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PWork initialization in c++ s-Function with 'Update Diagram' routine

Asked by Mathias Hakenberg on 5 Mar 2019
Latest activity Edited by Mark McBroom on 7 Mar 2019
I am using a c++ s-Function with persistent objects, that are stored in the PWork vector, following the documentation example. The callback methods look like this:
1.Initialize Sizes:
static void mdlInitializeSizes(SimStruct *S)
{
ssSetNumPWork(S, 1);
}
2. Start Model:
static void mdlStart(SimStruct *S)
{
ssGetPWork(S)[0] = new uint8_T(0); // some persistent variable
}
3. Do something with the variable:
static void mdlOutputs(SimStruct *S, int_T tid)
{
uint8_T* aliveCounter = static_cast<uint8_T*>(ssGetPWorkValue(S,0));
(*aliveCounter)++;
}
4. Free the memory after model finishes:
static void mdlTerminate(SimStruct *S)
{
uint8_T* aliveCounter = static_cast<uint8_T*>(ssGetPWorkValue(S, 0));
delete aliveCounter;
}
Everything works fine, if the model is simulated. However, if the Simulink model just gets updated (e.g. via Simulation -> Update Diagram), Matlab crashes with a Memory error. The reason for this is, that 'Update Diagram' only invokes the mdlInitializeSizes and mdlTerminate callbacks. Thus, the PWork vector-size is set to 1, but the value is not initialized, because mdlStart is not called. (Initialization of PWork in mdlInitializeSizes is not possible).
Finally, when the mdlTerminate function is called ssGetPWorkValue(S, 0) trys to access an undefined memory block and Matlab crashes.
Do you have any recommendations, how to avoid this? Is there any way of distinguishing, whether the function is called at the end of a simulation, or just as part of a 'Update Diagram' routine?

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Release

R2017b

1 Answer

Answer by Mark McBroom on 6 Mar 2019

Check aliveCounter for NULL before trying to delete it.

  2 Comments

The program crashes when executing ssGetPWorkValue(S, 0). That is before checking the aliveCounter for NULL.
In more detail:
ssGetPWorkValue(S, 0) expands to S->work.pWork[0] , but pWork is not initialized.
Try code like this:
#define IS_SIMULATION_TARGET(S) (ssRTWGenIsAccelerator(S) || ssIsRapidAcceleratorActive(S) || ssRTWGenIsModelReferenceSimTarget(S) || (ssGetSimMode(S)==SS_SIMMODE_NORMAL) || (ssGetSimMode(S)==SS_SIMMODE_SIZES_CALL_ONLY) || !((ssRTWGenIsCodeGen(S) || ssGetSimMode(S)==SS_SIMMODE_EXTERNAL) && GetRTWEnvironmentMode(S)==0))
static void mdlOutputs(SimStruct *S, int_T tid)
{
if (IS_SIMULATION_TARGET(S)) {
uint8_T* aliveCounter = static_cast<uint8_T*>(ssGetPWorkValue(S, 0));
}

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