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C99 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 eight-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name \unnnn designates the character whose four-digit short identifier is nnnn (and whose eight-digit short identifier is 0000nnnn).

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

In general, universal character names should be avoided in identifiers unless absolutely necessary. The basic character set should suffice for almost every identifier.

Noncompliant Code Example

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

#define assign(uc1, uc2, uc3, uc4, val) \
  uc1##uc2##uc3##uc4 = val;

int \U00010401\U00010401\U00010401\U00010402;
assign(\U00010401, \U00010401, \U00010401, \U00010402, 4);

Compliant Solution

This code solution is compliant.

#define assign(ucn, val) ucn = val;

int \U00010401\U00010401\U00010401\U00010402;
assign(\U00010401\U00010401\U00010401\U00010402, 4);

Risk Assessment

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

Related Vulnerabilities

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

References

\[[ISO/IEC 10646-2003|AA. C References#ISO/IEC 10646-2003]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 5.1.1.2, "Translation phases," Section 6.4.3, "Universal character names," and Section 6.10.3.3, "The ## operator"

PRE10-C. Wrap multi-statement macros in a do-while loop      01. Preprocessor (PRE)       PRE31-C. Never invoke an unsafe macro with arguments containing assignment, increment, decrement, volatile access, or function call