[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: 19980921: max_var_dims & max-dims ?

>To: address@hidden
>cc: address@hidden
>From: Paul LE TEXIER <address@hidden>
>Subject: max_var_dims & max-dims ?
>Organization: CEA/CESTA
>Keywords: 199809211702.LAA01141

Hi Paul,

>  Why in the implementation limits you fix
> for the fortran version binding (same for C & C++):
> 
> nf_max_var_dims = nf_max_dims = 100
> and
> nf_max_vars = 2000
> 
>  It seems to me that I've missed something !
>  Ok for, nf_max_var_dims = 100
>  but why, nf_max_dims = 100 ? => Only 100 dims in a DATASET !
> 
>  Maybe it should be better if:
> nf_max_dims = nf_max_vars  X  nf_max_var_dims

Usually variables in a single dataset share some of their dimensions,
for example if multiple variables are defined on the same grid.  If each
variable has dimensions that are completely disjoint from every other
variable, it might be better to store them in separate datasets.

However, I conceded that NF_MAX_DIMS may be set too low for some of the
datasets for which netCDF would be useful.  The only reason there is any
limit at all is for making it easier to write generic netCDF
applications in languages like Fortran 77 that don't permit dynamic
memory allocation.  If NF_MAX_DIMS were set to 200000, such applications
would be bloated for most uses.  But as it says in the comments in
netcdf.h:

 /*
  * These maximums are enforced by the interface, to facilitate writing
  * applications and utilities.  However, nothing is statically allocated to
  * these sizes internally.
  */
 #define NC_MAX_DIMS    100      /* max dimensions per file */
 ...

So you can set NC_MAX_DIMS in libsrc/netcdf.h and the parameter
NF_MAX_DIMS in fortran/netcdf.inc to a much higher number, if you like,
and rebuild the library.  It will still work fine, but it's possible
that if you write a netCDF dataset with more than 100 dimensions,
someone else who has the library compiled with the smaller limit may not
be able to read it without recompiling their library with the larger
limits too.

If you are using many dimensions for character array variables, you can
use fewer dimensions at the cost of some extra space in the file by
using a single maxstringlen dimension instead of a lot of custom
stringlen dimensions for each string variable.

Please let us know if you need more information.

--Russ

_____________________________________________________________________

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