Re: Return struct causes segfault

I see. So -- sorry to drag you through this -- if I create an instance of
PhotoInfo, which is a subclass of ctypes.STRUCTURE, and then pass that in
as a parameter to the C function, I could then have the C function assign
the array object to the PhotoInfo variables? But the array objects can' t
be pointers, right? The problem is, I don't know how big the array objects
will be, so I've been using vectors and then retrieving their array
pointers.

Thanks for your help,

Nicholas Earl
Graduate Research Assistant
San Diego State University


On Sun, Sep 29, 2013 at 11:17 PM, Thor Andreas Tangen <[email protected]>wrote:

> 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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