Debugging Python ctypes segmentation fault

Question

I am trying to port some Python ctypes code from a Windows-specific program to link with a Linux port of my library. The shortest Python code sample that describes my problem is shown below. When I try to execute it, I receive a segmentation fault in examine_arguments() in Python. I placed a printf statement in my library at the crashing function call, but it is never executed, which leads me to think the problem is in the ctypes code.

import ctypes

avidll = ctypes.CDLL("libavxsynth.so")


class AVS_Value(ctypes.Structure, object):
    def __init__(self, val=None):
        self.type=ctypes.c_short(105) # 'i'
        self.array_size = 5
        self.d.i = 99


class U(ctypes.Union):
    _fields_ = [("c", ctypes.c_void_p),
                ("b", ctypes.c_long),
                ("i", ctypes.c_int),
                ("f", ctypes.c_float),
                ("s", ctypes.c_char_p),
                ("a", ctypes.POINTER(AVS_Value))]


AVS_Value._fields_ = [("type", ctypes.c_short),
                      ("array_size", ctypes.c_short),
                      ("d", U)]


avs_create_script_environment = avidll.avs_create_script_environment
avs_create_script_environment.restype = ctypes.c_void_p
avs_create_script_environment.argtypes = [ctypes.c_int]

avs_set_var = avidll.avs_set_var
avs_set_var.restype = ctypes.c_int
avs_set_var.argtypes = [ctypes.c_void_p, ctypes.c_char_p, AVS_Value]


env = avs_create_script_environment(2)
val = AVS_Value()
res = avs_set_var(env, b'test', val)

My library has the following in its headers, and a plain-C program doing what I describe above (calling create_script_environment followed by set_var ) runs fine. Looking at logging information my library is putting onto the console, the crash happens when I try to enter avs_set_var .

typedef struct AVS_ScriptEnvironment AVS_ScriptEnvironment;
typedef struct AVS_Value AVS_Value;
struct AVS_Value {
  short type;  // 'a'rray, 'c'lip, 'b'ool, 'i'nt, 'f'loat, 's'tring, 'v'oid, or 'l'ong
               // for some function e'rror
  short array_size;
  union {
    void * clip; // do not use directly, use avs_take_clip
    char boolean;
    int integer;
    float floating_pt;
    const char * string;
    const AVS_Value * array;
  } d;
};
AVS_ScriptEnvironment * avs_create_script_environment(int version);
int avs_set_var(AVS_ScriptEnvironment *, const char* name, AVS_Value val);

I tried backtracing the call from GDB, but I don't understand how to interpret the results nor really much about using GDB.

#0  0x00007ffff61d6490 in examine_argument () from /usr/lib/python2.7/lib-dynload/_ctypes.so
#1  0x00007ffff61d65ba in ffi_prep_cif_machdep () from /usr/lib/python2.7/lib-dynload/_ctypes.so
#2  0x00007ffff61d3447 in ffi_prep_cif () from /usr/lib/python2.7/lib-dynload/_ctypes.so
#3  0x00007ffff61c7275 in _ctypes_callproc () from /usr/lib/python2.7/lib-dynload/_ctypes.so
#4  0x00007ffff61c7aa2 in PyCFuncPtr_call.2798 () from /usr/lib/python2.7/lib-dynload/_ctypes.so
#5  0x00000000004c7c76 in PyObject_Call ()
#6  0x000000000042aa4a in PyEval_EvalFrameEx ()
#7  0x00000000004317f2 in PyEval_EvalCodeEx ()
#8  0x000000000054b171 in PyRun_FileExFlags ()
#9  0x000000000054b7d8 in PyRun_SimpleFileExFlags ()
#10 0x000000000054c5d6 in Py_Main ()
#11 0x00007ffff68e576d in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#12 0x000000000041b931 in _start ()

I'm at a loss as to how to approach this problem. I've looked at the details of the calling types, but I don't see anything obviously incorrect there. Am I falling into any platform-specific usages of types?

Edit It seems there's a problem with 32-bit vs 64-bit architectures in the ctypes module. When I tested this again with a 32-bit build of my library and 32-bit Python, it ran successfully. On 64-bit, it segfaults at the same place.

Answer 1

Try using c_void_p for the opaque AVS_ScriptEnvironment* :

avs_create_script_environment.restype = c_void_p

and:

avs_set_var.argtypes=[c_void_p,ctypes.c_char_p,AVS_Value]