NetCDF Driver picks up zero length files

[kyngchaos@…]

When NetCDF support is built into GDAL, the Idrisi driver can't write RST files. Reading works. When using gdal_translate to write an Idrisi file (or in the gdalautotest), it gives an assertion error:

gdal_translate -of RST rgbsmall.tif asdf.rst
Input file size is 50, 50
0posixio.c:396: failed assertion `extent != 0'
Abort trap

I thought it may have something to do with OS X threads (from the posicio.c reference), but disabling threads in gdal didn't help - same error.

[warmerdam]

I have tried to reproduce this on linux with FWTools 1.1.2 unsuccessfully.
The same translation works fine. 

I will test on MacOS X...

[warmerdam]

I was able to reproduce the problem on MacOS X, though I think it is only 
occuring there because the asserts() are compiled in. 

I think the problem is that ncopen() is called on a zero length file. 

I think the appropriate solution is to modify netcdfdataset::Open() to only
try and open files with the magic code "CDF\001" at the beginning.  

[warmerdam]

This change will be a bit involved, so I am deferring it for now.

[kyngchaos@…]

A workaround I found is to reorder the loading of formats in GDALAllRegister() so that netcdf is loaded AFTER idrisi.  This only works when both are built into GDAL, instead of my practice of having netcdf as a plugin.

[kyngchaos]

Any chance of some fix making it into 1.4.1? At least the reordering in GDALAllRegister?

[warmerdam]

I have added the following check in netCDFDataset::Open() which I think will do the trick. Currently the change is only in trunk, but I'll move it into the 1.4 branch once I have a chance to add a test suite entry for netcdf.

/* -------------------------------------------------------------------- */ /* Does this appear to be a netcdf file? */ /* -------------------------------------------------------------------- */

if( !EQUALN(poOpenInfo->pszFilename,"NETCDF:",7)

&& ( poOpenInfo->nHeaderBytes < 5

!EQUALN((const char *) (poOpenInfo->pabyHeader),"CDF\001",5)))

return NULL;

[warmerdam]

Once more with format control:

/* -------------------------------------------------------------------- */
/*      Does this appear to be a netcdf file?                           */
/* -------------------------------------------------------------------- */
    if( !EQUALN(poOpenInfo->pszFilename,"NETCDF:",7)
        && ( poOpenInfo->nHeaderBytes < 5 
             || !EQUALN((const char *) (poOpenInfo->pabyHeader),"CDF\001",5)))
        return NULL;

[kyngchaos]

I put that right before "Try opening the dataset." in my 3/28 snapshot. That seems to work - no idrisi errors.

[warmerdam]

change migrated into 1.4 branch as well, and a small netcdf test script has been added to autotest/gdrivers (trunk only).