Hi,
(dual posted to sage and cython)
A few of us (ipython and mpi4py devs) are wondering what the
best/safest way of allocating dynamic memory in a local scope
(method/function) is when using cython. An example would be if you
need an array of c ints that is locally scoped.
The big question is how to make sure that the memory gets freed - even
if something goes wrong in the function/method. That is, you want to
prevent memory leaks. It looks like in sage, the
sage_malloc/sage_free functions are used for this purpose:
from sage/graphs/graph_isom.pyx:
176 def incorporate_permutation(self, gamma):
202 cdef int *_gamma = <int *> sage_malloc( n * sizeof(int) )
203 if not _gamma:
204 raise MemoryError("Error allocating memory.")
205 for k from 0 <= k < n:
206 _gamma[k] = gamma[k]
207 self._incorporate_permutation(_gamma, n)
208 sage_free(_gamma)
Because sage_malloc is #defined to malloc in stdsage.h, I think there
is a significant potential for memory leaks in code like this. Are we
thinking correctly on this issue? Isn't this a huge problem?
Lisandro Dalcin (author of mpi4py) came up with the following trick
that, while more complicated, prevents memory leaks:
cdef extern from "Python.h":
object PyString_FromStringAndSize(char*,Py_ssize_t)
char* PyString_AS_STRING(object)
cdef inline object pyalloc_i(int size, int **i):
if size < 0: size = 0
cdef Py_ssize_t n = size * sizeof(int)
cdef object ob = PyString_FromStringAndSize(NULL, n)
i[0] = <int*> PyString_AS_STRING(ob)
return ob
and now
def foo(sequence):
cdef int size = len(sequence),
cdef int *buf = NULL
cdef object tmp = pyalloc_i(size, &buf)
This could probably be adapted into a malloc-like function. What do
people think?
Thanks,
Brian
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