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

fusl/src/multibyte/mbsrtowcs.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 <stdint.h>
+#include <wchar.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+#include "internal.h"
+
+size_t mbsrtowcs(wchar_t *restrict ws, const char **restrict src, size_t wn, mbstate_t *restrict st)
+{
+        const unsigned char *s = (const void *)*src;
+        size_t wn0 = wn;
+        unsigned c = 0;
+
+        if (st && (c = *(unsigned *)st)) {
+                if (ws) {
+                        *(unsigned *)st = 0;
+                        goto resume;
+                } else {
+                        goto resume0;
+                }
+        }
+
+        if (MB_CUR_MAX==1) {
+                if (!ws) return strlen((const char *)s);
+                for (;;) {
+                        if (!wn) {
+                                *src = (const void *)s;
+                                return wn0;
+                        }
+                        if (!*s) break;
+                        c = *s++;
+                        *ws++ = CODEUNIT(c);
+                        wn--;
+                }
+                *ws = 0;
+                *src = 0;
+                return wn0-wn;
+        }
+
+        if (!ws) for (;;) {
+                if (*s-1u < 0x7f && (uintptr_t)s%4 == 0) {
+                        while (!(( *(uint32_t*)s | *(uint32_t*)s-0x01010101) & 0x80808080)) {
+                                s += 4;
+                                wn -= 4;
+                        }
+                }
+                if (*s-1u < 0x7f) {
+                        s++;
+                        wn--;
+                        continue;
+                }
+                if (*s-SA > SB-SA) break;
+                c = bittab[*s++-SA];
+resume0:
+                if (OOB(c,*s)) { s--; break; }
+                s++;
+                if (c&(1U<<25)) {
+                        if (*s-0x80u >= 0x40) { s-=2; break; }
+                        s++;
+                        if (c&(1U<<19)) {
+                                if (*s-0x80u >= 0x40) { s-=3; break; }
+                                s++;
+                        }
+                }
+                wn--;
+                c = 0;
+        } else for (;;) {
+                if (!wn) {
+                        *src = (const void *)s;
+                        return wn0;
+                }
+                if (*s-1u < 0x7f && (uintptr_t)s%4 == 0) {
+                        while (wn>=5 && !(( *(uint32_t*)s | *(uint32_t*)s-0x01010101) & 0x80808080)) {
+                                *ws++ = *s++;
+                                *ws++ = *s++;
+                                *ws++ = *s++;
+                                *ws++ = *s++;
+                                wn -= 4;
+                        }
+                }
+                if (*s-1u < 0x7f) {
+                        *ws++ = *s++;
+                        wn--;
+                        continue;
+                }
+                if (*s-SA > SB-SA) break;
+                c = bittab[*s++-SA];
+resume:
+                if (OOB(c,*s)) { s--; break; }
+                c = (c<<6) | *s++-0x80;
+                if (c&(1U<<31)) {
+                        if (*s-0x80u >= 0x40) { s-=2; break; }
+                        c = (c<<6) | *s++-0x80;
+                        if (c&(1U<<31)) {
+                                if (*s-0x80u >= 0x40) { s-=3; break; }
+                                c = (c<<6) | *s++-0x80;
+                        }
+                }
+                *ws++ = c;
+                wn--;
+                c = 0;
+        }
+
+        if (!c && !*s) {
+                if (ws) {
+                        *ws = 0;
+                        *src = 0;
+                }
+                return wn0-wn;
+        }
+        errno = EILSEQ;
+        if (ws) *src = (const void *)s;
+        return -1;
+}