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

fusl/src/stdio/__lockfile.c

+#include "stdio_impl.h"
+#include "pthread_impl.h"
+
+int __lockfile(FILE *f)
+{
+        int owner, tid = __pthread_self()->tid;
+        if (f->lock == tid)
+                return 0;
+        while ((owner = a_cas(&f->lock, 0, tid)))
+                __wait(&f->lock, &f->waiters, owner, 1);
+        return 1;
+}
+
+void __unlockfile(FILE *f)
+{
+        a_store(&f->lock, 0);
+
+        /* The following read is technically invalid under situations
+         * of self-synchronized destruction. Another thread may have
+         * called fclose as soon as the above store has completed.
+         * Nonetheless, since FILE objects always live in memory
+         * obtained by malloc from the heap, it's safe to assume
+         * the dereferences below will not fault. In the worst case,
+         * a spurious syscall will be made. If the implementation of
+         * malloc changes, this assumption needs revisiting. */
+
+        if (f->waiters) __wake(&f->lock, 1, 1);
+}