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From: Rich Felker <dalias@libc.org>
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Subject: Re: [setlocale]: return only one copy if all six parts of
 locale are same
Date: Sat, 18 Mar 2017 08:33:35 -0400
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On Sat, Mar 18, 2017 at 07:37:57AM +0000, He X wrote:
> As i suggest on IRC, here's the patch.

> --- musl-1.1.16/src/locale/setlocale.c	2017-03-17 17:49:15.767952411 +0000
> +++ musl-1.1.16/src/locale/setlocale.c	2017-03-17 17:49:15.767952411 +0000
> @@ -48,16 +48,33 @@
>  			}
>  		}
>  		char *s = buf;
> -		for (i=0; i<LC_ALL; i++) {
> + 		const struct __locale_map *flm =
> + 			libc.global_locale.cat[0];
> + 		const char *fpart = flm ? flm->name : "C";
> + 		size_t l = strlen(fpart);
> + 		memcpy(s, fpart, l);
> + 		s[l] = 0;
> + 		i=1;
> + 		do {
>  			const struct __locale_map *lm =
>  				libc.global_locale.cat[i];
>  			const char *part = lm ? lm->name : "C";
> -			size_t l = strlen(part);
> -			memcpy(s, part, l);
> -			s[l] = ';';
> -			s += l+1;
> + 			if (strcmp(s, part)) break;
> + 			i++;
> + 		} while (i<LC_ALL);
> + 
> + 		if (i != LC_ALL) {
> + 			for (i=0; i<LC_ALL; i++) {
> + 				const struct __locale_map *lm =
> + 					libc.global_locale.cat[i];
> + 				const char *part = lm ? lm->name : "C";
> + 				size_t l = strlen(part);
> + 				memcpy(s, part, l);
> + 				s[l] = ';';
> + 				s += l+1;
> + 			}
> + 			*--s = 0;
>  		}
> -		*--s = 0;
>  		UNLOCK(lock);
>  		return buf;
>  	}

I think the results of this patch can be achieved much more simply.
Just compare lm pointers (rather than string contents) for equality at
each iteration in the current loop, counting how many were equal to
libc.global_locale.cat[0]. If that number equals LC_ALL, return
libc.global_locale.cat[0]->name (or "C" if the lm is null) rather than
buf.

Rich