Two consecutive question marks signify the start of a trigraph sequence.
According to the C99 Standard:
All occurrences in a source file of the following sequences of three characters (ie. trigraph sequences) are replaced with the corresponding single character.
??=
#
??)
]
??!
|
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="
...
338bca3f-
...
dc30-
...
4f2e-
...
8115-
...
abb0fbd444f4"><ac:plain-text-body><![CDATA[
??(
[
??'
^
??>
}
]]></ac:plain-text-body></ac:structured-macro>
??/
\
??<
{
??-
~
Non-compliant Code Example
...