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

fusl/src/multibyte/mbrtowc.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 <stdlib.h>
+#include <wchar.h>
+#include <errno.h>
+#include "internal.h"
+
+size_t mbrtowc(wchar_t *restrict wc, const char *restrict src, size_t n, mbstate_t *restrict st)
+{
+        static unsigned internal_state;
+        unsigned c;
+        const unsigned char *s = (const void *)src;
+        const unsigned N = n;
+        wchar_t dummy;
+
+        if (!st) st = (void *)&internal_state;
+        c = *(unsigned *)st;
+        
+        if (!s) {
+                if (c) goto ilseq;
+                return 0;
+        } else if (!wc) wc = &dummy;
+
+        if (!n) return -2;
+        if (!c) {
+                if (*s < 0x80) return !!(*wc = *s);
+                if (MB_CUR_MAX==1) return (*wc = CODEUNIT(*s)), 1;
+                if (*s-SA > SB-SA) goto ilseq;
+                c = bittab[*s++-SA]; n--;
+        }
+
+        if (n) {
+                if (OOB(c,*s)) goto ilseq;
+loop:
+                c = c<<6 | *s++-0x80; n--;
+                if (!(c&(1U<<31))) {
+                        *(unsigned *)st = 0;
+                        *wc = c;
+                        return N-n;
+                }
+                if (n) {
+                        if (*s-0x80u >= 0x40) goto ilseq;
+                        goto loop;
+                }
+        }
+
+        *(unsigned *)st = c;
+        return -2;
+ilseq:
+        *(unsigned *)st = 0;
+        errno = EILSEQ;
+        return -1;
+}