Run-time pointer NULL error in C with OGR

Question

I want to find out the geometry column and then want to export the geometry to WKB format from a shapefile. I wrote a code in C to find the geometry column. But a run-time error occurred. Can you please explain?

#include <stdio.h>
#include <stdlib.h>
#include <ogr_api.h>
#include <cpl_conv.h>
#include <cpl_port.h>
#include <gdal.h>
#include <string.h>
#include <gdal_utils.h>
#include <cpl_progress.h>
#include <cpl_minixml.h>
#include <ogr_core.h>
int main()
{
    char path1[200],filename1[]="/1.shp";
    GDALDatasetH inShapeDS;
    OGRLayerH inShapeLayer;
    OGRFeatureH inShapeFeature;

    GDALAllRegister();

    getcwd(path1, sizeof(path1));
    strcat(path1,filename1);

    inShapeDS = GDALOpenEx(path1, GDAL_OF_VECTOR, NULL, NULL, NULL );
    if( inShapeDS == NULL )
    {
        printf( "Open failed.\n" );
        exit( 1 );
    }

    inShapeLayer = GDALDatasetGetLayerByName(inShapeDS, path1);

    printf("number of layers:%d\n",GDALDatasetGetLayerCount(inShapeDS));    

    printf("geom col name:%s",OGR_L_GetGeometryColumn(inShapeLayer));
    GDALClose(inShapeDS);

    return 0;
}

Error:

number of layers:1 
ERROR 10: Pointer 'hLayer' is NULL in 'OGR_L_GetGeometryColumn'.


geom col name:(null)

Answer 1

I would guess that the name of the layer getting passed into GDALDatasetGetLayerByName is incorrect. With a shapefile the layer name is the filename without the extension. In this case the layer name should be 1.

inShapeLayer = GDALDatasetGetLayerByName(inShapeDS, "1");

It is easier to just fetch the first layer (since there will only ever be one layer in a shapefile) instead.

inShapeLayer = GDALDatasetGetLayer(inShapeDS, 0);

GDALDatasetGetLayerCount is called against the inShapeDS not the actual layer so this call will work regardless of whether the call to GDALDatasetGetLayer* returned the layer or not.