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02_fill.rst_
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YYFILL with padding ------------------- In the default case (when EOF rule is not used) ``YYFILL`` is a function-like primitive that accepts a single argument and does not return any value. ``YYFILL`` invocation is triggered by condition ``(YYLIMIT - YYCURSOR) < n`` in default API and ``YYLESSTHAN(n)`` in generic API. The argument passed to ``YYFILL`` is the minimal number of characters that must be supplied. If it fails to do so, ``YYFILL`` must not return to the lexer (for that reason it is best implemented as a macro that returns from the calling function on failure). In case of a successful ``YYFILL`` invocation the limit position must be set either to one after the last input position in buffer, or to the end of ``YYMAXFILL`` padding (in case ``YYFILL`` has successfully read at least ``n`` characters, but not enough to fill the entire buffer). The pictures below show the relative locations of input positions in buffer before and after ``YYFILL`` invocation (``YYMAXFILL`` padding on the second picture is marked with ``#`` symbols). .. code-block:: text <-- shift --> <-- need --> >-A------------B---------C-----D-------E---F--------G-> buffer token marker cursor limit >-A------------B---------C-----D-------E---F--------G-> buffer, marker cursor limit token <-- shift --> <-- need --> >-A------------B---------C-----D-------E-F (EOF) buffer token marker cursor limit >-A------------B---------C-----D-------E-F############### buffer, marker cursor limit token <- YYMAXFILL -> Here is an example of a program that reads input file ``input.txt`` in chunks of 4096 bytes and uses bounds-checking with padding.