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
The C Standard, 184.108.40.206, 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 (220.127.116.11), 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:
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
This compliant solution uses a universal character name but does not create it by using token concatenation:
Creating a universal character name through token concatenation results in undefined behavior.
|Macro uses |
## follows # operator
|LDRA tool suite|
|CERT_C-PRE30-a||Avoid token concatenation that may produce universal character names|
|CERT C: Rule PRE30-C||Checks for universal character name from token concatenation (rule fully covered)|