NOTE: The netcdf-hdf mailing list is no longer active. The list archives are made available for historical reasons.
Hi Ed,
Bitfields are a black sheep in the datatype family and aren't terribly
well documented (which we're trying to work on). Say something if you think
we've got a terrible gap about them somewhere.
> Is there an example somewhere about using bitfields in HDF5?
Hmm, you can look in the test/dtypes.c for some examples of using them.
Search for "H5T_STD_B"...
> Or can you just tell me what functions would be used to create a
> bitfield?
The H5Tset_precision() routine determines the number of bits in a datatype
that are significant within it.
> Limits on number of bits?
Up to the size of the datatype that contains it (which is defined for up
to 64-bit datatypes currently).
> How are these stored then? Any sort of padding or what?
We currently don't pack them, so a 13-bit field in a 32-bit datatype still
takes up 4 bytes of space. Frankly, I think this is a bit of a bug, but
it's a fairly complicated problem to pack the bits on disk (in light of using
bitfields in compound, array and variable-length datatypes mostly) and noone
has whined strongly about it, so its been the status quo for a while now. :-/
Quincey
netcdf-hdf archives: