How to "free" or "destroy" pointer array of mxArray?

Hi,

I am using mexCallMATLAB in mex. So I need to construct pointer array for multi input or output. Like the below tmp variable. I would like to know how to free it.

 mxArray *tmp[3];
 mexCallMATLAB(3,tmp,1,A,"find");
 % must I do it in this way
 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
 % or
 mxDestroyArray(tmp);

Answer 1

[James Tursa]

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.

[wei zhang]

Thank you for you reply. But I have a further problem. If I need to reuse the variable tmp with different size. Like below,

 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
% some code 
 mxArray *tmp[5];

How to "clear" the variable tmp? Without clear, it gives an error as below.

"tmp" has already been declared in the current scope

[James Tursa]

You can't redefine tmp in C/C++. Once it has been defined that is what it is. If you need different sizes of tmp there are two approaches.

1) Make tmp big enough to handle all of your needs. The unused array elements don't hurt you. E.g.,

 mxArray *tmp[100]; // make it big enough to hold 100 mxArray pointers
 mexCallMATLAB(3,tmp,1,A,"find"); // using only three spots is OK
 // use the three spots of tmp here
 // then destroy the three spots you used
 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
 mexCallMATLAB(5,tmp,1,A,"some_other_function"); // use only five spots here
 // use the five spots of tmp here
 // then destroy the five spots you used
 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
 mxDestroyArray(tmp[3]);
 mxDestroyArray(tmp[4]);

2) Make tmp a pointer to pointer to mxArray and allocate it. E.g.,

 mxArray **tmp;
 tmp = (mxArray **) mxMalloc(3 * sizeof(*tmp)); // allocate space for three spots
 mexCallMATLAB(3,tmp,1,A,"find"); // using three spots here
 // use the three spots of tmp here
 // then destroy the three spots you used
 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
 mxFree(tmp); // free the three spots
 tmp = (mxArray **) mxMalloc(5 * sizeof(*tmp)); // allocate space for five spots
 mexCallMATLAB(5,tmp,1,A,"some_other_function"); // use five spots here
 // use the five spots of tmp here
 // then destroy the five spots you used
 mxDestroyArray(tmp[0]);
 mxDestroyArray(tmp[1]);
 mxDestroyArray(tmp[2]);
 mxDestroyArray(tmp[3]);
 mxDestroyArray(tmp[4]);
 mxFree(tmp); // free the five spots

[wei zhang]

Thank you. The nested pointer works well.