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

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 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).

Subclause 5.1.1.2, paragraph 4, of the C Standard [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 of Annex J.)

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, 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 complains of a "stray \," referring to the universal character fragment in the invocation of the assign macro.

Compliant Solution

This code solution is compliant because it does not form an identifier through concatenation involving a universal character name:

#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

Related Vulnerabilities

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

Related Guidelines

Bibliography