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Re: [Bug-readline] rl_point, multibyte strings, and the cursor position


From: Ulf Magnusson
Subject: Re: [Bug-readline] rl_point, multibyte strings, and the cursor position
Date: Tue, 17 Feb 2015 21:30:42 +0100

On Tue, Feb 17, 2015 at 9:28 PM, Ulf Magnusson <address@hidden> wrote:
> Thanks for the feedback!
>
> The -1 comparison should be safe in practice on non-exotic systems
> where the size (rank) of size_t is at least that of int, but yeah,
> it's kinda pointless and stupid to leave out the cast.
>
> I think I'll roll the mbrtowc -2 case into the error case as wc_len <
> 0 for now. It'd be weird MB_CUR_MAX gave -2, but it's worth checking
> for at least.
>
> I added control character handling by doing the following btw:
>
> width += iswcntrl(wc) ? 2 : max(0, wcwidth(wc));
>
> Guess that might catch more characters than it should though.
>
> I also noticed that readline outputs things like "~Z" for some (meta?)
> characters. Might want to get back to that later...
>
> /Ulf

(Excuse the top-posting by the way. Gmail keeps tripping me up. :P)

/Ulf

>
> On Tue, Feb 17, 2015 at 5:44 PM, Chet Ramey <address@hidden> wrote:
>> On 2/16/15 4:52 PM, Ulf Magnusson wrote:
>>> On Mon, Feb 16, 2015 at 4:43 PM, Ulf Magnusson <address@hidden> wrote:
>>>> I'll try it. Thanks for the suggestion!
>>>>
>>>> /Ulf
>>>>
>>>
>>> Here's what I came up with in case someone else runs into the same
>>> problem. I'm sure there's more stuff to handle (not sure what to do
>>> for non-printable characters for example), but it seems to handle
>>> multibyte (tested using åäö's and Chinese) and combining characters
>>> correctly for UTF-8 at least:
>>
>> This is basically what an implementation of wcswidth looks like.  A couple
>> of suggestions:
>>
>>> // Returns the total width (in columns) of the characters in the 'n'-byte
>>> // prefix of the null-terminated multibyte string 's'. If 'n' is larger than
>>> // 's', returns the total width of the string. Suitable for calculating a
>>> // cursor position.
>>> //
>>> // Makes a guess for malformed strings.
>>> static size_t strnwidth(const char *s, size_t n) {
>>>     mbstate_t shift_state;
>>>     wchar_t wc;
>>>     size_t wc_len;
>>>     size_t width = 0;
>>>
>>>     // Start in the initial shift state.
>>>     memset(&shift_state, '\0', sizeof shift_state);
>>>
>>>     for (size_t i = 0; i < n; i += wc_len) {
>>>         // Extract the next multibyte character.
>>>         wc_len = mbrtowc(&wc, s + i, MB_CUR_MAX, &shift_state);
>>>         if (wc_len == 0)
>>>             // Reached the end of the string.
>>>             break;
>>>         if (wc_len == -1)
>>
>> wc_len is a size_t, which is usually unsigned.  You need to cast the -1
>> to (size_t)-1.  You also need to handle mbrtowc returning (size_t)-2.
>>
>>
>> --
>> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>>                  ``Ars longa, vita brevis'' - Hippocrates
>> Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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