Andrew, what an interesting question! I can't answer the question, but would like to add an observation that might help finding an answer.
HDF5 attributes and datasets represent different uses of what some would refer to as array variables. What is the value of such an array variable? An (HDF5) array variable is characterized by its datatype (T) and dataspace (S). It's value is a function v: S -> T, or it's graph G(v) := [(s,t) in SxT | v(s) = t] . For H5S_NULL, S is the empty set. The graph (the value of the array variable) is also the empty set. How a particular API deals with that is another matter. In my opinion, it shouldn't be an error to read the empty set. G. -----Original Message----- From: Hdf-forum [mailto:hdf-forum-boun...@hdfgroup.org] On Behalf Of Andrew Collette Sent: Tuesday, March 19, 2013 2:32 PM To: hdf-forum@hdfgroup.org Subject: [Hdf-forum] How do people use H5S_NULL? Hi all, I am looking for information on the real-world use of the H5S_NULL dataspace flavor. My question was sparked by this discussion on the h5py mailing list: https://groups.google.com/forum/?fromgroups=#!topic/h5py/jbZmsUmDbdo We are trying to figure out what to do at a high level in Python when reading a string attribute created with H5S_NULL. Right now the two main choices are to return an empty string, or to fail with an error. Both have their merits but I thought I'd ask how other programs handle this, e.g. Java, C++, etc. In the HDF5 C interface, it's legal to perform a read from NULL -> SCALAR, but obviously nothing actually happens (the output buffer is untouched). I am particularly interested if H5S_NULL is being commonly used "in the wild" as shorthand for an empty string. Thanks! Andrew Collette _______________________________________________ Hdf-forum is for HDF software users discussion. Hdf-forum@hdfgroup.org http://mail.hdfgroup.org/mailman/listinfo/hdf-forum_hdfgroup.org _______________________________________________ Hdf-forum is for HDF software users discussion. Hdf-forum@hdfgroup.org http://mail.hdfgroup.org/mailman/listinfo/hdf-forum_hdfgroup.org