Re: 980217: Incompatibility of netcdf 2.3.2 run on OSF3.2 41 and

>To: address@hidden
>From: "Bryan C. Weare" <address@hidden>
>Subject: Re: 980217: Incompatibility of netcdf 2.3.2 run on OSF3.2 41 and 
>netcdf 3.3.1 on OSF4.0 464
>Organization: U.C. Davis
>Keywords: 199802171929.MAA13356

Bryan,

>       I don't think my problem is with getting the correct library. I have 
> copied 
> libnetcdf.a to a local directory and compiled the following program using 
>               f77 test.f libnetcdf.a
> It fails because the ncopn returns ncid as 3 not 0 as expected.

Aha!  It sounds like the library is working OK and the problem is the
assumption that "ncopn" would return some particular value.  All the
documentation says for either netcdf 2 or netcdf 3 is that the return
value from ncopn is a small integer, but your program should not in any
way depend on which small integer.  If you open files in a different
order, you will get different ncid values returned.  In netcdf 3, ncopn
doesn't return 0 as the ncid of the first opened file, it returns 3,
because ncid is now the file descriptor of the open netcdf file, and
file descriptors 0, 1, and 2, are already in use for stdin, stdout, and
stderr.  We could have tried to hide this difference, but did not think
this would be an incompatibility, because we never documented what
values of ncid meant; they were to be treated as opaque file "handles"
for use in other netcdf function calls.

>       I notice that the libnetcdf.a for version 3 is much larger than for 
> version 2. 
> Exactly how large should it be? I also notice that netcdf.inc is much larger 
> in the newer 
> version. Is there ANYTHING else that is needed for FORTRAN compilations that 
> might have 
> changed?

The library for version 3 is larger because:

 - it doesn't depend on the vendor's XDR library, but provides a
   new extended XDR implementation, improving portability and
   performance; 
 - it provides a new C interface with 134 functions, as well as
   providing backward compatibility for the old netcdf-2 C interface of
   34 functions;
 - it provides a new Fortran interface with 110 functions, as well as
   providing backward compatibility with the old Fortran interface of 41
   functions; and
 - it provides new automatic type conversion facilities, letting you
   ask for values as DOUBLE, for example, even if they are stored as
   integers or reals.

Similarly, netcdf.inc is larger because it must declare the netcdf-2
Fortran interfaces and parameters as well as all the new netcdf-3
interfaces and parameters.

It's too bad that we didn't achieve complete backward compatibility so
that you could just use old programs without even recompiling them, by
linking against the new library.  Instead, it's necessary to recompile
as well as relink to get the benefits of the better performance and
added functionality.  Your executables will also be larger than with
netcdf-2, which is another part of the trade-off.

Sorry you ran into an incompatibility with an undocumented property.
I'm also sorry this reply sounds like a lecture; I don't mean it that
way, and have made the same mistake myself (inadvertently depending on
undocumented properties of an implementation) numerous times.

--Russ

_____________________________________________________________________

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