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Re: 20030724: netCDF

>To: address@hidden
>From: "Martin Kolleck" <address@hidden>
>Subject: netCDF
>Organization: Deutsches Zentrum fuer Luft- und Raumfahrt
>Keywords: 200307240932.h6O9W5Ld018295 netCDF

Hi Martin,

> for the application I am writing I'd like to know a little mor in depth, how
> the netCDF library works (especially for C). The main point I am considering
> is wether it is a good idea to read all the structuring information about the
> file (like information about dimensions and variables) into memory for faster
> access or if that is already done by the netCDF library. So that inquiries of
> the form nc_inq_ are obtaining their information from memory or from disk. I
> assume that the nc_get_var(...) functions are working with disk-access, but I
> am not sure wether it is the same with the nc_ing_.
> I would be glad if you could give some detailed information about this matter.

The C library reads all the metadata into memory data structures when
the file is first opened, and also on any subsequent calls to
nc_sync().  So there are no disk accesses involved in any of the
nc_inq_ calls.

I found this information in the User's Guide, although it was hard to
find (in "4.3 Reading a netCDF Dataset with Unknown Names") and not
where I would have expected it (in "9 NetCDF File Structure and
Performance"):

   All the inquire functions are inexpensive to use and require no
   I/O, since the information they provide is stored in memory when a
   netCDF dataset is first opened.

Maybe some reorganization of the documentation is in order, the next
time we revise it.

--Russ

_____________________________________________________________________

Russ Rew                                         UCAR Unidata Program
address@hidden                      http://my.unidata.ucar.edu