MATLAB: C S-function problem when trying to get a pointer to a structure

c s-functionspointerpwork vectorssimulink

Hello all,

I created a structure within the mdlStart method and a pointer to it as follows:

   SomeStructureType  myStruct;
   SomeStructureType* myStruct_ptr;
   myStruct_ptr = &myStruct;

Then I saved the pointer into a (previously defined) PWork vector:

   ssSetPWorkValue(S, 0, (void *)myStruct_ptr);

When I retrieve the pointer, within mdlStart, as follows:

   anotherStruct_ptr= (SomeStructureType*)ssGetPWorkValue(S,0);

everything works fine. anotherStruct_ptr points to myStruct. But when I try to get the same pointer from the mdlTerminate method, I obtain a wrong one:

   SomeStructureType *Test_ptr;
   Test_ptr = (SomeStructureType*)ssGetPWorkValue(S,0);

Test_ptr does not point to myStruct.

Avoiding the cast gives the same result. I also tried to use ssSetUserData and ssGetUserData instead of ssSetPWorkValue and ssGetPWorkValue, respectively, but the result is still wrong.

I already used PWork vectors without any problem, but this is the first time I use it to hold a pointer to a structure. Am I doing something wrong?

Best Answer

You need to use:

   SomeStructureType* myStruct_ptr = new SomeStructureType;

to create the pointer. If you simply assign it to a variable on the stack, that memory is free'd at the end of mdlStart, and any further references to it are pointing to corrupt memory and can cause a segmentation violation.

Also, remember to use delete to free that memory in mdlTerminate.

Related Solutions

MATLAB: How to allocate and free memory for a PWork vector in a C-MEX S-function

First, the S-Function needs to specify how many PWork vectors it needs (using ssSetNumPWork) either in mdlInitializeSizes or in mdlSetWorkWidths. Simulink calls the above functions in the S-Function at the beginning of the model initialization stage (internally we refer to this part of the initialization stage as the compile phase). Towards the end of model initialization stage (internally we refer to this part of the initialization stage as the link phase), Simulink allocates the specified number of PWorks. Upon completing the model initialization stage, Simulink calls the S-Function's mdlStart. In the mdlStart function, the S-Function can allocate memory and store a pointer to it in the PWork array. The following code snippet illustrates specifying, allocating, using, and freeing memory using PWorks.

mdlInitializeSizes(SimStruct* S) 
{ 
/* snip */
ssSetNumPWork(S, 1);
/* snip */ 
}
mdlStart(SimStruct* S) 
{ 
/* snip */
void* ptr = malloc(1024 /* number of bytes */); 
if (ptr == NULL) { 
/* report memory allocation error */ 
ssSetErrorStatus(S, "Memory allocation error"); 
return; 
} 
ssSetPWorkValue(S, 0, ptr);
/* snip */ 
}
mdlOutputs(SimStruct* S, int_T tid) 
{ 
void* ptr = ssGetPWorkValue(S, 0);
/* snip */ 
}
mdlTerminate(SimStruct* S) 
{ 
/* snip */
/* free the memory allocated in mdlStart */ 
if (ssGetPWork(S) != NULL) { 
void* ptr = ssGetPWorkVale(S,0); 
if (ptr != NULL) free(ptr); 
}
/* snip */ 
}

For a more complete example on using PWork vectors please look at the following example in the documentation.

http://www.mathworks.com/help/releases/R2009b/toolbox/simulink/sfg/bre1pj2.html#bre1ps0

MATLAB: Why Matlab crashed when I use persistent C++ COM Object in Sfunction

I suspect it is because your declaration of the variable ISWrapper Iw; goes out of scope at the end of mdlStart. Perhaps you should instead write something like:

      static void mdlStart(SimStruct *S)
      {
         CoInitialize(NULL);  
         ssSetPWorkValue(S, 0, (void *) (new ISWrapper)); 
      }
      static void mdlOutputs(SimStruct *S, int_T tid)
      {
         const real_T *u = (const real_T*) ssGetInputPortSignal(S,0);
         real_T    *y = (real_T *) ssGetOutputPortSignal(S,0);
         ISWrapper* Iw = (ISWrapper*)ssGetPWorkValue(S,0);
         Iw->GetPtr()->Connect();
      }

Don't forget to delete the ISWrapper pointer in mdlTerminate.

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