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

fusl/src/stdio/vsnprintf.c

Index: fusl/src/stdio/vsnprintf.c
diff --git a/fusl/src/stdio/vsnprintf.c b/fusl/src/stdio/vsnprintf.c
new file mode 100644
index 0000000000000000000000000000000000000000..be2c44eb176d70a5d867b9f339dd58afe5953ec7
--- /dev/null
+++ b/fusl/src/stdio/vsnprintf.c
@@ -0,0 +1,42 @@
+#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;
+}