>To: <support@xxxxxxxxxxxxxxxx>,
>From: "Steve Loch" <sgl@xxxxxxxxxx>
>Subject: Attribute modification in Java2.1 NetCDF API
>Organization: British Oceanographic Data Centre
>Keywords: 200404141623.i3EGNSCT021650 netCDF
Hi Steve,
>I think I was overly brief in my query. If the attribute doesn't
>require additional space then it should, in my view, be possible to
>update the file without regenerating from scratch. [I have always been
>surprised that the APIs (or rather the part of the NetCDF APIs that are
>documented for public consumption) don't allow the addition of free
>space within the header at definition time which would ameliorate the
>problem. Perhaps it's the complexities of garbage collection that led to
>the simpler choice.]
Actually the C, C++, and Fortran APIs provide functions for the
addition of free space within the header at definition time, although
they are documented in the man page reference and not in the User
Guides. See the documentation for nc__enddef, nf__enddef, etc. (note
the double underbar in the function names):
int nc__enddef(int ncid, size_t h_minfree, size_t v_align,
size_t v_minfree, size_t r_align)
Like nc_enddef() but has additional performance tuning
parameters.
Caution: this function exposes internals of the netcdf
version 1 file format. It may not be available on fu-
ture netcdf implementations.
The current netcdf file format has three sections, the
"header" section, the data section for fixed size vari-
ables, and the data section for variables which have an
unlimited dimension (record variables). The header be-
gins at the beginning of the file. The (offset) of the
beginning of the other two sections is contained in the
header. Typically, there is no space between the sec-
tions. This causes copying overhead to accrue if one
wishes to change the size of the sections, as may hap-
pen when changing names of things, text attribute
values, adding attributes or adding variables. Also,
for buffered i/o, there may be advantages to aligning
sections in certain ways.
The minfree parameters allow one to control costs of
future calls to nc_redef(), nc_enddef() by requesting
that minfree bytes be available at the end of the sec-
tion. The h_minfree parameter sets the pad at the end
of the "header" section. The v_minfree parameter sets
the pad at the end of the data section for fixed size
variables.
--Russ
