[musl] [PATCH] fgetwc: fix errno when character span aver buffer end

[musl] [PATCH] fgetwc: fix errno when character span aver buffer end

[Pinghao Wu]

If attempt on converting character from buffer failed, errno is set to
EILSEQ by mbtowc. As a result, if further byte-by-byte conversion
succeeds, fgetwc will return a valid wchar with a misleading EILSEQ as
errno. This fixes it by saving errno before the from buffer attempt, and
restore if it fails.

This also fixes fgetws which find the misleading EILSEQ and fails in
this case.
---
 src/stdio/fgetwc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/stdio/fgetwc.c b/src/stdio/fgetwc.c
index aa10b818..9dad0505 100644
--- a/src/stdio/fgetwc.c
+++ b/src/stdio/fgetwc.c
@@ -8,6 +8,7 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
 	wchar_t wc;
 	int c;
 	size_t l;
+	int errno_save = errno;
 
 	/* Convert character from buffer if possible */
 	if (f->rpos != f->rend) {
@@ -16,6 +17,7 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
 			f->rpos += l + !l; /* l==0 means 1 byte, null */
 			return wc;
 		}
+		errno = errno_save;
 	}
 
 	/* Convert character byte-by-byte */
-- 
2.35.1

Re: [musl] [PATCH] fgetwc: fix errno when character span aver buffer end

[Rich Felker]

On Thu, Feb 10, 2022 at 01:55:57AM +0800, Pinghao Wu wrote:
> If attempt on converting character from buffer failed, errno is set to
> EILSEQ by mbtowc. As a result, if further byte-by-byte conversion
> succeeds, fgetwc will return a valid wchar with a misleading EILSEQ as
> errno. This fixes it by saving errno before the from buffer attempt, and
> restore if it fails.
> 
> This also fixes fgetws which find the misleading EILSEQ and fails in
> this case.

errno is only meaningful after failure. A program which is checking it
when the last function called did not fail is incorrect and needs to
be fixed.

> ---
>  src/stdio/fgetwc.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/src/stdio/fgetwc.c b/src/stdio/fgetwc.c
> index aa10b818..9dad0505 100644
> --- a/src/stdio/fgetwc.c
> +++ b/src/stdio/fgetwc.c
> @@ -8,6 +8,7 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
>  	wchar_t wc;
>  	int c;
>  	size_t l;
> +	int errno_save = errno;
>  
>  	/* Convert character from buffer if possible */
>  	if (f->rpos != f->rend) {
> @@ -16,6 +17,7 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
>  			f->rpos += l + !l; /* l==0 means 1 byte, null */
>  			return wc;
>  		}
> +		errno = errno_save;
>  	}
>  
>  	/* Convert character byte-by-byte */
> -- 
> 2.35.1