How to reuse memory pointer of GDAL Memory driver

Question

I am doing the same as in [GDAL API]: can't save image in some formats

That is, saving a JPEG image with GDAL. I know I'd have to create a MEM buffer and then CreateCopy to JPEG. However, I'd like to avoid the allocation of two buffers since my image might be big and I use my own memory buffer.

Is there a way to pass the data memory address that I'd like to use to the MEM driver?

I've seen this mysterious article http://www.gdal.org/frmt_mem.html which references the DATAPOINTER option. I've seen another reference here which does

sprintf(filename,"MEM:::DATAPOINTER=%d,PIXELS=%d,LINES=%d,BANDS=1,DATATYPE=%d,PIXELOFFSET=0,LINEOFFSET=0,BANDOFFSET=0",datpt,pixels,lines,datatype);
dataset = GDALOpen(filename,GA_Update);

How would this translate to c++? Maybe

    papszOptions = CSLSetNameValue( papszOptions, "DATAPOINTER", datapointer );

and so on?

Answer 1

The issue was resolved in the gdal-dev mailing list, here's how:

1. create the MEM driver and create the mem image with 0 bands
2. Get the pointer to your allocated memory with CPLPrintPointer
3. Use AddBand from the dataset to add the bands

e.g. (error checking omitted)

unsigned int numbands = 0;
char **papszOptions = NULL;
GDALDriver *poDriver  = GetGDALDriverManager()->GetDriverByName("MEM");

GDALDataset* poDstDS = poDriver->Create( "dummyname",(int)width, (int)height, numbands, GDT_Byte, papszOptions);

for(unsigned int b = 0; b < 3; b++)
{
    char szPtrValue[128] = { '\0' };
    int nRet = CPLPrintPointer( szPtrValue,
                                reinterpret_cast<void*>(abyRasters[b]),
                                sizeof(szPtrValue) );
    szPtrValue[nRet] = 0;
    papszOptions = CSLSetNameValue(papszOptions,
                                   "DATAPOINTER", szPtrValue);
    poDstDS->AddBand(GDT_Byte, papszOptions);
}
CSLDestroy(papszOptions);

//jpeg write test:
{
    char **papszOptions2 = NULL;
    papszOptions2 = CSLSetNameValue( papszOptions2, "QUALITY", "40" );
    GDALDriver* jpegDriver = GetGDALDriverManager()->GetDriverByName("JPEG");
    GDALDataset* jpeg_ds = jpegDriver->CreateCopy("somename.jpg", poDstDS, false, papszOptions2, GDALDummyProgress, NULL);
    CSLDestroy(papszOptions2);
    GDALClose( (GDALDatasetH) jpeg_ds );
}

GDALClose( (GDALDatasetH) poDstDS );