[Gdal-dev] Error while writing to .shp file.

[Mateusz Loskot]

Vidhiyadharan Nadarajah wrote:
> Hi,
> 
> I want to add a feature to an existing polygon layer. Iam getting errors as
> follows.
> 
> ERROR 3: Error in fseek() or fwrite() writing object to .shp file.
> ERROR 4: Failure writing .shp header.

Hi,

The problem is that you're opening datasource in read-only mode.


> Please find below the code Iam using and indicate me what is wrong.
> 
> *********** start********
> 
> #include "stdafx.h"
> #include "ogrsf_frmts.h"
> 
> 
> int main()
> 
> {
> 
> 	OGRRegisterAll();
>     OGRDataSource       *poDS;
>     poDS =
> GRSFDriverRegistrar::Open( "C:\\Projects\\Test\\GDALTest\\shp\\india_ds.shp"
> , FALSE );

Note 1: There is no class called GRSFDriverRegistrar  but
OGRSFDriverRegistrar.

Note 2: Change the second parameter from FALSE to TRUE.


>     if( poDS == NULL )
>     {
>         printf( "Open failed.\n%s" );
>         exit( 1 );
>     }
> 
>     OGRLayer  *poLayer;
>     poLayer = poDS->GetLayerByName( "india_ds" );
> 
>     poLayer->ResetReading();
> 	if (NULL == poLayer)
> 	 {
> 		 printf("Layer not found\n");
> 		 OGRDataSource::DestroyDataSource(poDS);
> 		 return -1;
> 	}
> 
> 
>     OGRFeature *poFeature = new OGRFeature( poLayer->GetLayerDefn() );
> 
> 	// set the field
>     poFeature->SetField( "State", "123" );
> 
> 	OGRPoint *poPoint = new OGRPoint();


Remember to destroy the OGRPoint object pointed by poPoint
to avoid memory leak.

The member function
void OGRLineString::setPoint( int iPoint, OGRPoint * poPoint )
does not transfer ownership to point passed in.
Honestly, for better understanding this prototype should look as:
void OGRLineString::setPoint( int iPoint, const OGRPoint * poPoint )
because the poPoint is read-only inside the setPoint() function.


> 	OGRLinearRing *poLinearRing = new OGRLinearRing() ;
> 	poLinearRing->setNumPoints(4);
> 
>     poPoint->setX( 13 );
>     poPoint->setY( 64 );
> 	poLinearRing->setPoint(0,poPoint);
> 
>     poPoint->setX( 13 );
>     poPoint->setY( 66 );
> 	poLinearRing->setPoint(1,poPoint);
> 
>     poPoint->setX( 15 );
>     poPoint->setY( 66 );
> 	poLinearRing->setPoint(2,poPoint);
> 
>     poPoint->setX( 15 );
>     poPoint->setY( 64 );
> 	poLinearRing->setPoint(3,poPoint);


So, here is the deallocation:

delete poPoint;


> 	poLinearRing->closeRings();
> 
> 
> 	OGRPolygon *poOGRPolygon = new OGRPolygon();
> 	poOGRPolygon->addRingDirectly(poLinearRing);
> 
> 
>     if(poFeature->SetGeometryDirectly( poOGRPolygon ) != OGRERR_NONE )
> 	{
>        printf( "Failed to Set Geometry.\n" );
>        exit( 1 );
> 
> 	}
> 
>     if( poLayer->CreateFeature( poFeature ) != OGRERR_NONE )
>     {
>        printf( "Failed to create feature in shapefile.\n" );
>        exit( 1 );
>     }
> 
>     delete poFeature;
> 
>     OGRDataSource::DestroyDataSource( poDS );
> 
> 	return 0;
> }


Best regards
-- 
Mateusz Loskot
http://mateusz.loskot.net