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diff --git a/lib/unistr/u8-uctomb-aux.c b/lib/unistr/u8-uctomb-aux.c
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+++ b/lib/unistr/u8-uctomb-aux.c
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+/* Conversion UCS-4 to UTF-8.
+ Copyright (C) 2002, 2006-2007, 2009-2022 Free Software Foundation, Inc.
+ Written by Bruno Haible <bruno@clisp.org>, 2002.
+
+ This file is free software: you can redistribute it and/or modify
+ it under the terms of the GNU Lesser General Public License as
+ published by the Free Software Foundation; either version 2.1 of the
+ License, or (at your option) any later version.
+
+ This file is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public License
+ along with this program. If not, see <https://www.gnu.org/licenses/>. */
+
+#include <config.h>
+
+/* Specification. */
+#include "unistr.h"
+
+#include "attribute.h"
+
+int
+u8_uctomb_aux (uint8_t *s, ucs4_t uc, ptrdiff_t n)
+{
+ int count;
+
+ if (uc < 0x80)
+ /* The case n >= 1 is already handled by the caller. */
+ return -2;
+ else if (uc < 0x800)
+ count = 2;
+ else if (uc < 0x10000)
+ {
+ if (uc < 0xd800 || uc >= 0xe000)
+ count = 3;
+ else
+ return -1;
+ }
+ else if (uc < 0x110000)
+ count = 4;
+ else
+ return -1;
+
+ if (n < count)
+ return -2;
+
+ switch (count) /* note: code falls through cases! */
+ {
+ case 4: s[3] = 0x80 | (uc & 0x3f); uc = uc >> 6; uc |= 0x10000;
+ FALLTHROUGH;
+ case 3: s[2] = 0x80 | (uc & 0x3f); uc = uc >> 6; uc |= 0x800;
+ FALLTHROUGH;
+ case 2: s[1] = 0x80 | (uc & 0x3f); uc = uc >> 6; uc |= 0xc0;
+ /*case 1:*/ s[0] = uc;
+ }
+ return count;
+}