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Seen in test_multiprocessing:
==31662== 44 bytes in 1 blocks are definitely lost in loss record 687 of 10,548
==31662== at 0x4C2154B: malloc (vg_replace_malloc.c:236)
==31662== by 0x41CC27: PyMem_Malloc (object.c:1699)
==31662== by 0x127D9F51: resize (callproc.c:1664)
==31662== by 0x5759D8: PyCFunction_Call (methodobject.c:81)
==31662== by 0x48E294: call_function (ceval.c:3980)
==31662== by 0x4895E1: PyEval_EvalFrameEx (ceval.c:2605)
==31662== by 0x48E67B: fast_function (ceval.c:4068)
==31662== by 0x48E3C7: call_function (ceval.c:4001)
==31662== by 0x4895E1: PyEval_EvalFrameEx (ceval.c:2605)
==31662== by 0x48C54F: PyEval_EvalCodeEx (ceval.c:3355)
==31662== by 0x48E786: fast_function (ceval.c:4078)
==31662== by 0x48E3C7: call_function (ceval.c:4001)

How did you obtain this? the resize() function is not called by test_multiprocessing.
And are you sure that it's not some kind of reference leak? (this pointer is tied to a CDataObject; its tp_alloc should free the memory)

I can reproduce this with:
valgrind --tool=memcheck --log-file=leaks.txt --leak-check=full --suppressions=Misc/valgrind-python.supp ./python -m test test_ctypes
Where as:
valgrind --tool=memcheck --log-file=leaks.txt --leak-check=full --suppressions=Misc/valgrind-python.supp ./python -m test test_multiprocessing
turns up nothing in 'ctypes.resize'.

> this pointer is tied to a CDataObject; its tp_alloc should free the 
> memory
The free in 'PyCData_clear' is conditional:
 if ((self->b_needsfree)
 && ((size_t)dict->size > sizeof(self->b_value)))
 PyMem_Free(self->b_ptr);
As written, 'PyCData_clear' has no way of knowing that memory has been 
{m,re}alloc'd in 'resize'. So in some cases memory will leak. Here is 
a small reproduction case extracted from 'test_varsize_struct.py'.
 from ctypes import *
 class X(Structure):
 _fields_ = [("item", c_int),
 ("array", c_int * 1)]
 x = X()
 x.item = 42
 x.array[0] = 100
 new_size = sizeof(X) + sizeof(c_int) * 5
 resize(x, new_size)
One potential fix is:
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -2440,7 +2440,7 @@ PyCData_clear(CDataObject *self)
 assert(dict); /* Cannot be NULL for CDataObject instances */
 Py_CLEAR(self->b_objects);
 if ((self->b_needsfree)
- && ((size_t)dict->size > sizeof(self->b_value)))
+ && (self->b_ptr != (char *)&self->b_value))
 PyMem_Free(self->b_ptr);
 self->b_ptr = NULL;
 Py_CLEAR(self->b_base);
I need to think about that more, though.

See patch in issue 16628.