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

fusl/src/stdio/ext2.c

+#include "stdio_impl.h"
+
+size_t __freadahead(FILE *f)
+{
+        return f->rend - f->rpos;
+}
+
+const char *__freadptr(FILE *f, size_t *sizep)
+{
+        size_t size = f->rend - f->rpos;
+        if (!size) return 0;
+        *sizep = size;
+        return (const char *)f->rpos;
+}
+
+void __freadptrinc(FILE *f, size_t inc)
+{
+        f->rpos += inc;
+}
+
+void __fseterr(FILE *f)
+{
+        f->flags |= F_ERR;
+}