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Re: 980106: Support for 64bit longs in netCDF 3.x?

>To: address@hidden
>From: address@hidden (Ethan Alpert)
>Subject: Support for 64bit longs in netCDF 3.x?
>Organization: .
>Keywords: 199801061814.LAA10344

Hi Ethan,

> I remember seeing a discussion about this but I can't seem to locate any
> on-line information about this. Does netCDF support 64bit longs? Can
> you point me to the documentation for it?

No, sorry, netCDF has no support for external storage of 64-bit longs,
although it does provide interfaces in C for storing and
retrieving data from and to longs to any of the 5 numeric external
netCDF data types (8-, 16-, and 32-bit ints, 32-, 64-bit floats).
If the native long type happens to be 64-bits, then this will convert to
and from 32-bit external ints, returning an error in case you try to put
an int value that cannot be contained in 32-bits.

Since some C/C++ compilers don't yet support 64-bit ints, writing 64-bit
int values into a file would be a portability problem, because the value
couldn't be read into a scalar variable on such a platform.

If you're asking because you wonder about file offsets in huge files,
netCDF *will* work with large files if the native "off_t" offset type
happens to be 64 bits, since it uses the off_t type in the
implementation.  However, if you create such a file, it may not be
readable on platforms that don't support files larger than 4 Gbytes.

For most purposes, a double is a better type than a 64-bit long for
numeric values, since it can represent a much wider range with a very
high precision.  I'm curious what application you have for 64-bit ints
as data values in a netCDF file (digital signatures? bit sets?).

64-bit ints will probably be added as a supported external data type
with the change to the netCDF data format in an upcoming release (netCDF
4?).  By then, it may be a supported data type ("long long") in most C
compilers and the C standard.

--Russ