Two consecutive question marks signify the start of a trigraph sequence.
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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.
??=
#
??)
]
??!
|
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??(
[
??'
^
??>
}
]]></ac:plain-text-body></ac:structured-macro>
??/
\
??<
{
??-
~
...