MATLAB: Pointer in a C mex sfunction

c codehandle the pointerspointers-function

Hello,

I have some problems to understand the syntax and the meaning of some pointers used in a C sfunction For example taken the output method of the TimesTwo sfunction

http://www.mathworks.com/support/tech-notes/1800/1819_files /timestwo.c

From which I reported an extract hereafter

      Int_T             i;
      InputRealPtrsType uPtrs = ssGetInputPortRealSignalPtrs(S,0);
      real_T            *y    = ssGetOutputPortRealSignal(S,0);
      int_T             width = ssGetOutputPortWidth(S,0);
      for (i=0; i<width; i++) {
          /*
           * This example does not implement complex signal handling.
           * To find out see an example about how to handle complex signal in 
           * S-function, see sdotproduct.c for details.
           */
          *y++ = 2.0 *(*uPtrs[i]); 
      }
  }

what is it supposed to mean *y++ (is the value to which the y pointer refers, isn't? but I'm wondering why there is the ++ notation following *y, I thought it's to point to the next memory area because we deal with a static vector). Am I right?

And what does it means (*uPtrs[i])? I thought it was the value of the variable to which the pointer points to.

is there anyone, who can help me to figure it out? can you suggest me some good references to understand how this stuff works?

thanks for reading

Best Answer

You don't show the definitions of Int_T, real_T, and InputRealPtrsType, so I am guessing a bit here.

y is a pointer to a real_T type.

*y is the real_T value that is being pointed to by y. I.e., it is the value at the memory address contained in the y variable.

The expression " *y++ = right_hand_side " means assign the value of right_hand_side to the real_T value that y is currently pointing to in memory, and then increment y to point to the next real_T value in memory. The ++ is the increment operator and the * is the dereference operator.

If uPtrs is a pointer to a pointer (or an array of pointers) to a real_T type, then uPtrs[i] is the i'th pointer in that array, and *uPtrs[i] is the real_T value at the memory address that uPtrs[i] is currently pointing to. The square brackets [ ] (array element) have higher precedence than the * operator (dereference).

So the breakdown is this:

*y++ = 2.0 *(*uPtrs[i]);

" Take the real_T value that uPtrs[i] is currently pointing to, multiply that by 2, assign this result to the memory location that y is currently pointing to, and then increment y to point to the next real_T value in memory."

If u0 is an array of pointers to a real_T type, then u0[0] is the first pointer in that array, and *u[0] is the real_T value that u[0] is pointing to in memory.

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: Call M-script in C/C++ Sfunction

I understand that you have written a S-Function in C and you want to call a matlab script from that function. I assume that you are using MATLAB R2016b.

You can do the above by using 'mexEvalString('Name_of_Script')' function in mdlOutputs method of S-Function. To illustrate, I am using timestwo example available at the following link http://in.mathworks.com/help/simulink/sfg/example-of-a-basic-c-mex-s-function.html and a dummy script plotline.m which plots a line. I am calling this MATLAB Script from timestwo S-function.

plotLine.m looks as below:

x = [1 2 3];
y = [1 2 3];
plot(x,y);

mdlOutputs method in timestwo.c looks as below:

static void mdlOutputs(SimStruct *S, int_T tid)
{
  int_T i;
  InputRealPtrsType uPtrs = ssGetInputPortRealSignalPtrs(S,0);
  real_T *y = ssGetOutputPortRealSignal(S,0);
  int_T width = ssGetOutputPortWidth(S,0);
    for (i=0; i<width; i++) {
        *y++ = 2.0 *(*uPtrs[i]);
    }
    mexEvalString("plotLine");
 }

Now use mex timestwo.c to compile this S-Function. Run the S-function and notice that it also executes the script now.

I hope that this helps.

Best Answer

Related Solutions

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

MATLAB: Call M-script in C/C++ Sfunction

Related Question