[postgis-devel] getPoint_internal dangerous?

[Björn Harrtell]

Thanks Paul, that makes it quite clear.

/Björn

2017-03-06 22:16 GMT+01:00 Paul Ramsey <pramsey at cleverelephant.ca>:

> It's no better, do feel free to write directly into the pointer. Just make
> sure you aren't doing so w/i the context of an lwgeom that has been built
> directly on top of database-allocated memory. Namely, writing into this
> lwgeom is dangerous:
>
> GSERIALIZED *gser = PG_GETARG_GSERIALIZED_P(0);
> LWGEOM *lwg = lwgeom_from_gserialized(gser);
>
> Writing into this one (lwgc) is OK:
>
> GSERIALIZED *gser = PG_GETARG_GSERIALIZED_P(0);
> LWGEOM *lwg = lwgeom_from_gserialized(gser);
> LWGEOM *lwgc = lwgeom_clone_deep(lwg);
>
> The first lwgeom is build with POINTARRAYs that point directly into the
> GSERIALIZED buffer. The second has POINTARRAYs that own their own buffers.
>
> P.
>
>
> On Mon, Mar 6, 2017 at 12:35 PM, Björn Harrtell <bjorn.harrtell at gmail.com>
> wrote:
>
>> Ok I see, I didn't realize I was this close to the database
>> storage/cache.
>>
>> But then I fail to see how a memcpy straight into the pointer of
>> a getPoint_internal can be any better? This is done all over the place.
>>
>> /Björn
>>
>> 2017-03-06 21:28 GMT+01:00 Paul Ramsey <pramsey at cleverelephant.ca>:
>>
>>> It's not a struct alignment thing (anymore) but there is a danger in
>>> mutating the point array storage directly, because in the database context
>>> it's not clear who owns it, and we've had problems in the past w/ functions
>>> that directly write into the database storage. If you're going to be
>>> changing data, the safest thing is always to take a copy at the start.
>>>
>>> P.
>>>
>>> On Mon, Mar 6, 2017 at 12:25 PM, Björn Harrtell <
>>> bjorn.harrtell at gmail.com> wrote:
>>>
>>>> I want to mutate one axis of the points in a POINTARRAY in the most
>>>> efficient way.
>>>>
>>>> Something similar to ptarray_longitude_shift, see:
>>>> https://github.com/postgis/postgis/blob/svn-trunk/liblwgeom/
>>>> ptarray.c#L1393
>>>>
>>>> I see memcpy being used there and other places and afaik it has
>>>> something to do with fears of struct misalignment.
>>>>
>>>> Does this mean I should most definitely not do this? :)
>>>>
>>>> POINT2D *p = (POINT2D *) getPoint_internal(pa, 0);
>>>> p->y = extent - p->y;
>>>>
>>>> Regards,
>>>> Björn
>>>>
>>>> _______________________________________________
>>>> postgis-devel mailing list
>>>> postgis-devel at lists.osgeo.org
>>>> https://lists.osgeo.org/mailman/listinfo/postgis-devel
>>>>
>>>
>>>
>>
>
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