Python Memory Leak using SWIG

[Pierre Sangouard]

cody314 at gmail.com wrote:
> Versions:
> Python 2.5.1 (r251:54863, May 14 2007, 10:50:04)
> SWIG Version 1.3.20
>
> Hello I have some code that wraps a C++ library so I may use it in
> python.  The code basically just gets some data and puts it in the
> PyArrayObject* which is returned as a PyObject*.
> I then call it from python like so:
> self.c = __f2.fdct2_wrapper(x,self.nbs,self.nba,self.ac)
>
> I then loop (which pretty much only calls this function) over and
> over.  I put the variable as a self.c hoping the garbage collector
> would know how to delete it after the class goes out of scope.  I also
> tried explicitly deleting the variable (del var) in the loop with no
> success.  In all cases quiet a large memory leak occurs (and grows
> rather quickly).
>
> I believe this is comming from the fact that the thing that is
> returned is a pointer to the data.  So the returning object is a
> pointer.  The python garbage collector then doesn't know how to delete
> this structure and probably (maybe) just deletes the pointer after the
> class goes out of scope.  Leave the data there and causing the memory
> leak issue.  I however doesn't know how to tell python that this
> variable is a pointer and to delete whats going to it.  Or perhaps
> tell SWIG to delete the data, and return the structure some other way?
>
> Here is the c++ wrapping code,  perhaps there is an easy way to fix
> this memory leak (I believe a lot of SWIG people probably do this)
> perhaps some function call from the python? or some special call from
> the SWIG?  Thanks a bunch!
>
> // set up the list output
> PyListObject* out;
> PyArrayObject* output;
> out = (PyListObject*) PyList_New(0);
> npy_intp dims[2];
> int i,j;
>
> for(i=0;i<g.size();i++)
> {
> // append a list for this scale
> PyList_Append((PyObject*) out,PyList_New(0));
>
> for(j=0;j<g[i].size();j++)
> {
> // set the dimensions for this scale & angle
> dims[0] = g[i][j].n();
> dims[1] = g[i][j].m();
>
> // make an array for this scale & angle's data
> output = (PyArrayObject*) PyArray_SimpleNewFromData(2, dims,
> PyArray_CDOUBLE,g[i][j]._data);
> Py_INCREF((PyObject*) output);
>
> // append this angle's data to the list for this scale
> PyList_Append(PyList_GetItem((PyObject*) out,i),(PyObject*)
> output);
>
> // zero the CpxNumMat for this scale & angle, hand ownership to
> numpy
> g[i][j]._data = NULL;
> g[i][j]._m = 0;
> g[i][j]._n = 0;
> output->flags = output->flags | NPY_OWNDATA;
> }
> }
>
> return (PyObject*) out;

I think %newobject swig directive is a solution to your problem.

Pierre