[gdal-dev] VSIGetMemFileBuffer Returns NULL When Copying Vector Dataset to Vsimem

[Even Rouault]

Hi,

size_t is an incorrect size for DataLength (if you run on 32 bit 
architecture). You should use vsi_l_offset.

Besides that the following standalone inspired from your code works fine 
for me:

#include <stdio.h>
#include "gdal_priv.h"

int main()
{
     GDALAllRegister();
     GDALDataset* Dataset 
=GDALDataset::FromHandle(GDALOpenEx("poly.shp", GDAL_OF_VECTOR | 
GDAL_OF_VERBOSE_ERROR, nullptr, nullptr, nullptr));
     GDALDataset* Copy = 
Dataset->GetDriver()->CreateCopy("/vsimem/foo.shp", Dataset, false, 
nullptr, nullptr, nullptr); //appears to produce a valid Dataset 
containing the same layers & features as its source.
     GDALClose(Dataset);
     GDALClose(Copy);
     vsi_l_offset DataLength = 0;
     GByte* Buffer = VSIGetMemFileBuffer("/vsimem/foo.shp", &DataLength, 
true);
     printf("%p %d\n", Buffer, int(DataLength)); // buffer not null and 
DataLength > 0
     CPLFree(Buffer);
     VSIUnlink("/vsimem/foo.dbf");
     VSIUnlink("/vsimem/foo.shx");
     return 0;
}

Even

Le 04/10/2023 à 06:26, Soren Saville Scott via gdal-dev a écrit :
> Hi,
>
> I'm integrating GDAL in C++ with a system that defines its own asset 
> types stored on disk, and am writing a wrapper for datasets using this 
> asset system. To 'import' a dataset I call GDALOpenEx on it, 
> CreateCopy() it to some "/vsimem/foo.shp" path, GDALClose() the 
> datasets to flush it, and call VSIGetMemFileBuffer with 
> bUnlinkAndSeize=true to take ownership of the underlying memory buffer.
>
> GDALDataset* Dataset = GDALOpenEx("foo.shp", GDAL_OF_VECTOR | 
> GDAL_OF_VERBOSE_ERROR, nullptr, nullptr, nullptr);
> GDALDataset* Copy = 
> Dataset->GetDriver()->CreateCopy("/vsimem/foo.shp", Dataset, false, 
> nullptr, nullptr, nullptr); //appears to produce a valid Dataset 
> containing the same layers & features as its source.
> GDALClose(Dataset);
> GDALClose(Copy);
> size_t DataLength;
> GByte* Buffer = VSIGetMemFileBuffer("/vsimem/foo.shp", &DataLength, 
> true); //<-- returns NULL
>
> For some reason VSIGetMemFileBuffer always fails, returning a nullptr. 
> This is basically identical to a similar problem I had, described 
> here: https://stackoverflow.com/questions/73644157/get-raw-buffer-for-in-memory-dataset-in-gdal-c-api 
> <https://stackoverflow.com/questions/73644157/get-raw-buffer-for-in-memory-dataset-in-gdal-c-api> which 
> I never really found a solution to. I have similar issues with other 
> VSI functions, e.g. calling GDALOpenEx on a path created by 
> VSIFileFromMemBuffer also fails.
>
> Any idea as to what the problem is here?
>
> Alternatively, maybe there's a better way to 'emplace' datasets inside 
> some other file?
>
> Thanks,
> Soren Saville Scott
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/gdal-dev

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