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Two consecutive question marks signify the start of a trigraph sequence.

 According to the C99 Standard \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]:

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="c5964e890ff4349d-ce3d7147-45c248bd-89e786a6-02c5fcdeba698cd52b1000e3"><ac:plain-text-body><![CDATA[	??(	[		??'	^		??>	}	]]></ac:plain-text-body></ac:structured-macro>
??/	\		??<	{		??-	~

...

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

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

...

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

This example prints Over 9000!| if a C99-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 */;
/* assignment of i */
if (i > 9000) {
   if (puts("Over 9000!?""?!") == EOF) {
     /* Handle Error */
   }
}

...

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).

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