Hi Sameh, HDF5WRITE only writes out fixed-length strings, as you have noticed. It's also been deprecated in favor of a new high-level function called H5WRITE, but it only supports simple numeric types because there are just too many permutations of HDF5 datatypes to try to cover with one function.
In order to create variable-length string datasets, I would suggest looking at the H5T package, as this will give you the flexibility you need. The HDF Group has posted example MATLAB codes on their website that may help you as well, please see http://www.hdfgroup.org/ftp/HDF5/examples/examples-by-api/api18-m.html and look for the variable string example at the bottom.
