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Two consecutive question marks signify the start of a trigraph sequence. Wiki Markup According to the C99 Standard \[C Standard, subclause 5.2.1.1 [ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]: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.

??=	#		??)	]		??!	|

<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="2433d87e-f025-4468-8040-5a37743f145e"><ac:plain-text-body><![CDATA[

??(	[		??'	^		??>	}

]]></ac:plain-text-body></ac:structured-macro>

??/	\		??<	{		??-	~

Noncompliant Code Example

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.:

// whatWhat is the value of a now??/
a++;

Compliant Solution

The following This compliant solution eliminates the accidental introduction of the trigraph by separating the ?'squestion marks:

// whatWhat is the value of a now? ?/
a++;

Noncompliant Code Example

This noncompliant code example includes the trigraph sequence ??!, which is replaced by the character |.:

size_t i;
 = /* assignmentSome ofinitial ivalue */;
if (i > 9000) {
   if (puts("Over 9000!??!") == EOF) {
     /* Handle Errorerror */
   }
}

This example prints Over 9000!| if a C99C-compliant compiler is used.

Compliant Solution

The 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 */;
/* assignmentAssignment of i */
if (i > 9000) {
   if (puts("Over 9000!?""?!") == EOF) {
     /* Handle Errorerror */
   }
}

The above code This code prints Over 9000!??!, as intended.

Risk Assessment

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 See MSC00-C. Compile cleanly at high warning levels.).

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	trigraph	Fully checked
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-PRE07
ECLAIR	Include Page ECLAIR_V ECLAIR_V	CC2.PRE07	Fully implemented
GCC	Include Page GCC_V GCC_V		Can detect violation of this recommendation when the -Wtrigraphs flag is used
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C3601
LDRA tool suite	Include Page LDRA_V LDRA_V	81 S	Fully implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-PRE07-a	Trigraphs shall not be used
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	584, 854, 9060	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. PRE07-C	Checks for use of trigraphs (rec. fully covered)
RuleChecker	Include Page RuleChecker_V RuleChecker_V	trigraph	Fully checked
SonarQube C/C++ Plugin	Include Page SonarQube C/C++ Plugin_V SonarQube C/C++ Plugin_V	TrigraphUsage

GCC provides a -Wtrigraphs option that warns when trigraphs are used. GCC also provides a --no-trigraph option that is enabled by default.

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added sequences" \[[MISRA 04|AA. C References#MISRA 04]\] Rule 4.2PRE06-C. Enclose header files in an inclusion guard - CERT Secure Coding Standards      01. Preprocessor (PRE)       Image Modified