>From dac3fc8a25bbc43a2c231dfb8c41b4fd6c55a8a5 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Mon, 14 Sep 2026 18:09:27 -0400
Subject: [PATCH 09/18] locale: make explicit request of unavailable locales
 fail

have __get_locale track whether the request is implicit (obtained via
environment for an argument of "") or explicit. if explicit, names
that fail to resove to a locale file will produce a return value of
LOC_MAP_FAILED, which setlocale and newlocale already interpret as an
error condition. if implicit, return C.UTF-8 for LC_CTYPE, and
otherwise a null pointer (interpreted as C locale).

to avoid breaking explicit requests intending to get the C.UTF-8
locale, the matching is relaxed so that C.* and POSIX.* are
interpreted as C.UTF-8. this is mainly to catch C.utf8 or other
spellings without having to enumerate any possibility someone might
come up with.
---
 src/locale/locale_map.c | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/src/locale/locale_map.c b/src/locale/locale_map.c
index ce56bc40..9ebf7ced 100644
--- a/src/locale/locale_map.c
+++ b/src/locale/locale_map.c
@@ -39,8 +39,10 @@ const struct __locale_map *__get_locale(int cat, const char *val)
 	const char *path = 0, *z;
 	char buf[256];
 	size_t l, n;
+	int implicit = 0;
 
 	if (!*val) {
+		implicit = 1;
 		(val = getenv("LC_ALL")) && *val ||
 		(val = getenv(envvars[cat])) && *val ||
 		(val = getenv("LANG")) && *val ||
@@ -49,16 +51,12 @@ const struct __locale_map *__get_locale(int cat, const char *val)
 
 	/* Limit name length and forbid leading dot or any slashes. */
 	for (n=0; n<LOCALE_NAME_MAX && val[n] && val[n]!='/'; n++);
-	if (val[0]=='.' || val[n]) val = "C.UTF-8";
-	int builtin = (val[0]=='C' && !val[1])
-		|| !strcmp(val, "C.UTF-8")
-		|| !strcmp(val, "POSIX");
-
-	if (builtin) {
-		if (cat == LC_CTYPE && val[1]=='.')
-			return (void *)&__c_dot_utf8;
+	if (val[0]=='.' || val[n])
+		goto fail;
+	if ((val[0]=='C' && !val[1]) || !strcmp(val, "POSIX"))
 		return 0;
-	}
+	if ((val[0]=='C' && val[1]=='.') || !strncmp(val, "POSIX.", 6))
+		goto utf8;
 
 	for (p=loc_head; p; p=p->next)
 		if (!strcmp(val, p->name)) return p;
@@ -92,9 +90,10 @@ const struct __locale_map *__get_locale(int cat, const char *val)
 		}
 	}
 
-	/* For LC_CTYPE, never return a null pointer unless the
-	 * requested name was "C" or "POSIX". */
-	if (!new && cat == LC_CTYPE) new = (void *)&__c_dot_utf8;
-
-	return new;
+	if (new) return new;
+fail:
+	if (!implicit) return LOC_MAP_FAILED;
+utf8:
+	if (cat==LC_CTYPE) return &__c_dot_utf8;
+	return 0;
 }
-- 
2.21.0