Re: [h5md-user] Specifying the data type

[Peter Colberg]

Hi Olaf, hi all!

Your proposal of explicitly specifying the data types of elements in
the specification is a very good idea. Before we work out the details,
I would like to add two minor remarks to the discussion, since there
seems to be some confusion over HDF5’s derived data types.

* Array

A dataset element of any data type can only be read or written
atomically. For the case of an Array data type, this means that the
element itself cannot be sliced using a hyperslab selection. The
ability to slice vectors is important for certain classes of parallel
simulations, e.g., with OpenCL, where particle coordinates are stored
in memory as 4-dimensional vectors for alignment reasons (e.g.,
cl_double3 has a size of 4×8 bytes), while only 3 components are
stored in the file.

* Variable Length

A variable length data type allows each dataset element to have a
variable size. This is not to be confused with a variable-length
dataspace, as used for time-dependent datasets. An example for
variable-length data types that implicitly appear in the specification
are variable-length strings. As mentioned earlier, I would prefer to
explicitly specify string attributes to be of variable length string
data type, which eases handling in low-level languages.

Regards,
Peter