[netCDF #HRB-457979]: Error accessing netcdf file via Matlab, in particular a CF-1.6 convention file.

Hi Dan,

I spoke with our DAP developer and here's what he had to say:

````

This has come up a couple of times very recently (probably
from same original source). Anyway, I have added fixes in the dap4
code to handle this. I have not yet figured out how to deal
with this in the DAP2 code. Note that for us, we need to handle it
both in the netcdf-c library and in the thredds server.
=Dennis
p.s. In looking at the DAP2 spec, it is unclear if this is even legal.
but IMO we need to handle it anyway.
````

So, it looks like this is definitely something we should address on our end; 
thanks for the bug report.  I'll try to follow up with you when it looks like 
we have a fix in place.

-Ward

> I'm working to track down a problem that PeterC is having, trying to read
> data from NetCDF files (served via DAP) that contain 0-length arrays,
> He's using Matlab but thru their NetCDF interface.  For the record the
> problem is the same accessing the data via Hyrax or TDS.
> 
> Anyway, here's an example Hyrax URL:
> 
> http://satdat4.gso.uri.edu/opendap/ARGO/dac/aoml/15854/15854_prof.nc
> 
> and TDS URL:
> 
> http://satdat4.gso.uri.edu/thredds/dodsC/test/15854_prof.nc.html
> 
> note: the very last variables contain zero-length dimensions.
> 
> His initial workaround once we updated the server was to try to read a
> few of the valid variables, like LATITUDE, LONGITUDE.  AsciiVal works
> fine as such, but using the nc_read() in Matlab crashes hard.
> 
> So, I downloaded the latest netcdf-c api and wrote a very simple c
> program to open and try to read LATITUDE and LONGITUDE.   Opening the
> file, inquiring about the variables, and closing the works fine, but
> reading these valid variables doesn't.   This is the error
> returned:
> 
> [Dans-MBP:local/src/argo-test] dan% gcc -o prof-test argo-prof-test.c
> -I/usr/local/include -L/usr/local/lib -lnetcdf
> [Dans-MBP:local/src/argo-test] dan% ./prof-test
> dim->dim.declsize0 > 0
> 
> Assertion failed: (dappanic("dim->dim.declsize0 > 0")), function
> dapvar2projection, file constraints.c, line 721.
> Abort
> 
> It appears that the NetCDF API is testing all the variables for
> validity, not just the requested variable.
> 
> Also, if I run this against the file itself, not via the URL, it works
> fine.
> 
> We've reported the bug to MathWorks though I'm not sure how far they'll
> get, eventually they'll end up at Unidata.   Granted, files like this
> with 0-length variables are BS IMO, but if it works accessing thru the
> file-API it's hard to explain why it doesn't via DAP access.
> 
> Here's my sample program, can't get much simpler than this.
> 
> and here's that data file.
> 
> No idea why they create files with zero-length dimensioned arrays but
> the NetCDF API does not complain when accessing the file directly,
> but does when trying to retrieve data via the DAP even though we aren't
> trying to access the zero-length variables themselves.
> 
> Dan
> 
> 
> 


Ticket Details
===================
Ticket ID: HRB-457979
Department: Support netCDF
Priority: Normal
Status: Closed
===================
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