Open up lex.l and scroll down to the defs of yycharno (well, they're on the
first screeful, whatever).

Do what happens in the lex input macro in the flex YY_INPUT macro.

Init things like yylinestartpos in YY_USER_INIT macro.  Specifically, init each
of the following:
    yycharno
    yycharpos
    yycharposnnl
    yylinestartpos
    yyprevlinestartpos
    yyfilename
    inAttribute
    inParts

See what fur flies.  Presumably there are some lex-specific functions further
down that may need to be tweaked.