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[netCDF #EAM-622298]: Missing: "unsigned long"

Ed,

> > > We have noticed that netCDF4 does not support the "unsigned long" datatyp
> e.
> > >
> > > The netCDF api supports the following:
> > > int nc_put_var_text  (int ncid, int varid, const char *tp);
> > > int nc_put_var_uchar (int ncid, int varid, const unsigned char *up);
> > > int nc_put_var_schar (int ncid, int varid, const signed char *cp);
> > > int nc_put_var_short (int ncid, int varid, const short *sp);
> > > int nc_put_var_int   (int ncid, int varid, const int *ip);
> > > int nc_put_var_long  (int ncid, int varid, const long *lp);
> > > int nc_put_var_float (int ncid, int varid, const float *fp);
> > > int nc_put_var_double(int ncid, int varid, const double *dp);
> > > int nc_put_var_ubyte (int ncid, int varid, const unsigned char *op);
> > > int nc_put_var_ushort(int ncid, int varid, const unsigned short *op);
> > > int nc_put_var_uint  (int ncid, int varid, const unsigned int *op);
> > > int nc_put_var_longlong (int ncid, int varid, const long long *op);
> > > int nc_put_var_ulonglong(int ncid, int varid, const unsigned long
> > > long *op);
> > > int nc_put_var_string(int ncid, int varid, const char **op);
> > > int nc_put_var       (int ncid, int varid,  const void *op);
> > >
> > > This list excludes the "unsigned long" datatype.  Is there a reason for
> > > this?  We would like to know this reason.  We want to avoid using the
> > > int datatype since we have found int to be implementation-defined: int
> > > is either 16 or 32 bits depending on the platform.
> > 
> > It looks like an oversight, and I'm surprised no one else has noticed it
> > or commented on it.  Thanks for pointing out this gap.  I'll see if it's
> > something we can fix in the C interface before the 4.1 release, but it may
> > be too late, in which case it will be in a subsequent dot release.
> > 
> > --Russ
> 
> This is a feature, not a bug!
> 
> The NC_LONG type is not a proper netCDF type at all. In fact, it is just the 
> NC_INT type. From the netcdf.h file:
> /*                                                                           
>                                                                              
>                        
>  * If and when 64 integer types become ubiquitous,                           
>                                                                              
>                        
>  * we would like to use NC_LONG for that.                                    
>                                                                              
>                        
>  * For now, define for backward compatibility.                               
>                                                                              
>                        
>  */
> #define NC_LONG NC_INT

No, Tony's question has nothing to do with NC_LONG and NC_INT, he's
asking about C functions and the C "long" type.  The C get_var and
put_var functions read and write into C types from whatever the netCDF
external types are, so there should be a function for
nc_get_vara_ulong() just as there is one for nc_get_vara_long() and even
nc_get_vara_schar() (even though there is no "NC_SCHAR" type).  The type
names used in the C functions are C types, not netCDF types.  We provide a
full set of 10 functions for the "long" data type already, and we should
provide the same set of funcions for "ulong":

  nc_get_var1_long, nc_get_var_long, nc_get_vara_long, nc_get_vars_long,
  nc_get_varm_long, nc_put_var1_long, nc_put_var_long, nc_put_vara_long,
  nc_put_vars_long, nc_put_varm_long

> This was based on the idea that C compilers would someday provide a
> 64-bit "int" type. That is, the meaning of "int" would change to a
> 64-bit, instead of a 32-bit integer, as is the case with most C
> compilers today. This may still happen. But it seems unlikely to
> me. The "long long" type is used for 64-bit integers, and that's part
> of ANSO I C too. (Of course, one could just respond that the standard
> does not actually specify that long long is 64-bits, just that it's
> twice the size of an "int".)
> 
> In theory (say the C purists), all the worlds C compilers could switch
> to 64- bit someday, and all the existing code would still work. This
> may be true for some code, but not for I/O code, since changing the C
> standard does not chan ge how many bits are used in disk archives. If
> such a change to C implementat ions were to take place, then the use
> of NC_LONG would seriously break backwa rd compatibility for netCDF
> files. All existing NC_LONG data would become unr eadable as "long"
> ints, any program that used NC_LONG or any of the nc_put|ge
> t_var*_long functions would find that their code was badly broken when
> they c hanged C compilers. (I believe *all* the int read/write
> functions of netCDF w ould break in such a case.)
> 
> In netCDF-4, the new 64-bit int types are supported with calls like
> this:
> 
> EXTERNL int nc_put_att_longlong(int ncid, int varid, const char *name,
> nc_type xtype, size_t len, const long long *op);
> 
> EXTERNL int nc_get_att_longlong(int ncid, int varid, const char *name, long 
> long *ip);
> 
> etc.

Thos are for the C "long long" type, but have nothing to do with the C
"long" type.

> As types, netcdf-4 uses the unambiguous NC_INT64 and NC_UINT64.
> 
> In fact, support for NC_LONG, required from netCDF-4 for backward compatibili
> ty, cost me considerable trouble. The problem is that the long type may be th
> e same as an int (and this is the usual case), or it may be the same as a "lo
> ng long." This is a real bummer, but if it wasn't hard they wouldn't need som
> eone like me to program it, I guess. ;-)
> 
> In summary then, there is no NC_ULONG and never shall be, because that would 
> just add confusion to the API and implementation. Users should use NC_UINT fo
> r 32-bit unsigned ints, and NC_UINT64 for 64-bit unsigned ints. NC_LONG is su
> pported only for backward compatibility reasons.

I agree there is no NC_ULONG and never will be, but that is not really
relevant to Tony's question.  The question was about support for
conversion to the C long and unsigned long type ...

--Russ



Ticket Details
===================
Ticket ID: EAM-622298
Department: Support netCDF
Priority: Normal
Status: Closed