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From: Rich Felker <dalias@libc.org>
Newsgroups: gmane.linux.lib.musl.general
Subject: Re: [PATCH] fix fgetwc when decoding a character that crosses
 buffer boundary
Date: Sat, 18 Nov 2017 20:12:57 -0500
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On Sat, Nov 18, 2017 at 05:51:48PM +0100, Szabolcs Nagy wrote:
> Update the buffer position according to the bytes consumed into st when
> decoding an incomplete character at the end of the buffer.
> ---
>  src/stdio/fgetwc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/src/stdio/fgetwc.c b/src/stdio/fgetwc.c
> index e455cfec..a00c1a86 100644
> --- a/src/stdio/fgetwc.c
> +++ b/src/stdio/fgetwc.c
> @@ -15,20 +15,21 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
>  	if (f->rpos < f->rend) {
>  		l = mbrtowc(&wc, (void *)f->rpos, f->rend - f->rpos, &st);
>  		if (l+2 >= 2) {
>  			f->rpos += l + !l; /* l==0 means 1 byte, null */
>  			return wc;
>  		}
>  		if (l == -1) {
>  			f->rpos++;
>  			return WEOF;
>  		}
> +		f->rpos = f->rend;
>  	} else l = -2;

Thanks, applying! Here is a test case that demonstrates the bug
reproducibly; feel free to adapt (it should probably use the framework
functions for getting a utf-8 locale and error reporting) & include it
in libc-test.

Rich

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#include <stdio.h>
#include <poll.h>
#include <locale.h>
#include <wchar.h>
#include <unistd.h>

int main()
{
	setlocale(LC_CTYPE, "");
	int p[2];
	pipe(p);
	write(p[1], "x\340\240", 3);
	dup2(p[0], 0);
	wchar_t wc;
	wc = fgetwc(stdin);
	write(p[1], "\200", 1);
	close(p[1]);
	wc = fgetwc(stdin);
	printf("got %x\n", wc);
}

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