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

fusl/src/multibyte/wcsnrtombs.c

+/* 
+ * This code was written by Rich Felker in 2010; no copyright is claimed.
+ * This code is in the public domain. Attribution is appreciated but
+ * unnecessary.
+ */
+
+#include <wchar.h>
+
+size_t wcsnrtombs(char *restrict dst, const wchar_t **restrict wcs, size_t wn, size_t n, mbstate_t *restrict st)
+{
+        size_t l, cnt=0, n2;
+        char *s, buf[256];
+        const wchar_t *ws = *wcs;
+
+        if (!dst) s = buf, n = sizeof buf;
+        else s = dst;
+
+        while ( ws && n && ( (n2=wn)>=n || n2>32 ) ) {
+                if (n2>=n) n2=n;
+                wn -= n2;
+                l = wcsrtombs(s, &ws, n2, 0);
+                if (!(l+1)) {
+                        cnt = l;
+                        n = 0;
+                        break;
+                }
+                if (s != buf) {
+                        s += l;
+                        n -= l;
+                }
+                cnt += l;
+        }
+        if (ws) while (n && wn) {
+                l = wcrtomb(s, *ws, 0);
+                if ((l+1)<=1) {
+                        if (!l) ws = 0;
+                        else cnt = l;
+                        break;
+                }
+                ws++; wn--;
+                /* safe - this loop runs fewer than sizeof(buf) times */
+                s+=l; n-=l;
+                cnt += l;
+        }
+        if (dst) *wcs = ws;
+        return cnt;
+}