[postgis-devel] getPoint_internal dangerous?

[Paul Ramsey]

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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