diff --git a/src/iperf_api.c b/src/iperf_api.c
index bad0a63ad..901aec301 100644
--- a/src/iperf_api.c
+++ b/src/iperf_api.c
@@ -603,25 +603,25 @@ iperf_set_mapped_v4(struct iperf_test *ipt, const int val)
 }
 
 void 
-iperf_set_on_new_stream_callback(struct iperf_test* ipt, void (*callback)())
+iperf_set_on_new_stream_callback(struct iperf_test* ipt, void (*callback)(struct iperf_stream *))
 {
         ipt->on_new_stream = callback;
 }
 
 void 
-iperf_set_on_test_start_callback(struct iperf_test* ipt, void (*callback)())
+iperf_set_on_test_start_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *))
 {
         ipt->on_test_start = callback;
 }
 
 void 
-iperf_set_on_test_connect_callback(struct iperf_test* ipt, void (*callback)())
+iperf_set_on_test_connect_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *))
 {
         ipt->on_connect = callback;
 }
 
 void 
-iperf_set_on_test_finish_callback(struct iperf_test* ipt, void (*callback)())
+iperf_set_on_test_finish_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *))
 {
         ipt->on_test_finish = callback;
 }
diff --git a/src/iperf_api.h b/src/iperf_api.h
index 2b71613e9..5e2519e47 100644
--- a/src/iperf_api.h
+++ b/src/iperf_api.h
@@ -213,10 +213,10 @@ void    iperf_set_dont_fragment( struct iperf_test* ipt, int dont_fragment );
 void    iperf_set_test_congestion_control(struct iperf_test* ipt, char* cc);
 void    iperf_set_test_mss(struct iperf_test* ipt, int mss);
 void    iperf_set_mapped_v4(struct iperf_test* ipt, const int val);
-void    iperf_set_on_new_stream_callback(struct iperf_test* ipt, void (*callback)());
-void    iperf_set_on_test_start_callback(struct iperf_test* ipt, void (*callback)());
-void    iperf_set_on_test_connect_callback(struct iperf_test* ipt, void (*callback)());
-void    iperf_set_on_test_finish_callback(struct iperf_test* ipt, void (*callback)());
+void    iperf_set_on_new_stream_callback(struct iperf_test* ipt, void (*callback)(struct iperf_stream *));
+void    iperf_set_on_test_start_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *));
+void    iperf_set_on_test_connect_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *));
+void    iperf_set_on_test_finish_callback(struct iperf_test* ipt, void (*callback)(struct iperf_test *));
 
 #if defined(HAVE_SSL)
 void    iperf_set_test_client_username(struct iperf_test *ipt, const char *client_username);