diff --git a/lib/config_helper.cpp b/lib/config_helper.cpp
index 9a4a8bc79..97ec9d3b8 100644
--- a/lib/config_helper.cpp
+++ b/lib/config_helper.cpp
@@ -137,7 +137,7 @@ int json_to_config(json_t *json, config_setting_t *parent)
 			json_array_foreach(json, i, json_value) {
 				type = json_to_config_type(json_typeof(json_value));
 
-				cfg = config_setting_add(parent, NULL, type);
+				cfg = config_setting_add(parent, nullptr, type);
 				ret = json_to_config(json_value, cfg);
 				if (ret)
 					return ret;
@@ -179,7 +179,7 @@ void json_object_extend_key_value_token(json_t *obj, const char *key, const char
 	while (token) {
 		json_object_extend_key_value(obj, key, token);
 
-		token = strtok_r(NULL, delim, &lasts);
+		token = strtok_r(nullptr, delim, &lasts);
 	}
 
 	free(str);
@@ -199,7 +199,7 @@ void json_object_extend_key_value(json_t *obj, const char *key, const char *valu
 	cpy = strdup(key);
 
 	key1 = strtok_r(cpy, ".", &lasts);
-	key2 = strtok_r(NULL, ".", &lasts);
+	key2 = strtok_r(nullptr, ".", &lasts);
 
 	while (key1 && key2) {
 		existing = json_object_get(subobj, key1);
@@ -271,8 +271,8 @@ success:
 json_t * json_load_cli(int argc, const char *argv[])
 {
 	const char *opt;
-	const char *key = NULL;
-	const char *value = NULL;
+	const char *key = nullptr;
+	const char *value = nullptr;
 	const char *sep;
 	char *cpy, *lasts;
 
@@ -284,8 +284,8 @@ json_t * json_load_cli(int argc, const char *argv[])
 		/* Long Option */
 		if (opt[0] == '-' && opt[1] == '-') {
 			/* Option without value? Abort. */
-			if (key != NULL)
-				return NULL;
+			if (key != nullptr)
+				return nullptr;
 
 			key = opt + 2;
 
@@ -295,24 +295,24 @@ json_t * json_load_cli(int argc, const char *argv[])
 				cpy = strdup(key);
 
 				key = strtok_r(cpy, "=", &lasts);
-				value = strtok_r(NULL, "", &lasts);
+				value = strtok_r(nullptr, "", &lasts);
 
 				json_object_extend_key_value_token(json, key, value);
 
 				free(cpy);
-				key = NULL;
+				key = nullptr;
 			}
 		}
 		/* Value */
 		else {
 			/* Value without key. Abort. */
-			if (key == NULL)
-				return NULL;
+			if (key == nullptr)
+				return nullptr;
 
 			value = opt;
 
 			json_object_extend_key_value_token(json, key, value);
-			key = NULL;
+			key = nullptr;
 		}
 	}
 
diff --git a/lib/hooks/average.cpp b/lib/hooks/average.cpp
index 82089959b..24a2f28cc 100644
--- a/lib/hooks/average.cpp
+++ b/lib/hooks/average.cpp
@@ -64,7 +64,7 @@ public:
 
 	virtual ~AverageHook()
 	{
-		vlist_destroy(&signal_names, NULL, true);
+		vlist_destroy(&signal_names, nullptr, true);
 
 		bitset_destroy(&mask);
 	}
@@ -91,7 +91,7 @@ public:
 			throw RuntimeError("Invalid signal mask");
 
 		/* Add averaged signal */
-		avg_sig = signal_create("average", NULL, SIGNAL_TYPE_FLOAT);
+		avg_sig = signal_create("average", nullptr, SIGNAL_TYPE_FLOAT);
 		if (!avg_sig)
 			throw RuntimeError("Failed to create new signal");
 
diff --git a/lib/hooks/cast.cpp b/lib/hooks/cast.cpp
index b21f5882f..7919ee4c3 100644
--- a/lib/hooks/cast.cpp
+++ b/lib/hooks/cast.cpp
@@ -103,9 +103,9 @@ public:
 
 		assert(state != STATE_STARTED);
 
-		const char *name = NULL;
-		const char *unit = NULL;
-		const char *type = NULL;
+		const char *name = nullptr;
+		const char *unit = nullptr;
+		const char *type = nullptr;
 
 		ret = json_unpack_ex(cfg, &err, 0, "{ s: o, s?: s, s?: s, s?: s }",
 			"signal", &json_signal,
@@ -122,7 +122,7 @@ public:
 				break;
 
 			case JSON_INTEGER:
-				signal_name = NULL;
+				signal_name = nullptr;
 				signal_index = json_integer_value(json_signal);
 				break;
 
diff --git a/lib/hooks/dp.cpp b/lib/hooks/dp.cpp
index b913b5ee0..32ae3f641 100644
--- a/lib/hooks/dp.cpp
+++ b/lib/hooks/dp.cpp
@@ -201,7 +201,7 @@ public:
 				break;
 
 			case JSON_INTEGER:
-				signal_name = NULL;
+				signal_name = nullptr;
 				signal_index = json_integer_value(json_signal);
 				break;
 
diff --git a/lib/hooks/drop.cpp b/lib/hooks/drop.cpp
index efe38fc28..ec0fc6960 100644
--- a/lib/hooks/drop.cpp
+++ b/lib/hooks/drop.cpp
@@ -45,7 +45,7 @@ public:
 	{
 		assert(state == STATE_PREPARED || state == STATE_STOPPED);
 
-		prev = NULL;
+		prev = nullptr;
 
 		state = STATE_STARTED;
 	}
@@ -90,7 +90,7 @@ public:
 
 		if (prev) {
 			sample_decref(prev);
-			prev = NULL;
+			prev = nullptr;
 		}
 	}
 };
diff --git a/lib/hooks/limit_rate.cpp b/lib/hooks/limit_rate.cpp
index 1cd436072..6886e4738 100644
--- a/lib/hooks/limit_rate.cpp
+++ b/lib/hooks/limit_rate.cpp
@@ -41,7 +41,7 @@ void LimitRateHook::parse(json_t *cfg)
 	assert(state != STATE_STARTED);
 
 	double rate;
-	const char *m = NULL;
+	const char *m = nullptr;
 
 	ret = json_unpack_ex(cfg, &err, 0, "{ s: F, s?: s }",
 		"rate", &rate,
diff --git a/lib/hooks/print.cpp b/lib/hooks/print.cpp
index a9b82a8c9..d002662b1 100644
--- a/lib/hooks/print.cpp
+++ b/lib/hooks/print.cpp
@@ -95,7 +95,7 @@ public:
 
 	virtual void parse(json_t *cfg)
 	{
-		const char *f = NULL, *p = NULL, *u = NULL;
+		const char *f = nullptr, *p = nullptr, *u = nullptr;
 		int ret;
 		json_error_t err;
 
diff --git a/lib/hooks/restart.cpp b/lib/hooks/restart.cpp
index b61d38d77..2592d51d7 100644
--- a/lib/hooks/restart.cpp
+++ b/lib/hooks/restart.cpp
@@ -45,7 +45,7 @@ public:
 	{
 		assert(state == STATE_PREPARED);
 
-		prev = NULL;
+		prev = nullptr;
 
 		state = STATE_STARTED;
 	}
diff --git a/lib/hooks/scale.cpp b/lib/hooks/scale.cpp
index 39141c9d5..929cf2740 100644
--- a/lib/hooks/scale.cpp
+++ b/lib/hooks/scale.cpp
@@ -89,7 +89,7 @@ public:
 				break;
 
 			case JSON_INTEGER:
-				signal_name = NULL;
+				signal_name = nullptr;
 				signal_index = json_integer_value(json_signal);
 				break;
 
diff --git a/lib/hooks/shift_ts.cpp b/lib/hooks/shift_ts.cpp
index 4886ed57f..85a413a8c 100644
--- a/lib/hooks/shift_ts.cpp
+++ b/lib/hooks/shift_ts.cpp
@@ -51,7 +51,7 @@ public:
 	virtual void parse(json_t *cfg)
 	{
 		double o;
-		const char *m = NULL;
+		const char *m = nullptr;
 		int ret;
 		json_error_t err;
 
diff --git a/src/villas-relay.cpp b/src/villas-relay.cpp
index 8ebe2fdd0..f52c8cd36 100644
--- a/src/villas-relay.cpp
+++ b/src/villas-relay.cpp
@@ -74,7 +74,7 @@ lws_protocols protocols[] = {
 		.per_session_data_size = sizeof(Connection),
 		.rx_buffer_size = 0
 	},
-	{ NULL /* terminator */ }
+	{ nullptr /* terminator */ }
 };
 
 /** List of libwebsockets extensions. */
@@ -89,18 +89,18 @@ static const lws_extension extensions[] = {
 		lws_extension_callback_pm_deflate,
 		"deflate_frame"
 	},
-	{ NULL /* terminator */ }
+	{ nullptr /* terminator */ }
 };
 
 static const lws_http_mount mount = {
-	.mount_next =		NULL, /* linked-list "next" */
+	.mount_next =		nullptr, /* linked-list "next" */
 	.mountpoint =		"/api/v1", /* mountpoint URL */
-	.origin =		NULL,	/* protocol */
-	.def =			NULL,
+	.origin =		nullptr,	/* protocol */
+	.def =			nullptr,
 	.protocol =		"http-api",
-	.cgienv =		NULL,
-	.extra_mimetypes =	NULL,
-	.interpret =		NULL,
+	.cgienv =		nullptr,
+	.extra_mimetypes =	nullptr,
+	.interpret =		nullptr,
 	.cgi_timeout =		0,
 	.cache_max_age =	0,
 	.auth_mask =		0,
@@ -109,7 +109,7 @@ static const lws_http_mount mount = {
 	.cache_intermediaries =	0,
 	.origin_protocol =	LWSMPRO_CALLBACK, /* dynamic */
 	.mountpoint_len =	7, /* char count */
-	.basic_auth_login_file =NULL,
+	.basic_auth_login_file =nullptr,
 };
 
 Session::Session(Identifier sid) :
@@ -120,7 +120,7 @@ Session::Session(Identifier sid) :
 
 	sessions[sid] = this;
 
-	created = time(NULL);
+	created = time(nullptr);
 
 	uuid_generate(uuid);
 }
@@ -196,7 +196,7 @@ Connection::Connection(lws *w) :
 
 	lws_get_peer_addresses(wsi, lws_get_socket_fd(wsi), name, sizeof(name), ip, sizeof(ip));
 
-	created = time(NULL);
+	created = time(nullptr);
 
 	console->info("New connection established: session={}, remote={} ({})", session->identifier, name, ip);
 }
@@ -465,13 +465,13 @@ check:		if (optarg == endptr) {
 	ctx_info.mounts = &mount;
 
 	context = lws_create_context(&ctx_info);
-	if (context == NULL) {
+	if (context == nullptr) {
 		console->error("WebSocket: failed to initialize server context");
 		exit(EXIT_FAILURE);
 	}
 
 	vhost = lws_create_vhost(context, &ctx_info);
-	if (vhost == NULL) {
+	if (vhost == nullptr) {
 		console->error("WebSocket: failed to initialize virtual host");
 		exit(EXIT_FAILURE);
 	}