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

fusl/src/multibyte/mbsnrtowcs.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 mbsnrtowcs(wchar_t *restrict wcs, const char **restrict src, size_t n, size_t wn, mbstate_t *restrict st)
+{
+        size_t l, cnt=0, n2;
+        wchar_t *ws, wbuf[256];
+        const char *s = *src;
+
+        if (!wcs) ws = wbuf, wn = sizeof wbuf / sizeof *wbuf;
+        else ws = wcs;
+
+        /* making sure output buffer size is at most n/4 will ensure
+         * that mbsrtowcs never reads more than n input bytes. thus
+         * we can use mbsrtowcs as long as it's practical.. */
+
+        while ( s && wn && ( (n2=n/4)>=wn || n2>32 ) ) {
+                if (n2>=wn) n2=wn;
+                n -= n2;
+                l = mbsrtowcs(ws, &s, n2, st);
+                if (!(l+1)) {
+                        cnt = l;
+                        wn = 0;
+                        break;
+                }
+                if (ws != wbuf) {
+                        ws += l;
+                        wn -= l;
+                }
+                cnt += l;
+        }
+        if (s) while (wn && n) {
+                l = mbrtowc(ws, s, n, st);
+                if (l+2<=2) {
+                        if (!(l+1)) {
+                                cnt = l;
+                                break;
+                        }
+                        if (!l) {
+                                s = 0;
+                                break;
+                        }
+                        /* have to roll back partial character */
+                        *(unsigned *)st = 0;
+                        break;
+                }
+                s += l; n -= l;
+                /* safe - this loop runs fewer than sizeof(wbuf)/8 times */
+                ws++; wn--;
+                cnt++;
+        }
+        if (wcs) *src = s;
+        return cnt;
+}