#Copy pointer to struct at address

215 messages · Page 1 of 1 (latest)

stuck nimbus
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If I have a FooStruct** at src - 4, how can I copy this to an allocation dest? I have the following, and it doesn't work:

// src is a BarStruct*
// src - 4 is a FooStruct**
// dest is a BazStruct*
*dest = *(src - 4);
bronze archBOT
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spare pawn
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you likely shouldn't copy a FooStruct* to a BarStruct*

stuck nimbus
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im keeping different datatypes in the same allocation

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src - 4 is a pointer to a FooStruct**, but src is a BarStruct*

spare pawn
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in a char* you mean ?

stuck nimbus
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yeah

spare pawn
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are you sure the -4 here is correct ?

stuck nimbus
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yes

spare pawn
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do you mean 4 char before, or 4*sizeof(FooStruct*) char before?

stuck nimbus
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4 chars before

spare pawn
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then it's not src-4

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unless src is of type char*

stuck nimbus
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i can show you what im referencing

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i need to copy the PyDictValues** to a seperate allocation

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whats wrong with this?

*((PyDictValues***) dest) = ((PyDictValues**) src) - 4;
atomic drift
stuck nimbus
atomic drift
stuck nimbus
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yes

atomic drift
stuck nimbus
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correct

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i already allocated dest somewhere else

atomic drift
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gotcha

stuck nimbus
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i just need to put the pointer in it

atomic drift
stuck nimbus
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PyObject*

atomic drift
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is it a triple PyDictValues pointer

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hm ok

stuck nimbus
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(src is also a PyObject*)

atomic drift
stuck nimbus
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this is the python c api

atomic drift
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Then why are you directly manipulating pointers src -4 the way u are

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what's your end goal

stuck nimbus
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im trying to artifically copy a PyObject* to an allocation

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which in order to do the PyObject* must have attribute values at loc - 4

atomic drift
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then ur just copying the reference you need a seperate instance

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read this

stuck nimbus
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im trying to copy the reference

atomic drift
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have you tried the Py_INCREF function

stuck nimbus
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that only increments the refcnt

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im trying to actually copy the entire pyobject

atomic drift
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u need to make a deep copy

stuck nimbus
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copy does not let me copy an object into an allocation

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is it just plain not possible to set the value from src - 4?

atomic drift
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u want to duplicate the object's content into somewhere else right

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somewhere already allocated

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u can do *dest = copiedObject

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if dest is a PyObject**

stuck nimbus
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but then dest wont include that important PyDictValues**

atomic drift
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typedef struct {
  PyObject* obj;
  PyDictValues **dictValues;
} thing;

Py_INCREF(src);
thing->obj = src;
thing->dictValues = /* Somehow obtain PyDictValues** for src, if src is a dict */;
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@stuck nimbus im not even sure what u are trying to do atp

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but this is my guess

stuck nimbus
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thing would not be a valid python object then

stuck nimbus
atomic drift
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ok

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that makes more sense

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i was a lil confused

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u would need to implement a custom constructor

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which would duplicate the contents of ur object

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but im not sure how

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u might want to ask someone more into python c api

stuck nimbus
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dest + 4 is the python object part, and dest should be the PyDictValues**

atomic drift
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no where can i find that PyDictValues is anywhere in the python c api

stuck nimbus
atomic drift
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have you tried using PyObject_SetAttrString or PyDict_Copy

stuck nimbus
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neither of those would help me here

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all i need to do is just copy that pointer to my allocation

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src - 4 to dest

atomic drift
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PyObject* src_dict = PyObject_GetAttrString(src, "__dict__");
if (src_dict && PyDict_Check(src_dict)) {
    PyObject* copied_dict = PyDict_Copy(src_dict);
    if (copied_dict) {
        PyObject* dest_obj = *((PyObject**)(dest + 4));
        PyObject_SetAttrString(dest_obj, "__dict__", copied_dict);
        Py_DECREF(copied_dict);
    }
    Py_DECREF(src_dict);
}
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i found this online asw

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an example implementation of copying (in your case)

stuck nimbus
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i dont want to copy the dict

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i want both objects to point to the same dict

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which is weird but im more concerned with whether i could than whether i should

atomic drift
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cant u just explicitly set the__dict__attribute of the second object to be the same as the first?

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or am i missing something

stuck nimbus
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__dict__ is stored under PyObject.ob_type->tp_dict

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which holds the types dict

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but the instance attributes are stored at obj - 4

atomic drift
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so do you want to copy the type dict

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the contents of

stuck nimbus
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no, the type dict is copied already

atomic drift
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just the pydictvalues?

stuck nimbus
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yup

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which needs to be at dest, and then the PyObject* is at dest + 4

atomic drift
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after thinking about it for a wihle i might be overthinking it

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have you tried using memcpy

stuck nimbus
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i did and it didnt work

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i probably messed something up though

atomic drift
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memcpy(*dest, &sourceValues, sizeof(PyDictValues*));
void *objectDest = (char*)*dest + sizeof(PyDictValues*);
memcpy(objectDest, &sourceObject, sizeof(PyObject*));
``` did you try something like this?
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where sourcevalues is ur src

stuck nimbus
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i did not

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lemme try it

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@atomic drift dest is a PyObject*, not a PyObject**

atomic drift
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oh

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hm

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char *b = (char*) dest;
memcpy(b, your_dict_values, sizeof(PyDictValues*));
memcpy(b + sizeof(PyDictValues*), &your_py_obj, sizeof(PyObject*));
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try that

stuck nimbus
atomic drift
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ye

stuck nimbus
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wouldnt that make it a triple pointer?

atomic drift
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oh its a double

stuck nimbus
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im assuming that your_py_obj shouldnt have the addressof either?

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your_py_object in my case is a PyObject*

atomic drift
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on your_dict_values

stuck nimbus
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got it

atomic drift
stuck nimbus
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ok

atomic drift
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did it work?

stuck nimbus
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hmm, src - 4 and dest are different pointers

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src - 4: 0x78027ecdaf90, dest: 0x59862056c250

atomic drift
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didn't you say they were the same

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oh u want them to be the same

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my b

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idk if u could achieve that

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i dont think u can

stuck nimbus
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why not?

atomic drift
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at least not the way i set it up

stuck nimbus
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do they both point to the same PyDictValues*?

atomic drift
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your_dict_values is copied at the start of dest, so whatever PyDictValues* it was pointing to, now a copy of that pointer is stored at dest.
your_py_obj's address is stored right after that, so you now have a pointer to your_py_obj stored next to it

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they dont both point to a pydictvalues *

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one is a pydictvalue * one is a pyobject*

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thats why im not sure u can do what u want to do

stuck nimbus
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i think youre overthinking a little

atomic drift
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you mean what its pointing too?

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because they aren't the same pointer type

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one struct could be bigger so -4 isn't guranteed

stuck nimbus
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dest should be a PyDictValues**, but dest + 4 is a PyObject*

atomic drift
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so ur saying u have an array of pydictvalues pointers and the 4th one is a pyobject *

stuck nimbus
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no

atomic drift
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you mean *(dest + 4) is a pyobject*

stuck nimbus
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yes

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or no

atomic drift
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ah

stuck nimbus
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dest + 4 itself is a PyObject*

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and PyDictValues** is at dest

atomic drift
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so what type is dest

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pydictvalues** ?

stuck nimbus
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yes

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and dest + 4 is a PyObject*

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the entire dest allocation itself contains multiple types

atomic drift
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PyDictValues **dest = array of PyDictValues pointers

stuck nimbus
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no

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its not an array

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its a pointer to a pointer to a struct

atomic drift
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so (PyObject*) (dest + 4) should be a PyObject *

stuck nimbus
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dest is just a void*, but the value at dest should be a PyDictValues**

stuck nimbus
atomic drift
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yea i mis understood u from the start then

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lol

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let me re read what u asked

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so you want to copy a pyobject * allocation to dest + 4

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correct?

stuck nimbus
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yes

atomic drift
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and you are sure dest + 4 holds a pyobject *

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if you are, then you can just do

PyObject * newObj; // whatever u want to copy over
(PyObject *) *(dest + 4) = newObj;
stuck nimbus
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and ive already implemented that

atomic drift
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sorry you need to dereference it because dest + 4 is still a double pointer

stuck nimbus
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it’s getting that PyDictValues** at dest that i can’t do

atomic drift
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*(dest + 4) is a PyObj *

atomic drift
stuck nimbus
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but i need to copy the PyDictValues** from src - 4

atomic drift
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yea because *(dest + 4) holds a PyObject*

stuck nimbus
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yup

stuck nimbus
atomic drift
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does it matter where in dest you put it?

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if dest is type void** you can just either allocate more space at the end and typecast it to PyDictValues** and directly copy the data there

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i dont know where u want to put the data specifically in dest from src-4

stuck nimbus
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it needs to be at the start

atomic drift
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so the first sizeof(PyDictValues**) bytes?

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in dest ?

stuck nimbus
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yes

atomic drift
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okay

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that can be achieved

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void **dest; (ur block of memory);
size_t newSize = sizeof(dest**) + sizeof(PyDictValues**);
PyDictValues **comesFirst; // your memory u want to come first
void** newMemory = malloc(newSize);
memcpy(newMemory, &comesFirst, sizeof(comesFirst));
memcpy((char*)newMemory + sizeof(comesFirst), dest, sizeof(dest));

dest = newMemory; // dest is still ur pointer
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@stuck nimbus try that

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this is what that is doing

stuck nimbus
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src - 4 and dest are still different pointers

atomic drift
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src is your PyDict** right

stuck nimbus
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no

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src is a PyObject*

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src - 4 is a PyDictValues**

atomic drift
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why do you need it to be in this specific order anyways?

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you want src-4 (your pydict) to be the first block of memory right? in dest

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@stuck nimbus

stuck nimbus
stuck nimbus
atomic drift
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are you doing
PyDictValues** stuff = src - 4;

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and then copying stuff over to dest

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if not try that ^

stuck nimbus
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I already tried that yeah

atomic drift
stuck nimbus
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sorry for late reply

atomic drift
stuck nimbus
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i dont, cpython does

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i just get the PyObject* later on

atomic drift
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they should be the same

atomic drift
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ill try and replicate the code give me a sec

stuck nimbus
atomic drift
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the addresses are very close but not the same

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im not sure if it's because of the type casting or what

stuck nimbus
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do they point to the same thing?

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when you dereference them i mean