Technical Reports

Unicode Standard Annex #19

UTF-32

Summary

This document specifies a Unicode transformation format that serializes a Unicode codepoint (from U+0000 to U+10FFFF) as a sequence of four bytes.

Status

This document has been reviewed by Unicode members and other interested parties, and has been approved by the Unicode Technical Committee as a Unicode Standard Annex. It is a stable document and may be used as reference material or cited as a normative reference from another document.

A Unicode Standard Annex (UAX) forms an integral part of the Unicode Standard, carrying the same version number, but is published as a separate document. Note that conformance to a version of the Unicode Standard includes conformance to its Unicode Standard Annexes.

A list of current Unicode Technical Reports is found on http://www.unicode.org/unicode/reports/. For more information about versions of the Unicode Standard, see http://www.unicode.org/unicode/standard/versions/.

The References provide related information that is useful in understanding this document. Please mail corrigenda and other comments to the author(s).

Contents

• 1 Introduction
• 2 Definitions
• 3 Relation to ISO/IEC 10646 and UCS-4
• References
• Modifications

1 Introduction

UTF-32 defines an encoding form for Unicode for representing Unicode code points with single 32-bit code units. With the addition of UTF-32, the Unicode Standard now has three sanctioned encoding forms: UTF-8, UTF-16, and UTF-32. These use 8-bit, 16-bit, and 32-bit code units, respectively.

Different encoding forms of Unicode are useful in different system environments. For example, UTF-32 is somewhat simpler in usage than UTF-16, but in almost all cases occupies twice the storage. A common strategy is to have internal string storage use UTF-16 or UTF-8, but to use UTF-32 for individual character datatypes.

Considerations of byte-order serialization lead to a further subdivision of this encoding form into 3 encoding schemes: UTF-32 (possibly using BOM), UTF-32BE, and UTF-32LE.

The following lists the important features of this encoding form:

• UTF-32 is restricted in values to the range 0..10FFFF₁₆, which precisely matches the range of characters defined in the Unicode Standard (and other standards such as XML), and those representable by UTF-8 and UTF-16.
• Over and above ISO 10646, the Unicode Standard adds a number of conformance constraints on character semantics (see The Unicode Standard, Version 3.0, Chapter 3). Declaring UTF-32 instead of UCS-4 allows implementations to explicitly commit to Unicode semantics.
• The term UTF-32 is parallel to UTF-16 and UTF-8, avoiding confusion among software developers — especially since the pronunciations of "UTF" and "UCS" are so very similar.

The code units for UTF-32 correspond exactly to Unicode code points. Compare this to UTF-8, where only for the ASCII repertoire (U+0000..U+007F) do code units numerically correspond to code points. In terms of frequency of use, the situation for UTF-16 is essentially the same as UTF-32. For the vast majority of computer text in the world, UTF-16 code units correspond to code points, since the frequency of characters with code points above U+FFFF is and will remain vanishingly small.

In any event, however, Unicode code points do not necessarily match user-expectations for "characters". For example, the following are not represented by a single code point: a combining character sequences such as <g, acute>; a conjoining jamo sequence; or the Devanagari conjunct "ksha". These are better matched by grapheme boundaries, as explained in Chapter 5, Implementation Guidelines and in UTR #18: Unicode Regular Expression Guidelines.

For a discussion of the general framework for understanding the Unicode character encoding and its relationship to the Unicode Transformation Formats, see UTR #17, Character Encoding Model [CharMod].

2 Definitions

The following define the UTF-32 Transformation Formats. Note that these rely on the conformance modifications introduced in Unicode 3.1 [U3.1].

3 Relation to ISO/IEC 10646 and UCS-4

ISO/IEC 10646 defines a 4-byte encoding form called UCS-4. Since UTF-32 is simply a subset of UCS-4 characters, it is conformant to ISO/IEC 10646 as well as to the Unicode Standard.

As of the recent publication of the second edition of ISO/IEC 10646-1, UCS-4 still assigns private use codepoints (E00000₁₆..FFFFFF₁₆ and 60000000₁₆..7FFFFFFF₁₆) that are not in the range of valid Unicode codepoints. To promote interoperability among the Unicode encoding forms JTC1/SC2/WG2 has approved a motion removing those private use assignments:

Resolution M38.6 (Restriction of encoding space) [adopted unanimously]

"WG2 accepts the proposal in document N2175 towards removing the provision for Private Use Groups and Planes beyond Plane 16 in ISO/IEC 10646, to ensure internal consistency in the standard between UCS-4, UTF-8 and UTF-16 encoding formats, and instructs its project editor [to] prepare suitable text for processing as a future Technical Corrigendum or an Amendment to 10646-1:2000."

While this resolution must still be approved as an Amendment to 10646-1:2000, the Unicode Technical Committee has every expectation that once the text for that Amendment completes its formal balloting it will proceed smoothly to publication as part of that standard.

Until the formal balloting is concluded, the term UTF-32 can be used to refer to the subset of UCS-4 characters that are in the range of valid Unicode code points. After it passes, UTF-32 will then simply be an alias for UCS-4 (with the extra requirement that Unicode semantics are observed).

References

Modifications

The following summarizes modifications from the previous version of this document.

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