I'm just starting to get myself familiar with Cython, trying to wrap some structs from a C library to Python methods and classes. What I do not really understand is how casting from (initialized) C structs to the corresponding Python class should work. What am I missing here:
A snippet from the C header file:
struct test_struct {
int _something;
complex_struct* _another;
};
typedef struct test_struct test;
test *test_new(void);
int some_method(test **n, int irrelevant);
Corresponding snippet from my .pxd:
cdef struct test_struct:
pass
ctypedef test_struct test
test* test_new()
int some_method(test **n, int irrelevant)
My .pyx:
def do_something(int irrelevant):
cdef test* t = test_new()
ret = some_method(&t, irrelevant)
# Here comes the problem...
return <Test?>t
cdef class Test:
cdef test* _t
# cinit here and some methods here. No members except _t
Everything up to the return statement works fine. I get a correct value in ret
, etc. But the cast in the return statement seems to be incorrect or its missing more information. When issuing t = do_something(42)
Python segfaults.
The segfault itself is not at all helpful:
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7a9e74b in internal_print () from /usr/lib/libpython2.7.so.1.0
(gdb) bt
#0 0x00007ffff7a9e74b in internal_print () from /usr/lib/libpython2.7.so.1.0
#1 0x00007ffff7a81adb in PyFile_WriteObject () from /usr/lib/libpython2.7.so.1.0
#2 0x00007ffff7b19acb in sys_displayhook () from /usr/lib/libpython2.7.so.1.0
#3 0x00007ffff7a64c23 in PyObject_Call () from /usr/lib/libpython2.7.so.1.0
#4 0x00007ffff7af2ff7 in PyEval_CallObjectWithKeywords () from /usr/lib/libpython2.7.so.1.0
#5 0x00007ffff7af6f3b in PyEval_EvalFrameEx () from /usr/lib/libpython2.7.so.1.0
#6 0x00007ffff7af91b0 in PyEval_EvalCodeEx () from /usr/lib/libpython2.7.so.1.0
#7 0x00007ffff7af92b2 in PyEval_EvalCode () from /usr/lib/libpython2.7.so.1.0
#8 0x00007ffff7b11f9f in run_mod () from /usr/lib/libpython2.7.so.1.0
#9 0x00007ffff7b13ec0 in PyRun_InteractiveOneFlags () from /usr/lib/libpython2.7.so.1.0
#10 0x00007ffff7b140ae in PyRun_InteractiveLoopFlags () from /usr/lib/libpython2.7.so.1.0
#11 0x00007ffff7b1470e in PyRun_AnyFileExFlags () from /usr/lib/libpython2.7.so.1.0
#12 0x00007ffff7b24bcf in Py_Main () from /usr/lib/libpython2.7.so.1.0
#13 0x00007ffff746f000 in __libc_start_main () from /usr/lib/libc.so.6
#14 0x000000000040073e in _start ()
As you can see, the do_something
method should return a Python object of type Test. What do I need to add to make the cast successful? Do I need to manually map the struct to the Python class? Is there some Cython magic I can use?
You need to make Test
into an actual wrapper around the C type. But you also cannot pass a C argument to a Python function (such as a constructor). So you'll need a factory function as well. Here's an example:
cdef class Test:
cdef test* _t
def __cinit__(self):
self._t = NULL
def _setup(self, test* t):
self._t = t
return self
property something:
def __get__(self):
return self._t._something
def __set__(self, int val):
self._t._something = val
cdef Test_create(test* t):
return Test()._setup(t)
Then in do_something()
:
return Test_create(t)
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