# RFC 37: User data callbacks in CPLError

Date:

  2011/10/25

Author:

  Howard Butler

Contact:

  hobu.inc at gmail dot com

Status:

  Implemented

Version:

  GDAL 1.9

Voting:

  +1 Frank, Howard, Tamas, Daniel, Even

Description: This RFC proposes to implement user context data in CPLErrorHandler callback functions. It does so without disrupting existing callback patterns already in use, and provides completely auxiliary functionality to CPLErrorHandler.

## Rationale

It could be argued that users could already manage user context of error handling functions with application-level globals that control its interaction. While this sentiment is technically true, this approach adds a ton of complication for library users. A scenario that has error callbacks pass back user context data means simpler code for users wishing to have the state of their application be returned along with errors from inside of GDAL.

The case for user data be passed in callbacks:

- It is a common idiom for signal-based APIs (of which CPLErrorHandler is one)

- It is simpler than requiring library users to manage the state of internal library error handling externally in their own applications

## Implementation Concerns

GDAL's (and OGR and OSR's) error handling callback mechanisms are in wide use and changes to the base library that were to break either the callback signatures or the behavior of existing callback operations should be rejected. Adding support for user data in the call back is to be provided in addition to existing functionality that already exists in the error handling, and an approach that mimics and looks similar to the existing operations is likely the best approach for GDAL -- if not the cleanest approach in general.

## Planned Changes

The first change will add a void* to CPLErrorHandlerNode:

    typedef struct errHandler
    {
        struct errHandler   *psNext;
        void                *pUserData;
        CPLErrorHandler     pfnHandler;
    } CPLErrorHandlerNode;

and to methods to add error handlers with user data will be provided:

    CPLErrorHandler CPL_DLL CPL_STDCALL CPLSetErrorHandlerEx(CPLErrorHandler, void*);
    void CPL_DLL CPL_STDCALL CPLPushErrorHandlerEx( CPLErrorHandler, void* );

`CPLSetErrorHandler` and `CPLPushErrorHandler` will simply use the `Ex` functions and pass NULL in for the pUserData member.

Finally, similar to `CPLGetLastErrorType` and `CPLGetLastErrorMsg` methods, a `CPLGetErrorHandlerUserData`

    void* CPL_STDCALL CPLGetErrorHandlerUserData(void);

### SWIG bindings consideration

The SWIG bindings will not be updated to provide access to user data for the currently active error handler for implementation of this RFC. SWIG bindings maintainers can take advantage of this new functionality at their discretion, however.

## Ticket History

http://trac.osgeo.org/gdal/ticket/4295 contains a patch that implements the proposed solution and provides context and discussion about this feature. http://trac.osgeo.org/gdal/attachment/ticket/4295/4295-hobu-rfc.patch contains the current patch to implemented the proposed functionality.

## Documentation

Documentation of the added functions is provided as part of the patch.

## Implementation

All code will be implemented in trunk by Howard Butler after passage of the RFC.