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Using UTF-8 Encoding in HDF5 Applications
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 encoding
(see note 1),
enabling the use of 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 and Unicode.
Any reader who is unfamiliar with UTF-8 encoding should read the
Wikipedia UTF-8 article
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 Is UTF-8 Supported in HDF5?
HDF5 uses characters in object names (which are actually link names,
but that’s a story for a different article), dataset raw data,
attribute names, and attribute raw 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
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) ;
dataset_id = H5Dcreate2(group_id, "datos_ñ", datatype_id, dataspace_id,
lcpl_id, H5P_DEFAULT, H5P_DEFAULT) ;
If the character encoding of an attribute name is unknown, the
combination of an
H5Aget_create_plist call and an
H5Pget_char_encoding call will reveal that information.
If the character encoding of an object name is unknown, the
information can be accessed through the object’s
H5L_info_t structure which can be obtained using
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
which sets the character encoding to be used in building a character
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:
datatype_id = H5Tcopy(H5T_C_S1) ;
error = H5Tset_cset(datatype_id, H5T_CSET_UTF8) ;
error = H5Tset_size(datatype_id, "8") ;
If a character or string datatype’s character encoding is unknown,
the combination of an
call and an
H5Tget_cset call can be used to determine that.
Caveats, Pitfalls, and Things to Watch For
Programmers who are accustomed to using ASCII text without accommodating
other text encodings will have to be aware of certain common issues as
they begin using UTF-8 encodings.
Since the HDF5 Library handles datatypes directly,
UTF-8 encoded text in dataset and attribute datatypes in a
well-designed HDF5 application and file should work transparently
The same should be true of handling names of groups, datasets,
committed datatypes, and attributes within a file.
Be aware, however, of system or application limitations
once data or other information has been extracted from an HDF5 file.
The application or system must be designed to accommodate
UTF-8 encodings if the information is then used elsewhere in the
application or system environment.
Data from a UTF-8 encoded HDF5 datatype, in either a dataset or an
attribute, that has been established within an HDF5 application
should “just work” within the HDF5 portions of the
Since file access is a system issue, filenames do not fall within
the scope of HDF5’s UTF-8 capabilities;
filenames are encoded at the system level.
Linux and Mac OS systems normally handle UTF-8 encoded filenames
correctly while Windows systems generally do not.
The Plain Text Illusion
Beware the use of the term plain text.
Plain text is at best ambiguous, but often misleading.
Many will assume that plain text means ASCII,
but plain text German or French, for example,
cannot be represented in ASCII.
Plain text is only unambiguous in the context of English (and even
then can be problematic!).
Programmers and data users accustomed to working strictly with
ASCII data generally make the reasonable assumption that 1 character,
be it in an object name or in data, requires 1 byte of storage.
This equation does not work when using UTF-8 or any other Unicode
With Unicode encoding, number of characters
is not synonymous with number of bytes.
One must get used to thinking in terms of number of characters
when talking about content, reserving number of bytes
for discussions of storage size.
When working with Unicode text, one can no longer assume
a 1:1 correspondence between the number of characters and
the data storage requirement.
Linux, Unix, and similar systems
generally handle UTF-8 encodings in correct and predictable ways.
There is an apparent consensus in the Linux community that
“UTF-8 is just the right way to go.”
Mac OS systems generally handle UTF-8 encodings
Windows systems use a different Unicode encoding,
UCS-2 (discussed in this
at the system level.
Within an HDF5 file and application on a Windows system,
UTF-8 encoding should work correctly and as expected.
Problems may arise, however, when that UTF-8 encoding is exposed
directly to the Windows system. For example:
- File open and close calls on files with UTF-8 encoded names
are likely to fail as the HDF5 open and close operations
interact directly with the Windows file system interface.
- Anytime an HDF5 command-line utility (
h5dump, for example) emits text output,
the Windows system must interpret the character encodings.
If that output is UTF-8 encoded, Windows will correctly
interpret only those characters in the ASCII subset of UTF-8.
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.
UTF-8 is the only Unicode
standard encoding supported in HDF5.
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