Kevin,
The information from appendix B is correct but incomplete. In netcdf-3
classic format, bytes 4-7 are "numrecs". This is a big-endian integer with
the current dimension size, i.e. number of elements, of the unlimited
dimension. For netcdf-3 files with no unlimited dimension, in other words
all fixed dimensions, numrecs is present, but the value is undefined. For
streaming files, numrecs is defined as all four bytes = FF hex.
The unlimited dimension means the same thing as the record dimension.
I recommend that you use only bytes 0-3 to identify netcdf-3 files.
You might also take a look at how format identification is done in a recent
version of the "file" utility in Linux distributions. My recent version of
"file" identifies netcdf-3 files as "NetCDF Data", and netcdf-4 files as
HDF5. My guess is that they look at only bytes 0-3 for netcdf-3, but I am
not sure.
--Dave
On Thu, Jan 19, 2017 at 2:20 PM, HAVENER, KEVIN F GS-12 USAF ACC 14 WS/WXED
<kevin.havener@xxxxxxxxx> wrote:
> I have what I am sure is a very basic question but I couldn't figure out
> how to search the archives for it, and the documentation left me befuddled.
>
> I am trying to pass a netCDF3v1 file through a virus detector-like
> software (more like a firewall-like thing) that checks for a few things to
> ascertain the file is really a netCDF3 file. The file is global lon x lat
> x time (1 time step) with 4 variables.
>
> So I've done an octal dump on the file and I'm curious about the value
> that is supposed to be in bytes 4-7, where bytes 0-3 are "C-D-F-1".
> Appendix B in the user's guide says these bytes are the numrecs=length of
> the record dimension. What is that? The unlimited dimension? My example
> file has "1" at byte 7, the example in the user's guide has 0. My
> intuition tells me that for my file, time is considered the record
> dimension, but it would also be OK to have 0 record dimensions in this file
> if I don't intend to append to it.
>
> Is my understanding correct?
>
> Kevin Havener, DAFC, 14WS/WXED
>
>
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