Two consecutive question marks signify the start of a trigraph sequence. According to the C Standard, 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.
??=
#
??)
]
??!
|
??(
[
??'
^
??>
}
??/
\
??<
{
??-
~
In this noncompliant code example, a++
is not executed because the trigraph sequence ??/
is replaced by \
, logically putting a++
on the same line as the comment:
// What is the value of a now??/ a++; |
This compliant solution eliminates the accidental introduction of the trigraph by separating the question marks:
// What is the value of a now? ?/ a++; |
This noncompliant code example includes the trigraph sequence ??!
, which is replaced by the character |
:
size_t i = /* Some initial value */; if (i > 9000) { if (puts("Over 9000!??!") == EOF) { /* Handle error */ } } |
This example prints Over 9000!|
if a C-compliant compiler is used.
This compliant solution uses string concatenation to concatenate the two question marks; otherwise, they are interpreted as beginning a trigraph sequence:
size_t i = /* Some initial value */; /* Assignment of i */ if (i > 9000) { if (puts("Over 9000!?""?!") == EOF) { /* Handle error */ } } |
This code prints Over 9000!??!
, as intended.
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 |
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 |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
SEI CERT C++ Coding Standard | VOID PRE07-CPP. Avoid using repeated question marks |
MISRA C:2012 | Rule 4.2 (advisory) |
[ISO/IEC 9899:2011] | Subclause 5.2.1.1, "Trigraph Sequences" |