MATLAB: What is the appropriate function to free memory when using MATLAB Engine API

MATLAB

In MATLAB 7.3 (R2006b), the documentation for the MATLAB Engine API function "engGetVariable" reads:

 engGetVariable reads the named mxArray from the MATLAB engine session associated with ep and returns a pointer to a newly allocated mxArray structure, or NULL if the attempt fails. engGetVariable fails if the named variable does not exist. Be careful in your code to free the mxArray created by this routine when you are finished with it.

The last line, in saying "free the mxArray", suggests to use "mxFree". More often, the proper way to free the memory is with "mxDestroyArray".

Best Answer

mxDestroyArray is used to free dynamic memory allocated by mxCreate. This applies to any time you create an mxArray, such as with most of the mxArray API functions (mxCreateDoubleMatrix, mxCreateCellMatrix, etc). This also applies to engGetVariable, because engGetVariable returns a pointer to a newly allocated mxArray. mxDestroyArray should be called to free that dynamic memory when finished.

mxFree is used to free dynamic memory allocated by mxCalloc, mxMalloc, or mxRealloc (similar to their malloc counterpart, except they use the MATLAB memory manager). Calls to mxFree are less common.

Related Solutions

MATLAB: How to “free” or “destroy” pointer array of mxArray

You must do each one. So

 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);

or you could put these in a loop.

Doing this

 mxDestroyArray(tmp);

would likely result in a seg fault, since tmp does not point to any dynamically allocated mxArray. In particular, it would definitely not destroy the three mxArrays you want destroyed.

MATLAB: How to resize an mxArray

You are missing a step. After you do the mxRealloc, you need to set this new pointer into the mxArray. E.g.,

a = mxRealloc(a, mxGetM(mxA)*sizeof(double));
mxSetPr(mxA,a);

Also, your test is inconsistent. The variable nrec is a 0-based index for C array indexing, but the result of mxGetM is 1-based for MATLAB array indexing. So you should change this:

   if (nrec > mxGetM(mxA))

to this:

   if (nrec == mxGetM(mxA))

I.e., with your current code nrec is allowed to be equal to mxGetM(mxA), but this is 1 index beyond the actual memory allocated for 0-based indexing.

Best Answer

Related Solutions

MATLAB: How to “free” or “destroy” pointer array of mxArray

MATLAB: How to resize an mxArray

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