[erlang-questions] Binary pattern matching inconsistencies with R12B

Rusty Klophaus <>
Fri Feb 29 19:00:29 CET 2008


Hi Rory,

You may want to try specifying a size for the variables D, N, and C.

For example: scan(<<C:8/integer, Rest/binary>>, TokAcc)

According to the manual: "In matching, this default value is only valid  
for the very last element. All other bit string or binary elements in the  
matching must have a size specification."  
(otp_doc_html_R12B-1/doc/reference_manual/expressions.html#6.16)

It's possible that the lack of a size is confusing things.

Hope that helps,
Rusty

-- 
Rusty Klophaus (http://rklophaus.com)


On Fri, 29 Feb 2008 12:43:44 -0500, Rory Byrne <> wrote:

> Hello
>
> I'm writing a scanner for a query language and I'm encountering
> intermittent segmentation faults and other odd errors. The
> code I'm working on appears to work fine on 11.b.2-4
> (linux/amd64), but gives problems on r12b-0 (linux/i386) and
> r12b-1 (linux/amd64). I didn't add any fancy options when I
> compiled r12b, just a --prefix.
>
> I'm an erlang newbie so highly likely I've written something
> stupid. Just hope it's obvious whatever it is!
>
> The scanner is quite large so I've reduced it down to two
> smaller programs which show similar symptoms. The first one
> just throws exceptions from time to time. The second program
> ends up dying as a result of a segmentation fault sooner
> of later.
>
> The following code won't make real sense. The full scanner
> makes sense but this is only a mutated 10% of that. Sorry
> the code is so unintelligible - but on the bright side
> it fails more frequently and predictably than the full
> scanner does.
>
>
> %% START OF CODE: weird.erl %%
>
> -module(weird).
> -compile(export_all).
>
> %% For testing - runs scanner N number of times with same input
> run(N) ->
>         lists:foreach(fun(_) ->
>                              scan(<<"region:whatever">>, [])
>                       end, lists:seq(1, N)).
>
> scan(<<>>, TokAcc) ->
>         lists:reverse(['$thats_all_folks$' | TokAcc]);
>
> scan(<<D, $\s, Rest/binary>>, TokAcc) when
>                         (D =:= $D) or (D =:= $d) ->
>         scan(Rest, ['AND' | TokAcc]);
>
> scan(<<D>>, TokAcc) when
>                         (D =:= $D) or (D =:= $d) ->
>         scan(<<>>, ['AND' | TokAcc]);
>
> scan(<<N, Z, Rest/binary>>, TokAcc) when
>                         (N =:= $N) or (N =:= $n),
>                         (Z =:= $\s)  ->
>         scan(<<Z, Rest/binary>>, ['NOT' | TokAcc]);
>
> scan(<<C, Rest/binary>>, TokAcc) when
>                                 (C >= $A) and (C =< $Z);
>                                 (C >= $a) and (C =< $z);
>                                 (C >= $0) and (C =< $9) ->
>         case Rest of
>                 <<$:, R/binary>> ->
>                         scan(R, [{'FIELD', C} | TokAcc]);
>                 _ ->
>                         scan(Rest, [{'KEYWORD', C} | TokAcc])
>         end.
>
> %% END OF CODE %%
>
>
> Here's what I see from the shell on an i386 machine:
>
> 1> c(weird).
> {ok,weird}
> 2> weird:run(1000).
> ok
> 3> weird:run(1000).
> ok
> 4> weird:run(1000).
> ok
> 5> weird:run(1000).
> ** exception error: no function clause
>                       matching weird:scan(<<"whatever">>,
>                                             [{'FIELD',110},
>                                              {'KEYWORD',111},
>                                              {'KEYWORD',105},
>                                              {'KEYWORD',103},
>                                              {'KEYWORD',101},
>                                              {'KEYWORD',114}])
>      in function  lists:foreach/2
> 6> weird:run(1000).
> ** exception error: no function clause
>                       matching weird:scan(<<"whatever">>,
>                                             [{'FIELD',110},
>                                              {'KEYWORD',111},
>                                              {'KEYWORD',105},
>                                              {'KEYWORD',103},
>                                              {'KEYWORD',101},
>                                              {'KEYWORD',114}])
>      in function  lists:foreach/2
> 7>
>
> It will then keep throwing exceptions from this point on. On an
> amd64 machine I'm getting similar output, but it usually has
> the sequence ok, error, ok, error... And if I bump it from
> 1,000 up to 10,000 iterations the errors usually stop (on amd64).
>
>
> The second block of code is:
>
> %% START OF CODE: scanner.erl %%
>
> -module(scanner).
> -compile(export_all).
>
> %% For testing - runs scanner N number of times with same input
> run(N) ->
>         lists:foreach(fun(_) ->
>                              scan(<<"region:whatever">>, [])
>                       end, lists:seq(1, N)).
>
> scan(<<>>, TokAcc) ->
>         lists:reverse(['$thats_all_folks$' | TokAcc]);
>
> scan(<<D, Z, Rest/binary>>, TokAcc) when
>                         (D =:= $D orelse D =:= $d) and
>                         ((Z =:= $\s) or (Z =:= $() or (Z =:= $))) ->
>         scan(<<Z, Rest/binary>>, ['AND' | TokAcc]);
>
> scan(<<D>>, TokAcc) when
>                         (D =:= $D) or (D =:= $d) ->
>         scan(<<>>, ['AND' | TokAcc]);
>
> scan(<<N, Z, Rest/binary>>, TokAcc) when
>                         (N =:= $N orelse N =:= $n) and
>                         ((Z =:= $\s) or (Z =:= $() or (Z =:= $))) ->
>         scan(<<Z, Rest/binary>>, ['NOT' | TokAcc]);
>
> scan(<<C, Rest/binary>>, TokAcc) when
>                                 (C >= $A) and (C =< $Z);
>                                 (C >= $a) and (C =< $z);
>                                 (C >= $0) and (C =< $9) ->
>         case Rest of
>                 <<$:, R/binary>> ->
>                         scan(R, [{'FIELD', C} | TokAcc]);
>                 _ ->
>                         scan(Rest, [{'KEYWORD', C} | TokAcc])
>         end.
>
> %% END OF CODE %%
>
>
> When I use this code in the shell (on i386) is usually works okay
> for a smaller number of iterations but when you get into the
> hundreds it dies fast:
>
> 1> c(scanner).
> {ok,scanner}
> 2> scanner:run(10).     % Start with 10
> ok
> 3> scanner:run(10).
> ok
> 4> scanner:run(100).    % Bumped up to 100
> ** exception error: no function clause
>                       matching weird:scan(<<"whatever">>,
>                                             [{'FIELD',110},
>                                              {'KEYWORD',111},
>                                              {'KEYWORD',105},
>                                              {'KEYWORD',103},
>                                              {'KEYWORD',101},
>                                              {'KEYWORD',114}])
>      in function  lists:foreach/2
> 5> scanner:run(100).
> Segmentation fault
>
>
> Anyone got any ideas?
>
> Cheers,
>
> Rory




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