diff --git a/libmctp-serial.h b/libmctp-serial.h
index dc7cb99..73a28a4 100644
--- a/libmctp-serial.h
+++ b/libmctp-serial.h
@@ -26,8 +26,10 @@
 void mctp_serial_open_fd(struct mctp_binding_serial *serial, int fd);
 
 /* direct function call IO */
-typedef int (*mctp_serial_tx_fn)(void *data, void *buf, size_t len)
-	__attribute__((warn_unused_result));
+typedef int
+	__attribute__((warn_unused_result)) (*mctp_serial_tx_fn)(void *data,
+								 void *buf,
+								 size_t len);
 void mctp_serial_set_tx_fn(struct mctp_binding_serial *serial,
 			   mctp_serial_tx_fn fn, void *data);
 int mctp_serial_rx(struct mctp_binding_serial *serial, const void *buf,