What constitutes a “valid” C Identifier?

Question

At @Zaibis suggestion (and related to my own answer to What are the valid characters for macro names?, as well as

Answer 1

As already mentioned, the C11 Standard defines several allowed Ranges of Unicode characters.

• 00A8, 00AA, 00AD, 00AF, 00B2−00B5, 00B7−00BA, 00BC−00BE, 00C0−00D6, 00D8−00F6, 00F8−00FF
• 0100−167F, 1681−180D, 180F−1FFF
• 200B−200D, 202A−202E, 203F−2040, 2054, 2060−206F
• 2070−218F, 2460−24FF, 2776−2793, 2C00−2DFF, 2E80−2FFF
• 3004−3007, 3021−302F, 3031−303F
• 3040−D7FF
• F900−FD3D, FD40−FDCF, FDF0−FE44, FE47−FFFD
• 10000−1FFFD, 20000−2FFFD, 30000−3FFFD, 40000−4FFFD, 50000−5FFFD, 60000−6FFFD, 70000−7FFFD, 80000−8FFFD, 90000−9FFFD, A0000−AFFFD, B0000−BFFFD, C0000−CFFFD, D0000−DFFFD, E0000−EFFFD

This also means there are several ranges of characters excluded from usage.
From your examples:

• ☎ is 260E and from the "Miscellaneous Symbols" block: 2600-26FF which means youre missing out on all of these
• ❌ is 274C and from the "Dingbats" block: 2700-27BF which is all of these but some of them are allowed (2776−2793)
• ⇧ is 21E7 and from the "Arrows " block: 2190-21FF which means youre missing out on all of these
• 〓 is 3013 and from the "CJK Symbols and Punctuation" block: 3000-303F which is all these but some of them are allowed.

Answer 2

The syntax for identifiers, which include macro names, is presented in section 6.4.2 of the C2011 standard, as interpreted in light of appendix D.1. These provisions hold that every identifier may contain underscores, upper- and lower-case Latin letters, decimal digits, sequences of characters constituting "universal character names" (subject to limitations), and any other character defined by the implementation.

Universal character names (UCNs) are Unicode escape sequences similar to those provided by Java, Python, and some other languages: they start with a backslash (\), which is followed by a u or U, and either four or eight hexadecimal digits, respectively. There are some limitations on the specific hex digit sequences that may be used, some general, others specific to identifier context. Note, however, that syntactically, the only additional character that the provision for UCNs allows to appear in identifiers is the backslash; all the other characters that can appear in a UCN are allowed in identifiers outside of UCN context, too.

Thus, speaking syntactically and restricting the discussion to the characters that the standard requires to be allowed in identifiers, the underscore, (unaccented) Latin letters, decimal digits, and the backslash are the only characters that C requires must be supported in identifiers. Support for the backslash is required only in the context of UCNs, and not all valid UCNs are allowed in identifiers. Additionally, the standard does not require support for digits as the first characters of identifiers.

On the other hand, the standard is quite liberal in allowing "other implementation-defined characters" in identifiers, including as the first character. Even decimal digits, which otherwise cannot be the first character in an identifier, could, in principle, be allowed at that position under this provision, at the discretion of the implementation. If you want your code to be portable among implementations then you will avoid relying on this provision anywhere. If you want to know which characters your particular implementation allows then you must consult its documentation.

Every standard-conforming implementation must document its behavior with respect to every detail the standard declares to be implementation defined. For example, GCC's documentation specifies that the dollar sign ($) is allowed in identifiers on most target architectures. You yourself linked to and quoted Clang's documentation of the same implementation-defined detail, which is more liberal -- it allows all the characters that can be represented in identifiers via UCNs to also be representable by UTF-8 byte sequences. In many cases, if you display or print source code containing such byte sequences, they will be rendered as a single display character.

Answer 3

As others have mentioned, Annex D of ISO/IEC 9899:2011 lists the hexadecimal values of characters valid for universal character names in C11. (I won't bother repeating it here.) I have been searching for an answer as to "why" this list was chosen.

Character set standards

First, there are two relevant standards defining a set of characters: ISO/IEC 10646 (defining UCS) and Unicode. To further confuse (or simplify) things, they both define the same characters since the ISO and Unicode keep them synchronized. UCS is essentially just a character map associating values to a set of characters ("repertoire"), while Unicode also gives further definitions such how to compare strings in an alphabetical sorting order (collation), which code points represent "canonically equivalent" characters (normalization), and a bidirectional algorithm for how to process characters in languages written right to left, and more.

Universal character names in C

Universal character names (UCN) was a feature newly added in C99 (ISO/IEC 9899:1999). In the "Rationale for International Standard---Programming Languages---C" (Rev. 2, Oct. 1999), the purpose was "to enable the use of any 'native' character in identifiers, string literals and character constants, while retaining the portability objective of C" (sec. 5.2.1). This section continues on about issues of how to encode these characters in C (the \U and \u forms versus multibyte characters or native encodings) and policy models of how to deal with it (p.14, see PDF page 22).

Rationale

I was hoping that the same "rationale" document from 1999 would give a reason of why each extended character range was selected as acceptable for C99's UCNs. The entirety of the rationale's Annex I is:

Annex I Universal character names for identifiers (normative)

A new feature of C9X.

This is not much of a rationale. They didn't even know what year the C standard would be published, so it's just called "C9X". A later rationale document from 2003 is slightly more enlightening:

Annex D Universal character names for identifiers (normative)

New feature for C99.

The intention is to keep current with ISO/IEC TR 10176.

ISO/IEC TR 10176 is "Guidelines for the preparation of programming language standards." It a basically a guidebook for people who write programming language standards. It includes guidelines for the use of character sets in programming languages as well as a "recommended extended repertoire for user-defined identifiers" (Annex A). But this quote from the 2003 rationale document is only an "intention to keep current," not a pledge of strict adherence to TR 10176.

There is a publicly available ISO/IEC TR 10176:2003 table of characters. The character values refer to ISO 10646. The table classifies ranges of characters from numerous languages as being "uppercase" Lu; "lowercase" Ll; "number, decimal digit" Nd, "punctuation, connector" Pc; etc. It should be clear what use such classifications have to a programming language.

An important reminder is that TR 10176 is a Technical Report, and not a standard. I have found several passing references to it on forums and in documents related to other programming languages, such as Ada, COBOL, and D language. Much of the discussion was about how closely standards of those languages should follow TR 10176 (not being a standard) and complaints that TR 10176 was lagging behind updates to ISO 10646.

Perhaps most enlightening is document WG21/N3146: "Recommendations for extended identifier characters for C and C++." It starts with a comment in 2010 to the standards body recommending restrictions on the initial characters of identifiers. It mentions similar complaints about C referencing TR 10176, and makes suggestions about what characters should be allowed as initial characters of an identifier based on restrictions from Unicode's Identifier and Pattern Syntax and XML's Common Syntactic Constructs. WG21/N3146 gives the proposed wording that later appeared in the C11 standard ISO/IEC 9899:2011. There is a table at the end of the document that helps shed light on the character ranges selected.

Characters allowed and not allowed in C11

Below is a compiled list of ranges for extended identifier characters. The boldface ranges are those given in C11 (ISO/IEC 9899:2011 Annex D). Some comments are added about the italicized ranges not listed in C11 (i.e. not allowed). They are either marked in WG21/N3146 as disallowed by Unicode's UAX#31 or XML's Common Syntactic Constructs, or prohibited by some other comment.

00A8, 00AA, 00AD, 00AF, 00B2-00B5, 00C0-00D6, 00D8-00F6, 00F8-00FF: (Various characters, such as feminine ª and masculine º ordinal indicators, vowels with diacritics, numeric characters such as superscript numbers, fractions, etc.)

(previous gaps): All disallowed by UAX31 and/or XML. (Generally punctuation type marks like «», monetary symbols ¥£, mathematical operators ×÷, etc.)

0100-167F: (Latin, Greek, Cyrillic, Arabic, Thai, Ethiopic, etc.---many others)

1680: "The Ogham block contains a script-specific space:  "

1681-180D: (Ogham, Tagalog, Mongolian, etc.)

180E: "The Mongolian block contains a script-specific space"

180F-1FFF: (More languages... phonetics, extended Latin & Greek, etc.)

2000: starts the "General Punctuation" block, but some are allowed:

200B−200D, 202A−202E, 203F−2040, 2054, 2060−206F: (selections from "General Punctuation" block)

2070−218F: "Superscripts and Subscripts, Currency Symbols, Combining Diacritical Marks for Symbols, Letterlike Symbols, Number Forms"

2190-245F: "Arrows, Mathematical Operators, Miscellaneous Technical, Control Pictures, Optical Character Recognition"

2460-24FF: "Enclosed Alphanumerics"

2500: starts "Box Drawing, Block Elements, Geometric Shapes", etc.

2776-2793: (some dingbats and circled dingbats)

2794-2BFF: (a different dingbat set, mathematical symbols, arrows, Braille patterns, etc.)

2C00-2DFF, 2E80-2FFF: "Glagolitic, Latin Extended-C, Coptic, Georgian Supplement, Tifinagh, Ethiopic Extended, Cyrillic Extended-A" (also CJK radical supplement)

3000: (start of "CJK Symbols and Punctuation", some selections allowed)

3004-3007, 3021-302F, 3031-303F: (allowed "CJK Symbols and Punctuation")

3040-D7FF: "Hiragana, Katakana," more CJK ideograms, radicals, etc.

D800-F8FF: (This starts the High and Low Surrogate Areas (number space needed for encodings), and Private Use)

F900-FD3D, FD40-FDCF, FDF0-FE44, FE47-FFFD: selections from "CJK Compatibility Ideographs," "Arabic Presentation Forms," etc. 10000−1FFFD, 20000−2FFFD, 30000−3FFFD, 40000−4FFFD, 50000−5FFFD, 60000−6FFFD, 70000−7FFFD, 80000−8FFFD, 90000−9FFFD, A0000−AFFFD, B0000−BFFFD, C0000−CFFFD, D0000−DFFFD, E0000−EFFFD: WG21/N3146 gives the rationale for these final ranges:

The Supplementary Private Use Area extends from F0000 through 10FFFF; both [AltId] and [XML2008] disallow characters in that range.

In addition, [AltId] disallows, as non-characters, the last two code positions of each plane, i.e. every position of the form PFFFE or PFFFF, for any value of P.

The "Ranges of characters disallowed initially" from C11 Annex D.2 are 0300−036F, 1DC0−1DFF, 20D0−20FF, FE20−FE2F.

With this WG21/N3146 placed next to the Annex D of the C11 standard, much can be inferred about how they line up. For example, mathematical operators and punctuation seem to be not allowed. I hope this sheds some light on "why" or "how" the allowed characters were chosen.

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TLDR; version

• Authoritative source for legal identifier characters is the C11 standard ISO/IEC 9899:2011 (See Annex D).
• This list is based on a technical report, ISO/IEC TR 10176, but with modifications.

Answer 4

C 2011 standard

6.4.2 Identifiers

6.4.2.1 General
...
3     Each universal character name in an identifier shall designate a character whose encoding in ISO/IEC 10646 falls into one of the ranges specified in D.1.⁷¹⁾ The initial character shall not be a universal character name designating a character whose encoding falls into one of the ranges specified in D.2. An implementation may allow multibyte characters that are not part of the basic source character set to appear in identifiers; which characters and their correspondence to universal character names is implementation-defined.
...

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^{71) On systems in which linkers cannot accept extended characters, an encoding of the universal character name may be used in forming valid external identifiers. For example, some otherwise unused character or sequence of characters may be used to encode the \u in a universal character name. Extended characters may produce a long external identifier.}
...

Annex D

(normative)

Universal character names for identifiers

1     This clause lists the hexadecimal code values that are valid in universal character names in identifiers.

D.1 Ranges of characters allowed

1     00A8, 00AA, 00AD, 00AF, 00B2−00B5, 00B7−00BA, 00BC−00BE, 00C0−00D6, 00D8−00F6, 00F8−00FF

2     0100−167F, 1681−180D, 180F−1FFF

3     200B−200D, 202A−202E, 203F−2040, 2054, 2060−206F

4     2070−218F, 2460−24FF, 2776−2793, 2C00−2DFF, 2E80−2FFF

5     3004−3007, 3021−302F, 3031−303F

6     3040−D7FF

7     F900−FD3D, FD40−FDCF, FDF0−FE44, FE47−FFFD

8     10000−1FFFD, 20000−2FFFD, 30000−3FFFD, 40000−4FFFD, 50000−5FFFD, 60000−6FFFD, 70000−7FFFD, 80000−8FFFD, 90000−9FFFD, A0000−AFFFD, B0000−BFFFD, C0000−CFFFD, D0000−DFFFD, E0000−EFFFD

D.2 Ranges of characters disallowed initially

1     0300−036F, 1DC0−1DFF, 20D0−20FF, FE20−FE2F