Two consecutive question marks signify the start of a trigraph sequence. According to the C Standard, section 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.
??=
#
??)
]
??!
|
??(
[
??'
^
??>
}
??/
\
??<
{
??-
~
...
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
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Bibliography
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| ] | Section 5.2.1.1, "Trigraph |
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| Sequences" |
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