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

fusl/src/stdio/vsnprintf.c

+#include "stdio_impl.h"
+#include <limits.h>
+#include <string.h>
+#include <errno.h>
+#include <stdint.h>
+
+static size_t sn_write(FILE *f, const unsigned char *s, size_t l)
+{
+        size_t k = f->wend - f->wpos;
+        if (k > l) k = l;
+        memcpy(f->wpos, s, k);
+        f->wpos += k;
+        /* pretend to succeed, but discard extra data */
+        return l;
+}
+
+int vsnprintf(char *restrict s, size_t n, const char *restrict fmt, va_list ap)
+{
+        int r;
+        char b;
+        FILE f = { .lbf = EOF, .write = sn_write, .lock = -1 };
+
+        if (n-1 > INT_MAX-1) {
+                if (n) {
+                        errno = EOVERFLOW;
+                        return -1;
+                }
+                s = &b;
+                n = 1;
+        }
+
+        /* Ensure pointers don't wrap if "infinite" n is passed in */
+        if (n > (char *)0+SIZE_MAX-s-1) n = (char *)0+SIZE_MAX-s-1;
+        f.buf_size = n;
+        f.buf = f.wpos = (void *)s;
+        f.wbase = f.wend = (void *)(s+n);
+        r = vfprintf(&f, fmt, ap);
+
+        /* Null-terminate, overwriting last char if dest buffer is full */
+        if (n) f.wpos[-(f.wpos == f.wend)] = 0;
+        return r;
+}