[gdal-dev] Under what (if any) conditions would GDALWriteBlock modify data buffer?

[Fitch, Simeon]

Thanks very much for the help on this!  Greatly appreciated!



On Mon, Dec 18, 2023 at 10:38 AM Even Rouault via gdal-dev <
gdal-dev at lists.osgeo.org> wrote:

>
> Le 18/12/2023 à 16:18, Andrew C Aitchison via gdal-dev a écrit :
> > On Mon, 18 Dec 2023, Even Rouault via gdal-dev wrote:
> >
> >> Hi,
> >>
> >> interesting question. No easy answer as it is highly driver
> >> dependent. I believe that all drivers make sure that the content of
> >> the buffer before and after the call is the same, but some drivers
> >> might temporarily modify it, to do byte swapping. For example the HFA
> >> driver does that when run on big-endian hosts for non-Byte data type.
> >> I wouldn't exclude that for formats with MSB-byte ordering, a similar
> >> situation would happen for little endian hosts. So it is definitely
> >> not safe to use WriteBlock() with a buffer that would come from a
> >> read-only section of the calling program. Doc updated to reflect that
> >> in
> >>
> https://github.com/OSGeo/gdal/commit/ea321723dfc69ef3a422b1e3fe4dc9ee0832861d
> >
> > Did you mean to say
> >   Note that even with eRWFlag==GF_Write, the content of the buffer
> >   might be temporarily modified during the execution of this method
> >   (and eventually restored back to its original content), so it is not
> >   safe to use a buffer stored in a read-only section of the calling
> >   program.
>
> Yes I meant that. I've less evidence in the RasterIO(GF_Write, ...) case
> than in the WriteBlock() case, but without checking all drivers, it is
> more prudent to assume that the buffer might be touched during
> RasterIO(GF_Write) by some drivers.
>
>
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