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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 21:28:42 +0100

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...


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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