Support Requests item #2407218, was opened at 2008-12-08 18:52
Message generated for change (Comment added) made by nobody
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Category: None
Group: None
Status: Closed
Priority: 5
Private: No
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: YYFill bounds checking insufficient
Initial Comment:
Looking at the generated code from re2c's examples directory, re2c calls
YYFILL(n) with an n value of at least the size of the longest possible
match. e.g.
/*!re2c
"a" { return 0; }
"b"{6} { return 1; }
*/
will generate:
if ((YYLIMIT - YYCURSOR) < 6) YYFILL(6);
If we use this value to check for end of buffer, re2c will quit before
matching "a" if it is in the last 5 characters of the buffer. If we do not
check this value, the generated code will exceed the buffer limits if it
matches 'b' in the last 5 characters, as there's no check against YYLIMIT
until another 6 bs are consumed.
It seems to me that re2c cannot generate a correct solution without an EOF
sentinel?
----------------------------------------------------------------------
Comment By: Nobody/Anonymous (nobody)
Date: 2008-12-08 21:39
Message:
with the code:
#define YYFILL(n) return 3
/*!re2c
"a" { return 0; }
"b"{6} { return 1; }
[^] { return 4; }
*/
If we try to scan "a" for 1 character, we get the incorrect result 3.
This is because YYFILL gets called when YYLIMIT - YYCURSOR < 6 before any
scanning even happens, so we'll never get a correct result for any buffers
that are smaller than the largest constant string.
----------------------------------------------------------------------
Comment By: Marcus Börger (helly)
Date: 2008-12-08 20:50
Message:
In this example you need an else rule to determine the forth case of:
1) a
2) bbbbbb
3) not enough input
4) anything else
the rule to use is:
[^] { return 4; }
Case 3 gets handled by:
#define YYFILL(n) return 3
----------------------------------------------------------------------
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