Two consecutive question marks signify the start of a trigraph sequence. According to the C Standard, section subclause 5.2.1.1 [ISO/IEC 9899:2011],
All occurrences in a source file of the following sequences of three characters (that is, trigraph sequences) are replaced with the corresponding single character.
??=
#
??)
]
??!
|
??(
[
??'
^
??>
}
??/
\
??<
{
??-
~
Noncompliant Code Example
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| Code Block | ||||
|---|---|---|---|---|
| ||||
// What is the value of a now??/ a++; |
Compliant Solution
The following This compliant solution eliminates the accidental introduction of the trigraph by separating the question marks:
...
Inadvertent trigraphs can result in unexpected behavior. Some compilers provide options to warn when trigraphs are encountered or to disable trigraph expansion. Use the warning options, and ensure your code compiles cleanly. (See MSC00-C. Compile cleanly at high warning levels.)
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
PRE07-C | Low | Unlikely | Medium | P2 | L3 |
Automated Detection
| Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Astrée |
| trigraph | Fully checked | ||||||
| Axivion Bauhaus Suite |
| CertC-PRE07 | |||||||
|
CC2.PRE07 | Fully implemented | ||||||
| GCC |
|
Can detect violation of this recommendation when the | |||||||||
| Helix QAC |
| C3601 | |||||||
| LDRA tool suite |
| 81 S | Fully implemented |
| Parasoft C/C++test |
| CERT_C-PRE07-a | Trigraphs shall not be used | ||||||
| PC-lint Plus |
| 584, 854, 9060 | Fully supported | ||||||
| Polyspace Bug Finder |
| CERT C: Rec. PRE07-C | Checks for use of trigraphs (rec. fully covered) | ||||||
| RuleChecker |
| trigraph | Fully checked | ||||||
| SonarQube C/C++ Plugin |
| TrigraphUsage |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
| SEI CERT C++ |
| Coding Standard | VOID PRE07-CPP. Avoid using repeated question marks |
| MISRA C:2012 | Rule 4.2 (advisory) |
Bibliography
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