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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="c7613a4a-0416-4a86-addd-73e8ebeb69b2"><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.

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

Automated Detection

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

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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" \[[MISRA 04|AA. C References#MISRA 04]\] Rule 4.2PRE06-A. Enclose header files in an inclusion guard      01. Preprocessor (PRE)       PRE08-A. Guarantee that header file names are unique