Unfortunately this file type validator checks into at least byte 19. Is there
any way from the file metadata to calculate the size of the file? One of the
errors that seems to be blocking this file might be " Size computed did not
match size in header” is that something that can be calculated? There are
three mystery values in bytes 48 - 63 that I don’t have an explanation for. I
see no evidence of file size anywhere in the octal dump.
Kevin Havener
-----Original Message-----
From: netcdfgroup-bounces@xxxxxxxxxxxxxxxx
[mailto:netcdfgroup-bounces@xxxxxxxxxxxxxxxx] On Behalf Of Dave Allured - NOAA
Affiliate
Sent: Thursday, January 19, 2017 6:40 PM
To: netcdfgroup@xxxxxxxxxxxxxxxx
Subject: Re: [netcdfgroup] Record Dimension Question
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 <mailto: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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