[gdal-dev] the /vsistdout mistery and the Update question

[Even Rouault]

Abel,

Le 23/02/2024 à 10:34, Abel Pau via gdal-dev a écrit :
>
> Hi,
>
> after reading 
> https://gdal.org/user/virtual_file_systems.html#vsistdout-standard-output-streaming 
> <https://gdal.org/user/virtual_file_systems.html#vsistdout-standard-output-streaming>I 
> have to say I don’t understand what it means.
>
> I which cases it’s used? Some example to consider?
>
/vsistdout/ also makes sense for formats that can be streamed in 
writing, that is you can generate the content as data flows in the 
driver and don't have to seek back to edit something in the header. So 
only a small subset of formats/drivers can natively support that. The 
canonical example is a CSV file. So I'd say don't worry about that use 
case for your driver.

> On the other hand, I am confused about the Update concept.
>
> My driver is capable of creating from zero a layer from other formats.
>
> But I haven’t programmed the capacity to insert a feature in a 
> pre-existent layer (I cannot simply add at the end of the file because 
> de format description is a little more complicated).
>
Same thing as above. A lot of drivers can only create + add features in 
the newly features, but can't (at least easily) edit or append to an 
existing file. So just return nullptr when Open() is called with the 
update flag with a CPLError() message.

Something like

     if (poDS != nullptr && poOpenInfo->eAccess == GA_Update)
     {
         CPLError(CE_Failure, CPLE_OpenFailed,
                  "SDTS Driver doesn't support update.");
         delete poDS;
         poDS = nullptr;
     }

You have been in a "development black hole" for a long time now, so I'd 
suggest you limit the functionality to what you've already implemented, 
and make it in good shape for review. The more code, the harder & longer 
effort for reviewers. You can always add new functionality in follow-up 
pull requests once a base has already been integrated.

Even

-- 

http://www.spatialys.com
My software is free, but my time generally not.
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