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

fusl/src/stdio/fflush.c

+#include "stdio_impl.h"
+
+static int __fflush_unlocked(FILE *f)
+{
+        /* If writing, flush output */
+        if (f->wpos > f->wbase) {
+                f->write(f, 0, 0);
+                if (!f->wpos) return EOF;
+        }
+
+        /* If reading, sync position, per POSIX */
+        if (f->rpos < f->rend) f->seek(f, f->rpos-f->rend, SEEK_CUR);
+
+        /* Clear read and write modes */
+        f->wpos = f->wbase = f->wend = 0;
+        f->rpos = f->rend = 0;
+
+        return 0;
+}
+
+/* stdout.c will override this if linked */
+static FILE *volatile dummy = 0;
+weak_alias(dummy, __stdout_used);
+
+int fflush(FILE *f)
+{
+        int r;
+
+        if (f) {
+                FLOCK(f);
+                r = __fflush_unlocked(f);
+                FUNLOCK(f);
+                return r;
+        }
+
+        r = __stdout_used ? fflush(__stdout_used) : 0;
+
+        for (f=*__ofl_lock(); f; f=f->next) {
+                FLOCK(f);
+                if (f->wpos > f->wbase) r |= __fflush_unlocked(f);
+                FUNLOCK(f);
+        }
+        __ofl_unlock();
+        
+        return r;
+}
+
+weak_alias(__fflush_unlocked, fflush_unlocked);