Add a "fork" of musl as //fusl.

fusl/src/stdio/fgetwc.c

+#include "stdio_impl.h"
+#include "locale_impl.h"
+#include <wchar.h>
+#include <errno.h>
+
+static wint_t __fgetwc_unlocked_internal(FILE *f)
+{
+        mbstate_t st = { 0 };
+        wchar_t wc;
+        int c;
+        unsigned char b;
+        size_t l;
+
+        /* Convert character from buffer if possible */
+        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;
+                }
+        } else l = -2;
+
+        /* Convert character byte-by-byte */
+        while (l == -2) {
+                b = c = getc_unlocked(f);
+                if (c < 0) {
+                        if (!mbsinit(&st)) errno = EILSEQ;
+                        return WEOF;
+                }
+                l = mbrtowc(&wc, (void *)&b, 1, &st);
+                if (l == -1) return WEOF;
+        }
+
+        return wc;
+}
+
+wint_t __fgetwc_unlocked(FILE *f)
+{
+        locale_t *ploc = &CURRENT_LOCALE, loc = *ploc;
+        if (f->mode <= 0) fwide(f, 1);
+        *ploc = f->locale;
+        wchar_t wc = __fgetwc_unlocked_internal(f);
+        *ploc = loc;
+        return wc;
+}
+
+wint_t fgetwc(FILE *f)
+{
+        wint_t c;
+        FLOCK(f);
+        c = __fgetwc_unlocked(f);
+        FUNLOCK(f);
+        return c;
+}
+
+weak_alias(__fgetwc_unlocked, fgetwc_unlocked);
+weak_alias(__fgetwc_unlocked, getwc_unlocked);