diff --git a/src/emq_client.cpp b/src/emq_client.cpp
index 25940d7..d52511d 100644
--- a/src/emq_client.cpp
+++ b/src/emq_client.cpp
@@ -46,7 +46,7 @@ extern int g_mqtt_port;
 
 // private variable definitions
 static bool force_disconnected = false;
-static PubSubClient* emqx_client = NULL;
+static PubSubClient* emqx_client = nullptr;
 
 // private function declarations
 
@@ -55,7 +55,7 @@ static PubSubClient* emqx_client = NULL;
 int connectToEMQXBroker(PubSubClient &mqtt_client) {
     int retry_times = 0;
 
-    if (NULL == emqx_client) {
+    if (nullptr == emqx_client) {
         emqx_client = &mqtt_client;
     }
     emqx_client->setServer(g_mqtt_server.c_str(), g_mqtt_port);
diff --git a/src/iot_hub.cpp b/src/iot_hub.cpp
index 8dcb697..9fbc7db 100644
--- a/src/iot_hub.cpp
+++ b/src/iot_hub.cpp
@@ -166,7 +166,7 @@ void checkIot() {
 
 void iotCallback(char *topic, uint8_t *data, uint32_t length) {
     INFOF("Downstream message received, topic = %s, length = %d\n", topic, length);
-    if (NULL != g_downstream_topic.c_str() && 0 == strcmp(topic, g_downstream_topic.c_str())) {
+    if (nullptr != g_downstream_topic.c_str() && 0 == strcmp(topic, g_downstream_topic.c_str())) {
         handleIrisKitMessage((const char*) data, length);
     }
 }
\ No newline at end of file
diff --git a/src/iris_client.cpp b/src/iris_client.cpp
index 2a6f3d7..662ee99 100644
--- a/src/iris_client.cpp
+++ b/src/iris_client.cpp
@@ -92,7 +92,7 @@ event_handler_t event_handler_table[] = {
 
 // public function definitions
 int getIRISKitVersion(char *buffer, int buffer_size) {
-    if (NULL == buffer) {
+    if (nullptr == buffer) {
         return -1;
     }
     memset(buffer, 0, buffer_size);
@@ -123,7 +123,7 @@ int authIrisKit(String credential_token,
 
     http_error_t http_ret = HTTP_ERROR_GENERIC;
 
-    if (NULL != strstr(iris_server_address, "http://")) {
+    if (nullptr != strstr(iris_server_address, "http://")) {
         protocol_prefix = true;
     }
     if (protocol_prefix) {
@@ -185,7 +185,7 @@ bool downloadBin(int remote_id) {
     String download_bin_url;
     http_error_t http_ret = HTTP_ERROR_GENERIC;
 
-    if (NULL != strstr(iris_server_address, "http://")) {
+    if (nullptr != strstr(iris_server_address, "http://")) {
         protocol_prefix = true;
     }
     if (protocol_prefix) {
@@ -223,7 +223,7 @@ void sendIrisKitHeartBeat() {
 void handleIrisKitMessage(const char* data, int length) {
     int ret = 0;
     char* payload = (char*) malloc(length + 1);
-    if (NULL != payload) {
+    if (nullptr != payload) {
         strncpy(payload, data, length);
         payload[length] = '\0';
         INFOF("--> %s\n", payload);
@@ -238,7 +238,7 @@ void handleIrisKitMessage(const char* data, int length) {
         }
     }
 
-    if (NULL != payload) {
+    if (nullptr != payload) {
         free(payload);
     }
 }
diff --git a/src/iris_kit.cpp b/src/iris_kit.cpp
index 72a6cbc..5f2b8f0 100644
--- a/src/iris_kit.cpp
+++ b/src/iris_kit.cpp
@@ -113,11 +113,11 @@ void setup() {
         INFOF("Password is empty ? %s\n", iris_kit_settings.password.isEmpty() ? "yes" : "no");
     }
     if (!iris_kit_settings.credential_token.isEmpty() &&
-        !iris_kit_settings.credential_token.equalsIgnoreCase("NULL") &&
+        !iris_kit_settings.credential_token.equalsIgnoreCase("nullptr") &&
         !iris_kit_settings.server_address.isEmpty() &&
-        !iris_kit_settings.server_address.equalsIgnoreCase("NULL") &&
+        !iris_kit_settings.server_address.equalsIgnoreCase("nullptr") &&
         !iris_kit_settings.password.isEmpty() &&
-        !iris_kit_settings.password.equalsIgnoreCase("NULL")) {
+        !iris_kit_settings.password.equalsIgnoreCase("nullptr")) {
         iris_kit_settings_loaded = true;
     }
 
@@ -210,7 +210,7 @@ void setup() {
     loadIRPin(send_pin, recv_pin);
 
     // prepare MQTT connection params
-    if (NULL != strstr(iris_server_address, "iris.irext.net")) {
+    if (nullptr != strstr(iris_server_address, "iris.irext.net")) {
         g_mqtt_server = String(MQTT_HOST_REL);
         g_runtime_env = RUNTIME_RELEASE;
     } else {
diff --git a/src/utils.cpp b/src/utils.cpp
index e78a66a..c9ed3f4 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -32,31 +32,31 @@
 MD5Builder md5_builder;
 
 int split_string(const String source, char* parts[], const char* delimiter) {
-    char* pch = NULL;
-    char* copy = NULL;
-    char* tmp = NULL;
+    char* pch = nullptr;
+    char* copy = nullptr;
+    char* tmp = nullptr;
     int i = 0;
 
     copy = strdup(source.c_str());
-    if (NULL == copy) {
+    if (nullptr == copy) {
         goto exit;
     }
 
     pch = strtok(copy, delimiter);
 
     tmp = strdup(pch);
-    if (NULL == tmp) {
+    if (nullptr == tmp) {
         goto exit;
     }
 
     parts[i++] = tmp;
 
     while (pch) {
-        pch = strtok(NULL, delimiter);
-        if (NULL == pch) break;
+        pch = strtok(nullptr, delimiter);
+        if (nullptr == pch) break;
 
         tmp = strdup(pch);
-        if (NULL == tmp) {
+        if (nullptr == tmp) {
             goto exit;
         }