New submission from Dan <dtam...@astro.princeton.edu>:
I have a C struct
typedef struct Effect {
void* ptr;
} Effect;
where when I allocate the memory, the void* gets initialized to NULL, and pass
back a pointer:
Effect* get_effect(){
Effect* pEffect = malloc(sizeof(*pEffect));
pEffect->ptr = NULL;
return pEffect;
}
In Python, I need to call the C function to initialize, and then pass a
REFERENCE to the pointer to another C function:
from ctypes import cdll, Structure, c_int, c_void_p, addressof, pointer,
POINTER, c_double, byref
clibptr = cdll.LoadLibrary("libpointers.so")
class Effect(Structure):
_fields_ = [("ptr", POINTER(c_double))]
clibptr.get_effect.restype = POINTER(Effect)
pEffect = clibptr.get_effect()
effect = pEffect.contents
clibptr.print_ptraddress(byref(effect.ptr))
But this prints an error, because effect.ptr is None, so byref(None) fails.
Below is full working code in the case where ptr is instead a double*, where
there is no problem.
As far as I can tell, there is no way to pass a c_void_p field by reference,
which would be very useful!
#include <stdio.h>
#include <stdlib.h>
#define PRINT_MSG_2SX(ARG0, ARG1) printf("From C - [%s] (%d) - [%s]: ARG0:
[%s], ARG1: 0x%016llX\n", __FILE__, __LINE__, __FUNCTION__, ARG0, (unsigned
long long)ARG1)
typedef struct Effect {
double* ptr;
} Effect;
void print_ptraddress(double** ptraddress){
PRINT_MSG_2SX("Address of Pointer:", ptraddress);
}
Effect* get_effect(){
Effect* pEffect = malloc(sizeof(*pEffect));
pEffect->ptr = NULL;
print_ptraddress(&pEffect->ptr);
return pEffect;
}
Python:
from ctypes import cdll, Structure, c_int, c_void_p, addressof, pointer,
POINTER, c_double, byref
clibptr = cdll.LoadLibrary("libpointers.so")
class Effect(Structure):
_fields_ = [("ptr", POINTER(c_double))]
clibptr.get_effect.restype = POINTER(Effect)
pEffect = clibptr.get_effect()
effect = pEffect.contents
clibptr.print_ptraddress(byref(effect.ptr))
gives matching addresses:
>From C - [pointers.c] (11) - [print_ptraddress]: ARG0: [Address of Pointer:],
>ARG1: 0x00007FC2E1AD3770 From C - [pointers.c] (11) - [print_ptraddress]:
>ARG0: [Address of Pointer:], ARG1: 0x00007FC2E1AD3770
----------
components: ctypes
messages: 330961
nosy: dtamayo
priority: normal
severity: normal
status: open
title: ctypes not possible to pass NULL c_void_p in structure by reference
type: behavior
versions: Python 3.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue35390>
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