[netCDF #FMK-929748]: checking NC_ERANGE for type conversion from schar to uchar

Wei,

> I notice a comment at line 336 in file nc_test/tst_atts3.c.
> 
> 336      /* Shouldn't this get an NC_ERANGE error for storing -128 into an 
> unsigned char?  Possible bug ... */
> 337      if (nc_get_att_uchar(ncid, NC_GLOBAL, ATT_SCHAR_NAME, uchar_in)) ERR;
> 
> The test runs without errors on this line. But shouldn't it return NC_ERANGE?
> 
> nc_get_att_uchar() traces down to ncx_getn_schar_uchar() in file libsrc/ncx.c
> at line 2201. I can see the function does not check for negative values in 
> *xpp,
> a signed char, which should cause the out of range error for an unsigned char 
> type.
> 
> Is this supposed to be a correct behavior?
> 
> (I can see the same for ncx_getn_int_uint() when X_SIZEOF_INT == SIZEOF_INT)

Here's some relevant documentation that used to be included in the
netCDF-3 C Users Guide in an appendix on type conversion in netCDF-3, 
and is also in the type conversion section of the current release 
candidate:

  http://www.unidata.ucar.edu/netcdf/docs_rc/data_type.html#type_conversion

  ... The _uchar and _schar functions were introduced in netCDF-3 to
  eliminate an ambiguity, and support both signed and unsigned byte
  data. In netCDF-2, whether the external NC_BYTE type represented
  signed or unsigned values was left up to the user. In netcdf-3, we
  treat NC_BYTE as signed for the purposes of conversion to short,
  int, long, float, or double. (Of course, no conversion takes place
  when the internal type is signed char.) In the _uchar functions, we
  treat NC_BYTE as if it were unsigned. Thus, no NC_ERANGE error can
  occur converting between NC_BYTE and unsigned char.

So I think the code is behaving as the last sentence in this 
documentation says was intended. I seem to recall that there 
were backward compatibility issues with programs that were 
converted from the old netCDF-2 API if we didn't just copy
the bits between unsigned bytes and signed bytes, rather than
checking for conversion errors ...

--Russ

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



Ticket Details
===================
Ticket ID: FMK-929748
Department: Support netCDF
Priority: Normal
Status: Closed