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Re: 20050302: netCDF General - Large file problem

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  • Subject: Re: 20050302: netCDF General - Large file problem
  • Date: Thu, 03 Mar 2005 16:42:22 -0700
>To: address@hidden
>From: "Jim Cowie" <address@hidden>
>Subject: netCDF General - Large file problem
>Organization: RAL
>Keywords: 200503022003.j22K3ZjW025634

Jim,

> In a curious twist of fate, it turns out that the application
> reading these last variables did not need that data after all, so
> I don't have a bunch of data to recover, which is good. And
> actually, the data is recoverable in the sense that I could
> regenerate it from the source without too much difficulty. I'm
> a little reluctant to switch to 3.6 due to the backward
> compatibility with existing (3.5.x) apps, even if the code
> changes are small. (I am more interested in your version 4.)
> Anyway, at least I know what the problem is and there are
> some options. Thanks a bunch,

You're welcome!  I'm really glad you don't have to do data recovery,
because that saves me work, too.

Just to be clear about the backward compatibility issues, any
programs you have that link with 3.6 libraries will produce "classic"
netCDF files that are compatible with existing applications.  The only
way you can generate a netCDF file in the new 64-bit offset format is
by going out of your way to add an extra (non-default) flag on the
nc_create() call.  You can safely use 3.6 without using that flag and
be assured you won't generate incompatible files, with one minor
exception discussed in the FAQ:

  Is it possible to create a "classic" format netCDF file with netCDF
  version 3.6.0 that cannot be accessed by applications compiled and
  linked against earlier versions of the library?

  No, classic files created with the new library should be compatible
  with all older applications, both for reading and writing, with one
  minor exception. The exception is due to a correction of a netCDF
  bug that prevented creating records larger than 4 GiB in classic
  netCDF files with software linked against versions 3.5.1 and
  earlier. This limitation in total record size was not a limitation
  of the classic format, but an unnecessary restriction due to the use
  of too small a type in an internal data structure in the library. If
  you want to always make sure your classic netCDF files are readable
  by older applications, make sure you don't exceed 4 GiBytes for the
  total size of a record's worth of data. (All records are the same
  size, computed by adding the size for a record's worth of each
  record variable, with suitable padding to make sure each record
  begins on a byte boundary divisible by 4.)

--Russ