The C Standard supports universal character names that may be used in identifiers, character constants, and string literals to designate characters that are not in the basic character set. The universal character name \Unnnnnnnn designates the character whose 8-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name \unnnn designates the character whose 4-digit short identifier is nnnn (and whose 8-digit short identifier is 0000nnnn).

The C Standard, 5.1.1.2, paragraph 4 [ISO/IEC 9899:2011], says

If a character sequence that matches the syntax of a universal character name is produced by token concatenation (6.10.3.3), the behavior is undefined.

See also undefined behavior 3.

In general, avoid universal character names in identifiers unless absolutely necessary.

Noncompliant Code Example

This code example is noncompliant because it produces a universal character name by token concatenation:

#define assign(uc1, uc2, val) uc1##uc2 = val

void func(void) {
  int \u0401;
  /* ... */
  assign(\u04, 01, 4);
  /* ... */
}

Implementation Details

This code compiles and runs with Microsoft Visual Studio 2013, assigning 4 to the variable as expected.

GCC 4.8.1 on Linux refuses to compile this code; it emits a diagnostic reading, "stray '\' in program," referring to the universal character fragment in the invocation of the assign macro.

Compliant Solution

This compliant solution uses a universal character name but does not create it by using token concatenation:

#define assign(ucn, val) ucn = val
 
void func(void) {
  int \u0401;
  /* ... */
  assign(\u0401, 4);
  /* ... */
}

Risk Assessment

Creating a universal character name through token concatenation results in undefined behavior.

Automated Detection

Tool	Version	Checker	Description
Astrée		universal-character-name-concatenation	Fully implemented
Axivion Bauhaus Suite		CertC-PRE30	Fully implemented
CodeSonar		LANG.PREPROC.PASTE LANG.PREPROC.PASTEHASH	Macro uses ## operator ## follows # operator
Helix QAC		C0905  C++0064,C++0080	Fully implemented
Klocwork		MISRA.DEFINE.SHARP	Fully implemented
LDRA tool suite		573 S	Fully implemented
Parasoft C/C++test		CERT_C-PRE30-a	Avoid token concatenation that may produce universal character names
Polyspace Bug Finder		CERT C: Rule PRE30-C	Checks for universal character name from token concatenation (rule fully covered)
RuleChecker		universal-character-name-concatenation	Fully checked

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography