Re: Unicode question

[Hans Hagen <pragma@wxs.nl>]

On 3/12/2015 6:57 PM, Manfred Lotz wrote:
> On Thu, 12 Mar 2015 16:41:59 +0100
> Ulrike Fischer <news3@nililand.de> wrote:
>
>> Am Thu, 12 Mar 2015 08:48:27 +0100 schrieb Manfred Lotz:
>>
>>> Hi all,
>>> If I run this minimal example
>>>
>>> \starttext
>>>
>>> �
>>>
>>> \stopluacode
>>>
>>> \stoptext
>>>
>>>
>>> I get
>>>
>>> tex error       > error on line 3 in file /data/tmp/u1.tex: ! String
>>> contains an invalid utf-8 sequence
>>>
>>> and some more lines.
>>>
>>>
>>> The character above is:
>>>
>>>    Character: �
>>>     Character name: REPLACEMENT CHARACTER
>>>          Charblock: Specials
>>>           Category: Other symbol
>>>            Unicode: U+fffd
>>>               UTF8: 0xefbfbd
>>>
>>> which is a valid utf8 character.
>>>
>>> Questions:
>>>
>>> 1. Why is it considered to be invalid?
>>
>> This is not a context question/problem but related to the binary
>> (you would get the same error with lualatex or plain)
>>
>
> Yes, I know.
>
>> The luatex code contains the lines (in unistring.w)
>>
>> if (val == 0xFFFD)
>>          utf_error();
>>      return (val);
>>
>> in a function str2uni. I didn't really try to understand the code
>> but it looks as if 0xFFFD is used as "invalid marker": If luatex
>> encounters something that isn't valid utf8 it maps val to 0xFFFD and
>> then test against 0xFFFD to rise an error.
>>
>
> Took me a while to find the repository but finally I got it.
>
>
>>> 2. Are there other valid utf8 characters which are considered
>>> invalid?
>>
>> The comment in the code says
>>
>> /* the 5- and 6-byte UTF-8 sequences generate integers
>>
>> that are outside of the valid UCS range, and therefore
>>
>> unsupported
>>           */
>>
>
> Well, it is called REPLACEMENT CHARACTER, and it seems that this
> character will be used to replace invalid characters.  Then it causes
>   if (val == 0xFFFD)
>          utf_error();
>
> the error message
>      tex_error("String contains an invalid utf-8 sequence", hlp);
>
> to be displayed.
>
> Ok, this answers my question.
>
> Thanks for the pointer.

it's actually a bug ... it is ok to map an invalid character in the 
input to 0xFFFD, halt and continue when permitted, but the method used 
in luatex thereby obscures a valid 0xFFFD in the input

Hans

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