Using UTF-8 Encoding in HDF5 Applications

Introduction

Text and character data are often discussed as though text means ASCII text. We even go so far as to call a file containing only ASCII text a plain text file. This works reasonably well for English (though better for American English than British English), but what if that plain text file is in French, German, Chinese, or any of several hundred other languages? This document introduces the use of UTF-8 Unicode, a much more extensive and flexible character set that can faithfully represent any of those languages.

This document assumes a working familiarity with UTF-8 Unicode (UTF-8). Any reader who is unfamiliar with UTF-8 encoding should read the Wikipedia UTF-8 article (https://en.wikipedia.org/wiki/UTF-8) before proceeding; it provides an excellent primer.

For our context, the most important UTF-8 concepts are:

Multi-byte and variable-size character encodings
Limitations of the ASCII character set
Risks associated with the use of the term plain text
Representation of multiple language alphabets or characters in a single document

More specific technical details will only become important if they affect the specifics of your application design or implementation.

How and Where May UTF-8 Be Used in HDF5?

HDF5 uses characters in object names (which are actually link names, but that’s a story for a different article), dataset data, attribute names, and attribute data. Though the mechanisms differ, you can use either ASCII or UTF-8 character sets in all of these situations.

Object and Attribute Names

By default, HDF5 creates object and attribute names with ASCII character encoding. An object or attribute creation property list setting is required to create object names with UTF-8 characters. This uses the function H5Pset_char_encoding, which sets the character encoding used for object and attribute names.

For example, the following call sequence could be used to create a dataset with its name encoded with the UTF-8 character set:

    lcpl_id = H5Pcreate(H5P_LINK_CREATE) ;
    error =   H5Pset_char_encoding(lcpl_id, H5T_CSET_UTF8) ;
    dset_id = H5Dcreate2(group_id, "datos_ñ", dtype_id, dspace_id, 
              lcpl_id, H5P_DEFAULT, H5P_DEFAULT) ;

If the character encoding of an object name is unknown, the combination of an H5Dget_create_plist call and an H5Pget_char_encoding call will reveal that information.

Character Datatypes in Datasets and Attributes

Like object names, HDF5 character data in datasets and attributes is encoded as ASCII by default. Setting up attribute or dataset character data to be UTF-8-encoded is accomplished while defining the attribute or dataset datatype. This makes use of the function H5Tset_cset, which sets the character encoding to be used in building a character datatype.

For example, the following commands could be used to create an 8-character, UTF-8 encoded, string datatype for use in either an attribute or dataset:

    utf8_8char_dtype_id = H5Tcopy(H5T_C_S1) ;
    error =    H5Tset_cset(utf8_8char_dtype_id, H5T_CSET_UTF8) ;
    error =    H5Tset_size(utf8_8char_dtype_id, "8") ;

If a character or string datatype’s character encoding is unkonwn, an H5Tget_cset call can be used to determine that.

Caveats, Pitfalls, and Things to Watch For

[ This section remains under development; it will be posted as soon as it is available. — FMB ]

Common Characters in UTF-8 and ASCII

One interesting feature of UTF-8 and ASCII is that the ASCII character set is a discrete subset of the UTF-8 character set and where they overlap, the encodings are identical. This means that a character string consisting entirely of members of the ASCII character set can be encoded in either ASCII or UTF-8, the two encodings will be indistinguishable, and the encodings will require exactly the same storage space.

For object and attibute names: `H5Pset_char_encoding` `H5Pget_char_encoding`	For dataset and attribute datatypes: `H5Tset_cset` `H5Tget_cset`
UTF-8 article on Wikipedia