MATLAB: Are static variables in C or FORTRAN MEX S-functions not reset when running successive simulations

simulink

I am running a Simulink model which contains a C-MEX or FORTRAN S-function which uses persistent, static and/or global variables. When I run repeated (identical) simulation runs, I receive varying results due to S-function/C-code static variables not being re-initialized properly (except for the first time) by MATLAB and Simulink.

Best Answer

In general, work vectors are recommended over static or global variables if your S-function needs persistent memory storage. For more information on this, see the "Writing S-Functions" manual.

However, if you need to use such variables, you should be aware that C-MEX or FORTRAN S-functions are shared libraries and are not unloaded from memory when a simulation stops. There are multiple workaround to re-initialize these variables in an S-function.

1. You can clear all loaded MEX-functions (this includes but is not limited to S-functions) with the following command:

clear mex

You should run this command between simulations.

2. An alternative is to clear only those MEX-functions that should be unloaded from memory before a simulation because of their static variables. If the function that should be unloaded is mySfunction, you can unload it from memory with:

clear mySfunction

This option is used in the example model testmodel_wo1 and it is entered in the InitFcn callback of this model, such that this is called right before the model is simulated.

3. If you would like to use your custom S-function in various models and do not want to use model-wide callback functions, it is also possible to unload the S-function from memory in a callback specific to the S-function. You can set the following command as the InitFcn callback of the S-function itself:

clear(get_param(gcb, 'FunctionName'))

This option is used in the example model testmodel_wo2.

4. Finally it is also possible to initialize static variables in the S-function itself. In the example model testmodel_wo3, this option is used. For the example models attached to this solution, the Legacy Code Tool is used to generate the S-functions for the example models. For this option the StartFcnSpec is used to specify a C function that will be called at the start of a simulation. This function initializes persistent variables and since it will be called at the start of every simulation as opposed to when the S-function is loaded, it will have the same state for each simulation run.

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MATLAB: Why clear MEX needed

Does your S-Function use persistent variables? In C or C++ those correspond to any variable that is declared outside of any function. In C, persistent variables that are shared between compilation units would be declared "extern" in all compilation units except one; persistent variables that are local to the compilation unit would be declared "static".

When a mex function is run, it is loaded into memory, which includes allocating memory for all persistent variables; running the code leads to whatever changes to those variables made by executing the code. Initial loading can end up initializing a variable if the declaration of the variable has an initializer.

When you run the model again without having exited MATLAB and without having done "clear all" or "clear mex", then the function is not re-loaded. Persistent variables that had an initializer are not re-initialized in this case. The code will find the variables to have whatever state they had when the previous run finished.

"clear mex" causes the loaded mex function to be unloaded from memory. The next time the mex is run, it will not be present in memory and it will be loaded again, including initialization of persistent variables that had an initializer.

This behaviour of leaving persistent variables the same is not specific to MATLAB: it is the way C and C++ define persistent variables to work. It is a feature rather than a bug, since it permits hardware resource acquisition or authentication protocols to take place only once and be available for re-use when the code is executed again.

Note: if you re-create the .mex file during a session, then you should "clear mex" to force the new version to be used.

MATLAB: Does subsequent/multiple runs produce different results when using S-function with the custom code in Simulink 8.0 (R2012b)

With respect to this issue, following is the reason for the outputs to be different :

In your custom C code 'my_CustomCode.c' : lines

int j;
int debug_t[2];

are defined as global variables.

In the function call in the Custom Code:

int * my_CustomCode(int u, int v)
{
    ...
     if (j==0)
    {
       ...// Reset other global variables like : debug_t[]
       j = 1;
     }
    ...
}

The first time j=0 (is picked by default, i.e., global variable initialized to 0), and within this IF statement value of j is changed to 1, and thereafter all subsequent calls has j=1, hence skipping the IF statement.

This happens during the first Simulink run, now when the next Simulink run happens, value of j is still 1 and that means the IF statement which resets values for other variables is not run, and hence the simulation continues from there in the next run. Similarly, the variable global variable, debug_t[] uses the last run values and continue doing so.

The explaination for this behavior of MEX file:

 ".mex" files are DLLs and MEX file variables are only initialized when the ".mex" file is loaded, not with every simulation. Mex file loads only the first time the Simulink Model is run.

Only way to reset values if to execute the following command :

>> clear mex

before every Simulink run, to reload the Mex file everytime model is run. You can put this code in the InitFcn callback of the Simulink model.

The results you observe is due to a combined nature of Global variables with Mex files. As a recommendation, do not use global variables within your custom C code.

Best Answer

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MATLAB: Why clear MEX needed

MATLAB: Does subsequent/multiple runs produce different results when using S-function with the custom code in Simulink 8.0 (R2012b)

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