diff --git a/src/mslot_class.h b/src/mslot_class.h
index 4f5a7ff..d248526 100644
--- a/src/mslot_class.h
+++ b/src/mslot_class.h
@@ -50,3 +50,4 @@
 uint8_t mslot_class_max();
 
 void mslot_fill_rx_mask(uint8_t mslot_class, uint8_t num_tx, uint8_t *rx_mask);
+int8_t find_free_usf(uint8_t usf_map);