[gdal-dev] Get error handler user data when removing CPL Error Handler

[Even Rouault]

Hi,

no there's no thread safe API to do what you want. You'd need a new function

CPLErrorHandler CPLSetErrorHandlerEx2( CPLErrorHandler 
pfnErrorHandlerNew, void* pUserData, void** ppOldUserData )

to do that.

But as you mention threads that might compete to set an error handler, 
using CPLSetErrorHandlerEx() is probably not the best strategy. You'd be 
better with CPLPushErrorHandler() / CPLPopErrorHandler() that only 
affects the current thread.

Even

Le 12/09/2021 à 16:03, Rajsekar Manokaran a écrit :
> Hi,
>
> In the gdal rust bindings (https://github.com/georust/gdal 
> <https://github.com/georust/gdal>), we're trying to facilitate the use 
> of CPLSetErrorHandlerEx and related APIs.  While setting a handler, we 
> may pass a heap allocated data pointer to the second argument, which 
> is then read via the CPLGetErrorHandlerUserData in the handler and 
> passed to the user.
>
> However, while removing or setting another handler, we're unable to 
> find a race-free method to get the associated user data of the 
> previous handler.   This is needed to properly deallocate the memory.
>
> Is there an atomic way to get both the previous handler (as returned 
> by CPLSetErrorHandler), along with the associated user data?  The 
> issue with making two calls, is that another thread might make changes 
> in between the two calls.
>
> We could synchronize in our API, but it still has the same issue if 
> the user parallely used the C APIs directly or via a different interface.
>
> Relevant PR in rust gdal bindings: 
> https://github.com/georust/gdal/pull/215 
> <https://github.com/georust/gdal/pull/215>
>
> -
> Regards
>
>
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