[gdal-dev] Catch a GDALOpen exception

[Even Rouault]

> This is interesting. Could you give example?
> How is checking if GDALOpen returned NULL and prematurely exiting
> different to throwing exception?

Mateusz,

The ::Open() method of GDAL drivers is supposed to make all necessary 
cleanups, even in case of errors, before returning NULL.

But if you install a custom CPL error handler that throws a C++ exception, and 
a CPLError() is emitted in GDAL driver code, it will directly unwind to the 
matching catch( xx ) { } in your code. This might not be a big deal in most 
situations, but in theory, there could be cases where some cleanups haven't 
been yet done.

... After a few seconds of digging, here's an example (and I'm sure there are 
many other similar cases). Let's look at the Open() method of the BMP driver

You have :

    if  (poDS->nRasterXSize <= 0 || poDS->nRasterYSize <= 0)
    {
        CPLError( CE_Failure, CPLE_AppDefined, 
                  "Invalid dimensions : %d x %d", 
                  poDS->nRasterXSize, poDS->nRasterYSize); 
        delete poDS;
        return NULL;
    }

So if in your custom error handler you throw a C++ exception, the dataset will 
not be destroyed, which means both a memory leak and a file descriptor leak in 
that instance.

> 
> It should be possible to react on GDAL errors and throw exception
> as long as certain cleanup is performed.
> 
> > (there's no guarantee that the GDAL error is emitted
> > before the cleanup of ressources has been done).
> 
> What error emission you mean? Returning NULL? Call to abort()?

An error emitted by CPLError().

> 
> I have written code intensively throwing exceptions around GDAL,
> even CPLErr is used one of exception type. Code is well-tested
> and I haven't noticed problems.
> 
> Hobu's solution might be incomplete, but gives rough idea which is valid
> if used properly. In any case, details are needed to be able to judge.
> 
> Best regards,