MATLAB: Usage of calllib, libpointer, and libstruct with DLL that returns memory

callliblibpointerlibstructloadlibraryunloadlibrary

Dear Folks,

I have three design goals based on speed, thread safety, and Application developer protection. The proposed solution is to have the Application hold all the memory required by the Algorithm that it calls inside an Application for loop. So the Algorithm has API functions that return I/O memory structures that the Application will hold onto (and pass to the Algorithm in each successive call). When the Application is done, it calls Deallocate API functions to release that memory. We would like all Applications to use the API in the same way, if possible. This includes our Matlab testing platform.

Is it possible to use calllib() and libpointer() in a Matlab function if the DLL is returning memory for Matlab to attach to the libpointer object? The DLL has "Allocate" and "Deallocate" functions that handle memory management for the I/O to the main DLL API interface function. It seems that I can use calllib() and libpointer() to pass data back-n-forth, but when I go to release the memory, I get these conditions:

1. If I release the memory by calling Deallocate() functions and passing the libpointer object and then followup with a "clear" or "unloadlibrary()", Matlab crashes.

2. If I release the memory by not using the DLL's Deallocate functions, but instead just rely on Matlab's "clear", then I get a memory leak that quickly builds up one iteration after another.

Can anyone explain to me if there is a workaround to this besides making an intermediate wrapper Application? Note that we are returning memory via return statements, rather than through the argument list.

Best regards, Kris

Best Answer

You should be able to do what you are trying to do but have a bug or design flaw.

It is best to keep all memory in a single structure owned by the same owner. This line: t.Value.waves = libpointer('doublePtr',matlabstruct.waves); places a MATLAB owned and managed memory object inside t that is owned and allocated by your DLL. You free t then clear it in MATLAB but MATLAB does not know it is deallocated and goes to free waves causing a crash.

Without seeing the definition of the structure (in c) you are returning and a bit more info on what is expected I may not be able to give a complete response but you could start by trying this sequence however I have doubts that you want to make the above pointer assignment to waves.

%correct order to free t given its construction that I have some doubts about.
t.Value.waves=[]; or = libpointer;
t.Value.range=[];
calllib('DLL32', 'DeallocateInput', t);
clear t;

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MATLAB: Problem in using mxSetPr

AP - the documentation for mxSetPr indicates that for the second input parameter to this function,

The array must be in dynamic memory; call mxCalloc to allocate this memory. Do not use the ANSI® C calloc function, which can cause memory alignment issues leading to program termination. If pr points to static memory, memory leaks and other memory errors can result.

Did you use mxCalloc to allocate memory to Ain and kernel? That being said, I did try the following test function

 void test()
 {
     mxArray* myMxArray  = 0;
     double*  myDblArray = 0;
     myMxArray  = mxCreateNumericMatrix( 0, 0, mxDOUBLE_CLASS, mxREAL );
     myDblArray = mxCalloc(4,sizeof(double));
     if (myMxArray!=NULL && myDblArray!=NULL)
     {
        mxSetPr(myMxArray,myDblArray);
        mxSetM(myMxArray,2);
        mxSetN(myMxArray,2);
        mxFree(myDblArray);
        mxDestroyArray(myMxArray); 
     }
 }

and the above crashed at the mxSetPr line. So I am a little puzzled by the above. James Tursa would most likely know what is wrong with the above code.

-------

The only time I've seen mxSetPr being used in for output (the lhs) of the mexFunction, so if I had to do something similar to what you are doing, I would have tried the following

 void myfun2(double *Ain   , int  AnRows    , int AnCols, 
             double *kernel, int kernelnRows, int kernelnCols,
             mxArray **lhs)
 {
     mxArray *rhs[3];
     double  *ptr;
     // create the mxArrays
     rhs[0] = mxCreateNumericMatrix( AnRows, AnCols, mxDOUBLE_CLASS, mxREAL );
     rhs[1] = mxCreateNumericMatrix( kernelnRows, kernelnCols, mxDOUBLE_CLASS, mxREAL );
     rhs[2] = mxCreateString       ( "same" );
     // copy the Ain and kernel data
     ptr = mxGetPr(rhs[0]);
     memcpy(ptr,Ain,sizeof(double)*AnRows*AnCols);
     ptr = mxGetPr(rhs[1]);
     memcpy(ptr,kernel,sizeof(double)*kernelnRows*kernelnCols);
     mexCallMATLAB(1, lhs, 3, rhs, "conv2");
     // destroy the mxArrays
     mxDestroyArray(rhs[0]);
     mxDestroyArray(rhs[1]);
     mxDestroyArray(rhs[2]);
 }

Note in the above how memcpy is used to copy the data from the Ain and Kernel arrays to the destination mxArrays. Note also that for every create of an mxArray, we must destroy or free memory allocated to that array.

For the case of the left-hand side result of the convolution, we pass an mxArray into this function as

 mxArray **lhs

As (presumably) memory will be dynamically allocated to it within the conv2 function, the caller of myfun2 does not need to allocate memory to this parameter, but the caller will be responsible for freeing the memory. For example,

 // do some work to create the Ain and Kernel matrices
 // declare the output matrix from the 2D convolution
 mxArray *convMtx = 0;
 // call the function to do the 2D convolution
 myfun2(Ain, AnRows, AnCols,kernel,kernelnRows,kernelnCols,&convMtx);
 if (convMtx!=NULL)
 {
    // do some stuff with convMtx
    // destroy mxArray
    mxDestroyArray(convMtx);
 }

See how we pass the convMtx into the function as &convMtx.

Best Answer

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