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

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

<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="78e8db38-cf69-4701-9472-fe255754b36d"><ac:plain-text-body><![CDATA[

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

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

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

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Noncompliant Code Example

In this non-compliant 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++;

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Noncompliant Code Example

This non-compliant 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 error */
   }
}

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 error */
   }
}

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

Implementation Details

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 MSC00-AC. Compile cleanly at high warning levels.)

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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

The LDRA tool suite V 7.6.0 is able to 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 Image Added sequences" \[Wikipedia\] ["C Trigraphs"|http://en.wikipedia.org/wiki/C_trigraph]PRE06-A. Enclose header file in an inclusion guard      01. Preprocessor (PRE)       PRE08-A. Guarantee that header filenames are unique