diff --git a/lib/formats/CMakeLists.txt b/lib/formats/CMakeLists.txt
index d8e3689cb..71b0d3127 100644
--- a/lib/formats/CMakeLists.txt
+++ b/lib/formats/CMakeLists.txt
@@ -71,5 +71,6 @@ list(APPEND FORMAT_SRC
 )
 
 add_library(formats STATIC ${FORMAT_SRC})
+target_compile_options(formats PRIVATE -x c)
 target_include_directories(formats PUBLIC ${INCLUDE_DIRS})
 target_link_libraries(formats INTERFACE ${LIBRARIES} PUBLIC villas-common)
diff --git a/lib/formats/json.cpp b/lib/formats/json.cpp
index cf3933ba4..3c2cbb444 100644
--- a/lib/formats/json.cpp
+++ b/lib/formats/json.cpp
@@ -67,8 +67,8 @@ static int json_unpack_timestamps(json_t *json_ts, struct sample *smp)
 {
 	int ret;
 	json_error_t err;
-	json_t *json_ts_origin = nullptr;
-	json_t *json_ts_received = nullptr;
+	json_t *json_ts_origin = NULL;
+	json_t *json_ts_received = NULL;
 
 	json_unpack_ex(json_ts, &err, 0, "{ s?: o, s?: o }",
 		"origin",   &json_ts_origin,
@@ -173,7 +173,7 @@ static int json_unpack_sample(struct io *io, json_t *json_smp, struct sample *sm
 {
 	int ret;
 	json_error_t err;
-	json_t *json_data, *json_value, *json_ts = nullptr;
+	json_t *json_data, *json_value, *json_ts = NULL;
 	size_t i;
 	int64_t sequence = -1;
 
diff --git a/lib/formats/json_reserve.cpp b/lib/formats/json_reserve.cpp
index 75f350604..4b54a6ae6 100644
--- a/lib/formats/json_reserve.cpp
+++ b/lib/formats/json_reserve.cpp
@@ -37,8 +37,8 @@ static int json_reserve_pack_sample(struct io *io, json_t **j, struct sample *sm
 {
 	json_error_t err;
 	json_t *json_data, *json_name, *json_unit, *json_value;
-	json_t *json_created = nullptr;
-	json_t *json_sequence = nullptr;
+	json_t *json_created = NULL;
+	json_t *json_sequence = NULL;
 
 	if (smp->flags & SAMPLE_HAS_TS_ORIGIN)
 		json_created = json_integer(time_to_double(&smp->ts.origin) * 1e3);
@@ -67,7 +67,7 @@ static int json_reserve_pack_sample(struct io *io, json_t **j, struct sample *sm
 		if (sig->unit)
 			json_unit = json_string(sig->unit);
 		else
-			json_unit = nullptr;
+			json_unit = NULL;
 
 		json_value = json_pack_ex(&err, 0, "{ s: o, s: f }",
 			"name", json_name,
@@ -94,7 +94,7 @@ static int json_reserve_pack_sample(struct io *io, json_t **j, struct sample *sm
 	*j = json_pack_ex(&err, 0, "{ s: o }",
 		"measurements", json_data
 	);
-	if (*j == nullptr)
+	if (*j == NULL)
 		return -1;
 #if 0
 #ifdef JSON_RESERVE_INTEGER_TARGET
@@ -124,8 +124,8 @@ static int json_reserve_unpack_sample(struct io *io, json_t *json_smp, struct sa
 	int ret, idx;
 	double created = -1;
 	json_error_t err;
-	json_t *json_value, *json_data = nullptr;
-	json_t *json_origin = nullptr, *json_target = nullptr;
+	json_t *json_value, *json_data = NULL;
+	json_t *json_origin = NULL, *json_target = NULL;
 	size_t i;
 
 	ret = json_unpack_ex(json_smp, &err, 0, "{ s?: o, s?: o, s?: o, s?: o }",
@@ -173,7 +173,7 @@ static int json_reserve_unpack_sample(struct io *io, json_t *json_smp, struct sa
 	smp->length = 0;
 
 	json_array_foreach(json_data, i, json_value) {
-		const char *name, *unit = nullptr;
+		const char *name, *unit = NULL;
 		double value;
 
 		ret = json_unpack_ex(json_value, &err, 0, "{ s: s, s?: s, s: F, s?: F }",
diff --git a/lib/formats/protobuf.cpp b/lib/formats/protobuf.cpp
index d60d992de..6ffd0652a 100644
--- a/lib/formats/protobuf.cpp
+++ b/lib/formats/protobuf.cpp
@@ -29,6 +29,10 @@
 #include <villas/plugin.h>
 #include <villas/formats/protobuf.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 static enum signal_type protobuf_detect_format(Villas__Node__Value *val)
 {
 	switch (val->value_case) {
@@ -128,14 +132,14 @@ int protobuf_sprint(struct io *io, char *buf, size_t len, size_t *wbytes, struct
 		goto out;
 
 	villas__node__message__pack(pb_msg, (uint8_t *) buf);
-	villas__node__message__free_unpacked(pb_msg, nullptr);
+	villas__node__message__free_unpacked(pb_msg, NULL);
 
 	*wbytes = psz;
 
 	return cnt;
 
 out:
-	villas__node__message__free_unpacked(pb_msg, nullptr);
+	villas__node__message__free_unpacked(pb_msg, NULL);
 
 	return -1;
 }
@@ -145,7 +149,7 @@ int protobuf_sscan(struct io *io, const char *buf, size_t len, size_t *rbytes, s
 	unsigned i, j;
 	Villas__Node__Message *pb_msg;
 
-	pb_msg = villas__node__message__unpack(nullptr, len, (uint8_t *) buf);
+	pb_msg = villas__node__message__unpack(NULL, len, (uint8_t *) buf);
 	if (!pb_msg)
 		return -1;
 
@@ -216,7 +220,7 @@ int protobuf_sscan(struct io *io, const char *buf, size_t len, size_t *rbytes, s
 	if (rbytes)
 		*rbytes = villas__node__message__get_packed_size(pb_msg);
 
-	villas__node__message__free_unpacked(pb_msg, nullptr);
+	villas__node__message__free_unpacked(pb_msg, NULL);
 
 	return i;
 }
@@ -233,3 +237,6 @@ static struct plugin p = {
 	}
 };
 REGISTER_PLUGIN(&p);
+#ifdef __cplusplus
+}
+#endif