How to Access Datastore Values in C MEX S-Function MATLAB?

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Accessing datastore values directly in a C MEX S-Function can enhance your MATLAB simulations by making data handling more efficient. This capability allows you to perform calculations and manipulate data within your S-Function without the need to define explicit input ports for every datastore. In this article, we will delve into how to effectively access and utilize datastore values in your C MEX S-Function, including practical code examples to illustrate the process.

Understanding the Role of Datastores in Simulink


Datastores in Simulink are used to share data between different blocks in your model. They allow for easy management of complex data across your simulation. However, direct access to these values from a C MEX S-Function can be tricky since sometimes you want to bypass the need for input ports for every value you wish to utilize.

Why Use C MEX S-Functions?


C MEX S-Functions provide flexibility and performance, as you can write S-Functions in C or C++ to enhance simulation speed and optimize memory usage. Leveraging datastores directly within these functions facilitates the creation of more dynamic and responsive models.

How to Access Datastore Values in Your C MEX S-Function

Step 1: Including Necessary Headers


To get started, make sure to include the necessary Simulink headers in your C MEX file. Here's how you can set this up:

#include "simstruc.h"
#include "mex.h"

Step 2: Define the S-Function Structure


In this step, you will define the basic structure of your S-Function. This includes setting up the parameters and setting the number of inputs and outputs. Even though you plan to access values from datastores, you’ll still need a minimal structure defined:

#define S_FUNCTION_NAME myMexSFunction
#define S_FUNCTION_LEVEL 2

static void mdlInitializeSizes(SimStruct *S) {
ssSetNumSFcnParams(S, 0); // No input parameters
ssSetNumInputPorts(S, 0); // No input ports
ssSetNumOutputPorts(S, 1); // Define one output port
ssSetOutputPortWidth(S, 0, 1); // Output port width
}

Step 3: Accessing the Datastore


To access a specific datastore, use the mexGetVariablePtr function. You’ll typically find the value of the datastore by name. Here's an example of how to achieve this inside your mdlOutputs function:

static void mdlOutputs(SimStruct *S, int_T tid) {
// Access the datastore named 'myDatastore'
mxArray *dataPtr = mexGetVariablePtr("base", "myDatastore");
if (dataPtr != NULL) {
double value = mxGetScalar(dataPtr); // Assuming it's a scalar
// Update the output
real_T *y = ssGetOutputPortRealSignal(S, 0);
y[0] = value;
} else {
// Handle the case where the datastore is not found
ssSetErrorStatus(S, "Datastore not found!");
}
}

Step 4: Finalizing the S-Function


Finally, implement the necessary functions for your S-Function to ensure it finalizes correctly. Most commonly, you'll need mdlTerminate to clean up resources:

static void mdlTerminate(SimStruct *S) {
// Cleanup code if necessary
}

#ifdef MATLAB_MEX_FILE
#include "simulink.c"
#else
#include "cg_sfun.h"
#endif

Conclusion


By following these steps, you can effectively access values from datastores within your C MEX S-Function in MATLAB. This approach allows you to run simulations more efficiently without having to define each datastore value as an input port.

Frequently Asked Questions

Can I access multiple datastore values?


, you can access multiple datastore values by calling mexGetVariablePtr for each datastore you wish to use.

What if the datastore not found error occurs?


If the specified datastore is not found, ensure that you reference its name correctly or check that it is present in the MATLAB workspace at the time of simulation.

Is it possible to write values back to the datastore?


, you can write values back using mexPutVariable to update the datastore in the MATLAB base workspace if necessary.

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