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

[Even Rouault]

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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