[gdal-dev] Gdalinfo.exe is magic, why not bindings ?

[Ari Jolma]

On 11/26/2011 06:55 PM, Even Rouault wrote:
> Le samedi 26 novembre 2011 14:21:35, Florent JITIAUX a écrit :
>> Hi,
>>
>> first thanks for all tools and bidings.
>>
>> I have a question about Gdalinfo.exe and gdal binding programs.
>> I have an USRP map but with some fields errors. When i try to load it with
>> the java Gdalinfo, the open failed (the dataset is null). When i use
>> gdalinfo.exe to read the map, the error mesage is the same but after
>> gdalinfo succeeds to identify the driver and get informations about the
>> map. I found the IdentyDriver method of Gdal class which is able to read
>> the driver of my map but impossible to find a way to read informations
>> with the java binding. That's why i say gdalinfo.exe is magic.
>> That's the same with Gdalbuildvrt. With the java binding it's impossible to
>> create a vrt because the dataset is null but it works with the .exe.
> There's no magic here, but it is true that Java, Python and perhaps other
> bindings will return none/null if an error is emitted in the Open() method of
> the driver, even if it  returned a non-null dataset. There was a rationale for
> this. See http://trac.osgeo.org/gdal/changeset/11529 /
> http://trac.osgeo.org/gdal/ticket/1635

It seems to me that a datasource can be opened using a datasource 
(class) method or driver (object) method. Only the first tests for the 
case ds not null and error, but usually the error state is always 
checked after a call to GDAL internals in the bindings. The 
UseExceptions/DontUseExceptions methods can perhaps be used to change 
that (I'm not sure as I always have the UseExceptions on).

Change 11529 did not touch the case when such a dataset is opened with 
OGR_Dr_Open. That might still be a bug.

In general I don't think it is a good practice to return a valid pointer 
when there is an error, and it seems impossible to get a valid object 
when using the bindings in such a case.

Ari

>
> What should be done probably is turn the CE_Failure into a CE_Warning if the
> dataset opened by gdalinfo is readable. What is the *exact* error message you
> are seing ?
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