blocks. As a workaround, I reload it every time, and re-do the operations, for example: static void mdlOutputs(SimStruct *S, int_T tid) {   MASK_INDEX = *mxGetPr(ssGetSFcnParam(S, 0)); (...) operations } How can I get a true "local variable", so that I don't have to repeat all this every time?

Answer 1

Hello,

If I'm interpreting your question correctly, you're basically saying that the line:

MASK_INDEX = *mxGetPr(ssGetSFcnParam(S, 0));

is repeated, and you want to avoid doing so.

Note that, as you suspect, the variable assigned (MASK_INDEX) is a global, and the assignment modifies it. As such, it's assignment is a side-effect of executing the functions containing it, so using a *local* variable will mean that the global state won't change, and will be incorrect.

So, unfortunately, the duplication is necessary. However, there are things you could do to reduce the typing. E.g. you could create a small function which does nothing but execute the statement, then call that function:

void LastPart(SimStruct *S) {

MASK_INDEX = *mxGetPr(ssGetSFcnParam(S, 0));

}

...

LastPart(s); // replace MASK_INDEX = *mxGetPr(ssGetSFcnParam(S, 0));