Re: Return struct causes segfault

I would allocate the memory on the python side. Allocating persistent
memory on the C-side makes it difficult to free, unless you do it
specifically. If you allocate it on the python side the GC would take care
of all memory management.

Thor Andreas


On Mon, Sep 30, 2013 at 8:10 AM, Nicholas Earl <[email protected]> wrote:

> Thor,
>
> Thanks for the response! Is there, then, a standard way of calling a C
> function, creating an object (struct) that can hold two arrays, and then
> return that data object to the python program?
>
> Thanks,
>
> Nicholas Earl
> Graduate Research Assistant
> San Diego State University
>
>
> On Sun, Sep 29, 2013 at 10:11 PM, Thor Andreas Tangen <[email protected]>wrote:
>
>> Hi,
>>
>> The vectors times and fluxes are destroyed when exiting the function
>> start, the memory allocated and pointed to be times and fluxes are invalid
>> upon exiting the function.
>>
>> Thor Andreas
>>
>>
>> On Sun, Sep 29, 2013 at 12:53 AM, Nicholas Earl <[email protected]>wrote:
>>
>>> I'm struggling with returning a struct value using ctypes. Mainly, it
>>> keeps segfaulting when I try and get the values from the returned object. I
>>> have my python script setup like this:
>>>
>>> import ctypes
>>> import numpy as np
>>> from numpy.ctypeslib import ndpointer
>>>
>>>
>>> class PhotoInfo(ctypes.Structure):
>>>     _fields_ = [
>>>         ('times', ctypes.POINTER(ctypes.c_double)),
>>>         ('fluxes', ctypes.POINTER(ctypes.c_double)),
>>>     ]
>>>
>>>
>>> def main():
>>>     lib = ctypes.cdll.LoadLibrary('cfile.so')
>>>
>>>     start = lib.start
>>>     start.restype = ctypes.POINTER(PhotoInfo)
>>>     start.argtypes = [
>>>         ctypes.c_int,
>>>         ndpointer(ctypes.c_double),
>>>     ]
>>>
>>>     N = 3
>>>     mass = np.array([0.00020335520, 5.977884E-05, 9.320397E-08])
>>>
>>>     phin = start(N, mass)
>>>
>>>     print phin[0].times[0]
>>>
>>> And in my c file:
>>>
>>> #include <iostream>
>>> #include <iomanip>
>>> #include <fstream>
>>> #include <sstream>
>>> #include <string>
>>> #include <vector>
>>> #include <math.h>
>>> #include <stdlib.h>
>>> #include <stdio.h>
>>>
>>> using namespace std;
>>>
>>> typedef struct {
>>>   double *times;
>>>   double *fluxes;
>>> } PhotoInfo;
>>>
>>> extern "C" {
>>>   PhotoInfo *start(int N, double *mass);
>>> }
>>>
>>> PhotoInfo *start(int N, double *mass) {
>>>   PhotoInfo phin;
>>>   vector<double> times;
>>>   vector<double> fluxes;
>>>
>>> // Here, I read in a file and fill times and fluxes with doubles using
>>> // times.push_back(t) and fluxes.push_back(f)\
>>>
>>>   phin.times = &times[0];
>>>   phin.fluxes = &times[0];
>>>   return &phin;
>>> }
>>>
>>> Any guidance on this would be greatly appreciated!
>>>
>>> Nicholas Earl
>>> Graduate Research Assistant
>>> San Diego State University
>>>
>>>
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>>>
>>
>

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