diff --git a/ble/att.h b/ble/att.h
index d22ba4cf0..6c22c9135 100644
--- a/ble/att.h
+++ b/ble/att.h
@@ -246,7 +246,7 @@ uint16_t att_prepare_handle_value_indication(att_connection_t * att_connection,
 /*
  * @brief transcation queue of prepared writes, e.g., after disconnect
  */
-void att_clear_transaction_queue();
+void att_clear_transaction_queue(att_connection_t * att_connection);
 
 // experimental client API
 uint16_t att_uuid_for_handle(uint16_t handle);
diff --git a/ble/att_server.h b/ble/att_server.h
index 6b83fd104..df51fe11d 100644
--- a/ble/att_server.h
+++ b/ble/att_server.h
@@ -63,7 +63,7 @@ void att_server_register_packet_handler(btstack_packet_handler_t handler);
  * @brief tests if a notification or indication can be send right now
  * @return 1, if packet can be sent
  */
-int  att_server_can_send();
+int  att_server_can_send(void);
 
 /*
  * @brief notify client about attribute value change
diff --git a/ble/gatt_client.h b/ble/gatt_client.h
index 972bf90b2..1878ac86d 100644
--- a/ble/gatt_client.h
+++ b/ble/gatt_client.h
@@ -237,7 +237,7 @@ typedef struct le_characteristic_descriptor_event{
 /** 
  * @brief Set up GATT client.
  */
-void gatt_client_init();
+void gatt_client_init(void);
 
 /** 
  * @brief Register callback (packet handler) for GATT client. Returns GATT client ID.
diff --git a/ble/le_device_db.h b/ble/le_device_db.h
index fb3609720..1d3676024 100644
--- a/ble/le_device_db.h
+++ b/ble/le_device_db.h
@@ -64,7 +64,7 @@ extern "C" {
 /**
  * @brief init
  */
-void le_device_db_init();
+void le_device_db_init(void);
 
 /**
  * @brief add device to db
diff --git a/ble/sm.c b/ble/sm.c
index db5d34f17..73b1f5802 100644
--- a/ble/sm.c
+++ b/ble/sm.c
@@ -246,12 +246,12 @@ static const stk_generation_method_t stk_generation_method[5][5] = {
     { PK_RESP_INPUT,   PK_RESP_INPUT,    PK_INIT_INPUT,   JUST_WORKS,    PK_RESP_INPUT },
 };
 
-static void sm_run();
+static void sm_run(void);
 static void sm_done_for_handle(uint16_t handle);
 static void sm_notify_client(uint8_t type, uint8_t addr_type, bd_addr_t address, uint32_t passkey, uint16_t index);
 static sm_connection_t * sm_get_connection_for_handle(uint16_t handle);
 static inline int sm_calc_actual_encryption_key_size(int other);
-static int sm_validate_stk_generation_method();
+static int sm_validate_stk_generation_method(void);
 
 static void log_info_hex16(const char * name, uint16_t value){
     log_info("%-6s 0x%04x", name, value);
@@ -2064,7 +2064,7 @@ static sm_connection_t * sm_get_connection_for_handle(uint16_t con_handle){
 }
 
 static sm_connection_t * sm_get_connection(uint8_t addr_type, bd_addr_t address){
-    hci_connection_t * hci_con = hci_connection_for_bd_addr_and_type(address, addr_type);
+    hci_connection_t * hci_con = hci_connection_for_bd_addr_and_type(address, (bd_addr_type_t)addr_type);
     if (!hci_con)  return NULL;
     return &hci_con->sm_connection;
 }
diff --git a/ble/sm.h b/ble/sm.h
index 3c6a2c158..e0ec5b0a3 100644
--- a/ble/sm.h
+++ b/ble/sm.h
@@ -127,7 +127,7 @@ typedef struct sm_event {
 /**
  * @brief Initializes the Security Manager, connects to L2CAP
  */
-void sm_init();
+void sm_init(void);
 
 /**
  * @brief Set secret ER key for key generation as described in Core V4.0, Vol 3, Part G, 5.2.2 
@@ -253,7 +253,7 @@ void sm_authorization_grant(uint8_t addr_type, bd_addr_t address);
  * @brief Support for signed writes, used by att_server.
  * @note Message and result are in little endian to allows passing in ATT PDU without flipping them first.
  */
-int  sm_cmac_ready();
+int  sm_cmac_ready(void);
 void sm_cmac_start(sm_key_t k, uint16_t message_len, uint8_t * message, uint32_t sign_counter, void (*done_handler)(uint8_t hash[8]));
 
 /**
diff --git a/src/hci.c b/src/hci.c
index cb77df02b..27f80f56f 100644
--- a/src/hci.c
+++ b/src/hci.c
@@ -79,7 +79,7 @@ static void hci_connection_timeout_handler(timer_source_t *timer);
 static void hci_connection_timestamp(hci_connection_t *connection);
 static int  hci_power_control_on(void);
 static void hci_power_control_off(void);
-static void hci_state_reset();
+static void hci_state_reset(void);
 
 // the STACK is here
 #ifndef HAVE_MALLOC
@@ -1476,7 +1476,7 @@ static void event_handler(uint8_t *packet, int size){
         case HCI_EVENT_DISCONNECTION_COMPLETE:
             if (packet[2]) break;   // status != 0
             handle = READ_BT_16(packet, 3);
-            hci_connection_t * conn = hci_connection_for_handle(handle);
+            conn = hci_connection_for_handle(handle);
             if (!conn) break;       // no conn struct anymore
             conn->state = RECEIVED_DISCONNECTION_COMPLETE;
             break;
diff --git a/src/sdp_client.c b/src/sdp_client.c
index 4b63a2c81..ccf6ff197 100644
--- a/src/sdp_client.c
+++ b/src/sdp_client.c
@@ -194,7 +194,7 @@ void sdp_packet_handler(uint8_t packet_type, uint16_t channel, uint8_t *packet,
             return;
         }
 
-        PDU_ID = packet[0];
+        PDU_ID = (SDP_PDU_ID_t)packet[0];
         log_info("SDP Client :: PDU ID. %u ,%u", PDU_ID, packet[0]);
         switch (PDU_ID){
 #ifdef HAVE_SDP_EXTRA_QUERIES
diff --git a/src/sdp_query_rfcomm.h b/src/sdp_query_rfcomm.h
index cbe4a2ee6..688249830 100644
--- a/src/sdp_query_rfcomm.h
+++ b/src/sdp_query_rfcomm.h
@@ -68,7 +68,7 @@ typedef struct sdp_query_rfcomm_service_event {
  */
 void sdp_query_rfcomm_register_callback(void(*sdp_app_callback)(sdp_query_event_t * event, void * context), void * context);
 
-void sdp_query_rfcomm_deregister_callback();
+void sdp_query_rfcomm_deregister_callback(void);
 
 /** 
  * @brief Searches SDP records on a remote device for RFCOMM services with a given UUID.