MATLAB: Why clear MEX needed

MATLABmemorymexsimulink

Hello Mathworks,

I am currently working with one Simulink model which has S-Function with .mexw64 file. Now when I run my model first time, I am getting correct result. But after than when ever I run Simulink model, the output is always zero. I have tried to check that after first Simulation, is there any new varible or anything which will be created that makes my output zero for every simulation after the first one?. But ther is not (I have checked that in MATLAB Workspace). Then when I use clear mex command in MATLAB and then tried to simulate the Simulink model 2nd time then it works. I dont know what exactly this command does? If it clears the mex then shouldnt it be in the workspace just like when we use command clearvars .. to clear specific variable?

If after 1st Simulation, MATLAB by default stores a memory to mex then where that can be seen? I mean what clears or delete clear mex command?

Any help would be appreciated.

Best Regard Jim

Best Answer

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.

Related Solutions

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

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.

MATLAB: Make static variables in mex keep their values between calls

In order to make the variable persistent in memory so as to save its state after every mex call you can use mexMakeMemoryPersistent, and to free the allocated memory you can use the following command,

clear mex

Best Answer

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

MATLAB: Make static variables in mex keep their values between calls

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