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

fusl/src/stdio/ext.c

+#define _GNU_SOURCE
+#include "stdio_impl.h"
+#include <stdio_ext.h>
+
+void _flushlbf(void)
+{
+        fflush(0);
+}
+
+int __fsetlocking(FILE *f, int type)
+{
+        return 0;
+}
+
+int __fwriting(FILE *f)
+{
+        return (f->flags & F_NORD) || f->wend;
+}
+
+int __freading(FILE *f)
+{
+        return (f->flags & F_NOWR) || f->rend;
+}
+
+int __freadable(FILE *f)
+{
+        return !(f->flags & F_NORD);
+}
+
+int __fwritable(FILE *f)
+{
+        return !(f->flags & F_NOWR);
+}
+
+int __flbf(FILE *f)
+{
+        return f->lbf >= 0;
+}
+
+size_t __fbufsize(FILE *f)
+{
+        return f->buf_size;
+}
+
+size_t __fpending(FILE *f)
+{
+        return f->wend ? f->wpos - f->wbase : 0;
+}
+
+int __fpurge(FILE *f)
+{
+        f->wpos = f->wbase = f->wend = 0;
+        f->rpos = f->rend = 0;
+        return 0;
+}
+
+weak_alias(__fpurge, fpurge);