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Re: NetCDF help

> To: address@hidden
> Subject: NetCDF help 
> Date: Mon, 04 Nov 96 10:32:22 -0500
> From: address@hidden

Hi Chris,

> I have what seems to be an easy problem but I can't come up with an
> effective solution.  I'm working with a C interface with netCDF and
> have an array of unsigned chars.  When I send the data to netCDF, it
> interprets them as signed chars and therefore misinterprets the data
> for larger values.
> 
> Is there an easy way to make netCDF recognize unsigned char using a C
> interface?  Thanks.

I'll need more information to help.

What version of netCDF are you using?  The latest released version
(2.4.3) doesn't support any type conversions in the interface, so
whatever 8-bit byte you store in an NC_BYTE type netCDF variable, you
will get the exact same 8 bits back when you read data from that netCDF
variable into a memory location.  If that memory location is for a
variable that's declared "unsigned char", it should be interpreted as
unsigned.

On the other hand, if you're using the netCDF-3.1a prerelease, there is
a new interface that lets you read 8-bit data as other numeric types.
For example, the function

  int nc_get_vara_int   (int ncid, int varid, const size_t *startp,
                         const size_t *countp, int *ip);

will let you retrieve a variable as an int, whether it's stored as a
byte, a short, an int, a long, a float, or a double.  In that case,
8-bit values are interpreted as signed for the purpose of any numeric
conversions that the library does.  But you can still retrieve the data
as unsigned bytes by using a different netCDF-3.1a function:

  int nc_get_vara_uchar (int ncid, int varid, const size_t *startp,
                         const size_t *countp, unsigned char *ip);

So you should be able to do what you want with either interface.  Let me
know if you still have questions about this.

--Russ

_____________________________________________________________________

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