PRE30-C. Do not create a universal character name through concatenation

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.

Non-Compliant Code Example

This code example is non-compliant 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 will result in undefined behavior.

Examples of vulnerabilities resulting from the violation of this rule can be found on the CERT website.

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"