RFC 10: OGR Open Parameters

Author: Andrey Kiselev

Contact: dron@ak4719.spb.edu

Status: Development

Summary

It is proposed that OGRSFDriver::Open() and OGRSFDriverRegistrar::Open() calls should be changed to accept additional parameter containing arbitrary additional parameters supplied by caller. OGROpenEx() function will be introduced to map this new functionality into C interface. In addition it is proposed to add an “update” flag to OGRSFDriverRegistrar::Open() call to avoid using OGRSFDriverRegistrar::OpenShared() method.

Open parameters

Sometimes it is needed to pass additional information to OGR driver along with the name of the dataset to be opened. It can be, for example, the style table name (some drivers allow to choose from the various style tables) or any other additional data. The old method for doing this was to encode the extra info in the dataset name string. It was inconvenient approach, so it proposed to use separate parameter in OGRSFDriver::Open() and OGRSFDriverRegistrar::Open() calls representing open options, just like it is implemented in OGRDataSource::CreateLayer() call.

It is supposed that open options will be supplied in form of NAME=VALUE pairs forming the string list.

In addition to options parameter the special “shared” flag will be added to OGRSFDriverRegistrar::Open() call, so there will be no need in separate OGRSFDriverRegistrar::OpenShared() method.

Implementation

All Open() functions will be changed in the following way:

static OGRDataSource *
OGRSFDriverRegistrar::Open( const char * pszName, int bUpdate,
                            OGRSFDriver ** ppoDriver,
                int bShared = FALSE,
                char **papszOptions = NULL );
OGRDataSource *
OGRSFDriverRegistrar::OpenShared( const char * pszName, int bUpdate,
                                  OGRSFDriver ** ppoDriver )
    { return Open( pszName, bUpdate, ppoDriver, TRUE, NULL ); }
virtual OGRDataSource
OGRSFDriver::*Open( const char *pszName, int bUpdate=FALSE,
                    char **papszOptions = NULL ) = 0;

The last change needs to be propagated in all OGR drivers. The change itself is pretty simple: one additional parameter should be added to function definition. But it has impact on third-party OGR drivers: they are not source compatible anymore and should be changed too.

Also appropriate C functions will be added:

OGRDataSourceH OGROpenEx( const char *pszName, int bUpdate,
                          OGRSFDriverH *pahDriverList,
              int bShared, char **papszOptions );
OGRDataSourceH OGR_Dr_OpenEx( OGRSFDriverH hDriver, const char *pszName,
                              int bUpdate, char **papszOptions );

New Options for OGR Utilities

Proposed functionality will be available in OGR utilities ogr2ogr and ogrinfo via the ‘-doo NAME=VALUE’ (“Datasource Open Option”) format specific parameter.

Backward Compatibility

Proposed additions will not have any impact on C binary compatibility. C++ binary interface will be broken, source level compatibility will be broken for third-party OGR drivers only. There will be no impact for high-level applications on source level.

Responsibility and Timeline

Andrey Kiselev is responsible to implement this proposal. New API will be available in GDAL 1.5.0.