added readme and config example

config example might missing config examples... ?! :D

.gitignore

@@ -1,2 +1,3 @@
 bin/
 .vscode/
+config.h

Makefile

@@ -6,7 +6,7 @@ command:
 	gcc -o ${BIN}/command network.c command.c
 
 readref:
-	gcc -o ${BIN}/readref network.c readref.c
+	gcc -o ${BIN}/readref network.c mqtt.c -lmosquitto readref.c -lm
 
 .PHONY: clean dirs
 clean:

README.md

@@ -0,0 +1,7 @@
+## build
+```bash
+apt install libmosquitto-dev
+cp config.h.example config.h
+# edit config.h
+make
+```

command.c

@@ -4,23 +4,27 @@
 
 #include "network.h"
 
-int main(int argc, char *argv[]) {
+int main(int argc, char *argv[])
+{
     int xPlaneSocket;
 
     // check argument
-	if (argc != 2) {
+    if (argc != 2)
+    {
         fprintf(stderr, "Usage: %s <command>\n", argv[0]);
         exit(EXIT_FAILURE);
     }
 
     // network
     xPlaneSocket = createSocket(SRC_PORT);
-	if (xPlaneSocket < 0) {
+    if (xPlaneSocket < 0)
+    {
         exit(EXIT_FAILURE);
     }
 
     // send payload
-	if (sendToXPlane(xPlaneSocket, DEST_SERVER, DEST_PORT, "CMND", argv[1], strlen(argv[1])) < 0) {
+    if (sendToXPlane(xPlaneSocket, DEST_SERVER, DEST_PORT, "CMND", argv[1], strlen(argv[1])) < 0)
+    {
         exit(EXIT_FAILURE);
     }
 

config.h.example

@@ -0,0 +1,2 @@
+#define MQTT_USER1 "<user>"
+#define MQTT_PASS "<pass>"

mqtt.c

@@ -0,0 +1,93 @@
+#include <stdio.h>
+
+#include <errno.h>
+#include <string.h>
+
+// https://mosquitto.org/api/files/mosquitto-h.html
+#include <mosquitto.h>
+
+struct mosquitto *connectMqtt(char *user, char *pass, void (*messageCallback)(struct mosquitto *, void *, const struct mosquitto_message *))
+{
+    int err;
+    struct mosquitto *mqtt;
+
+    err = mosquitto_lib_init();
+    if (err)
+    {
+        fprintf(stderr, "Error initalizing mosquitto lib (%d): %s\n", err, mosquitto_strerror(err));
+        return NULL;
+    }
+
+    mqtt = mosquitto_new(user, true, NULL);
+    if (mqtt == NULL)
+    {
+        fprintf(stderr, "Error creating mosquitto instance (%d): %s\n", errno, strerror(errno));
+        return NULL;
+    }
+
+    err = mosquitto_username_pw_set(mqtt, user, pass);
+    if (err)
+    {
+        fprintf(stderr, "Error setting username & password (%d): %s\n", err, mosquitto_strerror(err));
+        return NULL;
+    }
+
+    // FIXME: I am hard coded :(
+    err = mosquitto_connect(mqtt, "openhab.sugarland.lan", 1883, 10);
+    if (err)
+    {
+        fprintf(stderr, "Error on connection of type ");
+        if (err == MOSQ_ERR_ERRNO)
+        {
+            fprintf(stderr, "system (%d): %s\n", errno, strerror(errno));
+        }
+        else
+        {
+            fprintf(stderr, "mosquitto (%d): %s\n", err, mosquitto_strerror(err));
+        }
+        return NULL;
+    }
+
+    mosquitto_message_callback_set(mqtt, messageCallback);
+
+    return mqtt;
+}
+
+int sendMqtt(struct mosquitto *mqtt, char *topic, char *payload, int payloadLength)
+{
+    int err;
+
+    err = mosquitto_publish(mqtt, NULL, topic, payloadLength, payload, 0, false);
+    if (err)
+    {
+        fprintf(stderr, "Error publishing message to %s (%d): %s\n", topic, err, mosquitto_strerror(err));
+        return -1;
+    }
+    else
+    {
+        return 0;
+    }
+}
+
+int recvMqtt(struct mosquitto *mqtt, char *topic)
+{
+    int err;
+    
+    err = mosquitto_subscribe(mqtt, NULL, topic, 0);
+    if (err) {
+        fprintf(stderr, "Error subscribing to %s (%d): %s", topic, err, mosquitto_strerror(err));
+        return -1;
+    }
+    else
+    {
+        return 0;
+    }
+}
+
+void disconnectMqtt(struct mosquitto *mqtt)
+{
+    // ignore errors
+    mosquitto_disconnect(mqtt);
+    mosquitto_destroy(mqtt);
+    mosquitto_lib_cleanup();
+}

mqtt.h

@@ -0,0 +1,4 @@
+struct mosquitto * connectMqtt(char *user, char *pass, void (*messageCallback)(struct mosquitto *, void *, const struct mosquitto_message *));
+int sendMqtt(struct mosquitto *mqtt, char *topic, char *payload, int payloadLength);
+int recvMqtt(struct mosquitto *mqtt, char *topic);
+void disconnectMqtt(struct mosquitto *mqtt);

network.c

@@ -15,18 +15,21 @@
 const int DGRAM_MSG_START = 0;
 const int DGRAM_PAYLOAD_START = DGRAM_MSG_START + DGRAM_MSG_LENGTH + DGRAM_NULL_LENGTH;
 
-int sendToXPlane(int sock, char *destAddr, int destPort, char *msg, void *payload, int lengthOfPayload) {
+int sendToXPlane(int sock, char *destAddr, int destPort, char *msg, void *payload, int lengthOfPayload)
+{
     char bytesToSend[SEND_BUFFER];
     struct sockaddr_in6 xPlaneAddress;
     memset(bytesToSend, 0, SEND_BUFFER);
 
     // msg is always 4 bytes long
-	if (strlen(msg) != DGRAM_MSG_LENGTH) {
+    if (strlen(msg) != DGRAM_MSG_LENGTH)
+    {
         return -10;
     }
 
     // payload shouldn't exceed buffer
-	if (lengthOfPayload > (SEND_BUFFER - DGRAM_MSG_LENGTH - DGRAM_NULL_LENGTH)) {
+    if (lengthOfPayload > (SEND_BUFFER - DGRAM_MSG_LENGTH - DGRAM_NULL_LENGTH))
+    {
         return -11;
     }
 
@@ -39,35 +42,55 @@ int sendToXPlane(int sock, char *destAddr, int destPort, char *msg, void *payloa
     inet_pton(AF_INET6, destAddr, &xPlaneAddress.sin6_addr.s6_addr);
 
     int numBytesSend = sendto(sock, bytesToSend, DGRAM_MSG_LENGTH + DGRAM_NULL_LENGTH + lengthOfPayload, 0, (struct sockaddr *)&xPlaneAddress, sizeof(xPlaneAddress));
-	if (numBytesSend >= 0) {
+    if (numBytesSend >= 0)
+    {
         return numBytesSend;
-	} else {
+    }
-		fprintf(stderr, "Failed to send message (%d): %s", errno, strerror(errno));
+    else
+    {
+        fprintf(stderr, "Failed to send message (%d): %s\n", errno, strerror(errno));
         return -1;
     }
 }
 
-int recvFromXPlane(int sock, char msg[DGRAM_MSG_LENGTH], void *payload, int maxLength) {
+int recvFromXPlane(int sock, char msg[DGRAM_MSG_LENGTH], void *payload, int maxLength)
+{
     char buf[maxLength + DGRAM_MSG_LENGTH];
     memset(buf, 0, sizeof(buf));
-	int recvBytes = recv(sock, buf, maxLength + DGRAM_MSG_LENGTH, 0);
+
-	if (recvBytes >= 0) {
+    strcpy(msg, "");
-		if (recvBytes >= DGRAM_MSG_LENGTH) {
+
+    int recvBytes = recv(sock, buf, maxLength + DGRAM_MSG_LENGTH, MSG_DONTWAIT);
+    if (recvBytes >= 0)
+    {
+        if (recvBytes >= DGRAM_MSG_LENGTH)
+        {
             memcpy(msg, &buf[0], DGRAM_MSG_LENGTH);
             memcpy(payload, &buf[DGRAM_MSG_LENGTH], maxLength);
             return recvBytes - DGRAM_MSG_LENGTH;
-		} else {
+        }
-			fprintf(stderr, "Received data without message header?!");
+        else
+        {
+            fprintf(stderr, "Received data without message header?!\n");
             return -11;
         }
-	} else {
+    }
-		fprintf(stderr, "Error receiving data (%d): %s", errno, strerror(errno));
+    else
+    {
+        if (errno != EAGAIN)
+        {
+            fprintf(stderr, "Error receiving data (%d): %s\n", errno, strerror(errno));
             return -1;
         }
-
+        else
+        {
+            return 0;
+        }
+    }
 }
 
-int createSocket(int srcPort) {
+int createSocket(int srcPort)
+{
     int xPlaneSocket;
     struct sockaddr_in6 localAddress;
 
@@ -75,7 +98,8 @@ int createSocket(int srcPort) {
 
     // create socket
     xPlaneSocket = socket(AF_INET6, SOCK_DGRAM, 0);
-	if (xPlaneSocket == -1) {
+    if (xPlaneSocket == -1)
+    {
         fprintf(stderr, "Error creating socket (%d): %s\n", errno, strerror(errno));
         return -1;
     }
@@ -87,7 +111,8 @@ int createSocket(int srcPort) {
 
     // bind to local port
     err = bind(xPlaneSocket, (struct sockaddr *)&localAddress, sizeof(localAddress));
-	if (err) {
+    if (err)
+    {
         fprintf(stderr, "Error binding [::]:%d (%d): %s\n", srcPort, errno, gai_strerror(errno));
         return -1;
     }
@@ -95,6 +120,7 @@ int createSocket(int srcPort) {
     return xPlaneSocket;
 }
 
-void closeSocket(int sock) {
+void closeSocket(int sock)
+{
     close(sock);
 }

readref.c

@@ -2,86 +2,240 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
+#include <math.h>
 
+#include <mosquitto.h>
 #include "network.h"
+#include "mqtt.h"
 
-struct dref_struct_in {
+#include "config.h"
-	int32_t dref_freq;
+
-	int32_t dref_sender_index;	// the index the customer is using to define this dataref
+#define MQTT_RREF_REQUEST_LENGTH_ID 3
-	char dref_string[400];
+#define MQTT_RREF_REQUEST_LENGTH_FREQ 3
+#define MQTT_RREF_REQUEST_LENGTH_SEPARATOR 1
+#define MQTT_RREF_REQUEST_SEPARATOR ' '
+
+const int MQTT_RREF_REQUEST_START_ID = 0;
+const int MQTT_RREF_REQUEST_START_FREQ = MQTT_RREF_REQUEST_START_ID + MQTT_RREF_REQUEST_LENGTH_ID + MQTT_RREF_REQUEST_LENGTH_SEPARATOR;
+const int MQTT_RREF_REQUEST_START_RREF = MQTT_RREF_REQUEST_START_FREQ + MQTT_RREF_REQUEST_LENGTH_FREQ + MQTT_RREF_REQUEST_LENGTH_SEPARATOR;
+
+const int MQTT_RREF_REQUEST_MIN_LENGTH = MQTT_RREF_REQUEST_LENGTH_ID + MQTT_RREF_REQUEST_LENGTH_FREQ + (2 * MQTT_RREF_REQUEST_LENGTH_SEPARATOR) + 1; // at least one char for rref
+
+int xPlaneSocket;
+struct mosquitto *mqtt;
+
+struct valueCacheElement
+{
+    float value;
+    bool force;
 };
 
-struct dref_struct_out {
+struct valueCacheElement *valueCache = NULL;
+int valueCacheCount = 0;
+
+#define XPLANE_RREF_MAX_LENGTH 400
+struct dref_struct_in
+{
+    int32_t dref_freq;
+    int32_t dref_sender_index; // the index the customer is using to define this dataref
+    char dref_string[XPLANE_RREF_MAX_LENGTH];
+};
+
+struct dref_struct_out
+{
     int32_t dref_sender_index;
     float dref_flt_value;
 };
 
-int main(int argc, char *argv[]) {
+int *strtoint(char *in, size_t start, size_t n)
-	int xPlaneSocket;
+{
-	char *wtf;
+    int *ret = malloc(sizeof(int));
+    *ret = 0;
+    int offset = start + n - 1;
 
-	// check argument
+    if (offset < 0)
-	if (argc != 1 && (argc - 1) % 3 != 0) {
+    {
-		fprintf(stderr, "Usage: %s [<dataref> <frequency> <id> [<dataref> <frequency> <id> ... ]]\n", argv[0]);
+        return NULL;
-		exit(EXIT_FAILURE);
     }
 
-	// network
+    for (int i = 0; i < n; i++)
-	xPlaneSocket = createSocket(SRC_PORT);
+    {
-	if (xPlaneSocket < 0) {
+        int position = offset - i;
-		exit(EXIT_FAILURE);
+        if (in[position] >= '0' && in[position] <= '9')
+        {
+            *ret += (in[position] - '0') * pow(10, i);
+        }
+        else
+        {
+            return NULL;
+        }
+    }
+
+    return ret;
+}
+
+void requestRrefFromXPlane(const struct mosquitto_message *message)
+{
+    int *index, *frequency;
+
+    if (message->payloadlen >= MQTT_RREF_REQUEST_MIN_LENGTH)
+    {
+        index = strtoint(message->payload, MQTT_RREF_REQUEST_START_ID, MQTT_RREF_REQUEST_LENGTH_ID);
+        frequency = strtoint(message->payload, MQTT_RREF_REQUEST_START_FREQ, MQTT_RREF_REQUEST_LENGTH_FREQ);
+
+        if (index == NULL || frequency == NULL)
+        {
+            return;
+        }
+
+        if (*index < valueCacheCount)
+        {
+            valueCache[*index].force = true;
         }
 
-//	char drefsToRead[413 * ((argc - 1) / 3)];
-//	memset(drefsToRead, 0, sizeof(drefsToRead));
-	for (int i = 1; i < argc; i = i + 3) {
         // prepare struct to send, make sure padding is with NULL bytes
         struct dref_struct_in drefToRead = {
-			.dref_freq = strtoimax(argv[i + 1], &wtf, 10),
+            .dref_freq = *frequency,
-			.dref_sender_index = strtoimax(argv[i + 2], &wtf, 10)
+            .dref_sender_index = *index};
-		};
         memset(drefToRead.dref_string, 0, sizeof(drefToRead.dref_string));
-		strcpy(drefToRead.dref_string, argv[i]);
+        strncpy(drefToRead.dref_string, &((char *)message->payload)[MQTT_RREF_REQUEST_START_RREF], message->payloadlen - MQTT_RREF_REQUEST_START_RREF);
 
-		// send payload
+        sendToXPlane(xPlaneSocket, DEST_SERVER, DEST_PORT, "RREF", (char *)&drefToRead, sizeof(drefToRead));
-		if (sendToXPlane(xPlaneSocket, DEST_SERVER, DEST_PORT, "RREF", (char *)&drefToRead, sizeof(drefToRead)) < 0) {
+    }
+}
+
+void sendCommandToXPlane(const struct mosquitto_message *message)
+{
+    char *command;
+
+    if (message->payloadlen > 0)
+    {
+        sendToXPlane(xPlaneSocket, DEST_SERVER, DEST_PORT, "CMND", (char *)message->payload, message->payloadlen);
+    }
+}
+
+void onMessage(struct mosquitto *mqtt, void *obj, const struct mosquitto_message *message)
+{
+    printf("Received message %.*s on topic %s\n", message->payloadlen, (char *)message->payload, message->topic);
+
+    if (strcmp(message->topic, "/xplane/meta/rref") == 0)
+    {
+        requestRrefFromXPlane(message);
+    }
+    else if (strcmp(message->topic, "/xplane/meta/cmnd") == 0)
+    {
+        sendCommandToXPlane(message);
+    }
+    else
+    {
+        fprintf(stderr, "Unkown topic %s\n", message->topic);
+    }
+}
+
+int main()
+{
+    // network
+    xPlaneSocket = createSocket(SRC_PORT);
+    if (xPlaneSocket < 0)
+    {
         exit(EXIT_FAILURE);
     }
 
-//		memcpy(&drefsToRead[((i - 1) / 3) * 413], &drefToRead, sizeof(drefToRead));
+    // mqtt
+    mqtt = connectMqtt(MQTT_USER1, MQTT_PASS, onMessage);
+    if (mqtt == NULL)
+    {
+        exit(EXIT_FAILURE);
     }
 
+    // subscribe meta channel
+    recvMqtt(mqtt, "/xplane/meta/#");
+
     // read from network
     char msg[DGRAM_MSG_LENGTH + 1];
     char payload[RECV_BUFFER];
-	while (1) {
+
+    char mqttTopic[100], mqttPayload[512];
+    while (1)
+    {
         int payloadBytes = recvFromXPlane(xPlaneSocket, msg, payload, RECV_BUFFER);
-		if (payloadBytes > 0) {
+        if (payloadBytes >= 0)
+        {
+            if (strcmp(msg, "RREF") == 0)
+            {
                 printf("Received message type %s\n", msg);
                 printf("Received message data (%d) ", payloadBytes);
-			for (int i = 0; i < payloadBytes; i++) {
+                for (int i = 0; i < payloadBytes; i++)
+                {
                     printf("%02x", payload[i]);
                 }
                 printf("\n");
-			if (strcmp(msg, "RREF") == 0) {
-				if ((payloadBytes - 1) % 8 == 0) {
-					// XXX: loop
-					struct dref_struct_out drefRead;
-					memcpy(&drefRead, &payload[1], 8);
 
+                // continue if payloadBytes is valid (1 byte + 8 bytes each dref)
+                if ((payloadBytes - 1) % sizeof(struct dref_struct_out) == 0)
+                {
+                    // loop throught all provided drefs
+                    for (int i = 1; i < payloadBytes; i += sizeof(struct dref_struct_out))
+                    {
+                        memset(mqttTopic, 0, sizeof(mqttTopic));
+                        memset(mqttPayload, 0, sizeof(mqttPayload));
+
+                        struct dref_struct_out drefRead;
+                        memcpy(&drefRead, &payload[i], sizeof(struct dref_struct_out));
+
+                        // add new element to array
+                        if (valueCache == NULL)
+                        {
+                            valueCacheCount = drefRead.dref_sender_index + 10;
+                            valueCache = calloc(valueCacheCount, sizeof(struct valueCacheElement));
+                        }
+                        else if (drefRead.dref_sender_index >= valueCacheCount)
+                        {
+                            valueCacheCount = drefRead.dref_sender_index + 10;
+                            valueCache = reallocarray(valueCache, valueCacheCount, sizeof(struct valueCacheElement));
+                        }
+                        else
+                        {
+                            // size of array is sufficient
+                        }
+
+                        if (valueCache[drefRead.dref_sender_index].force || valueCache[drefRead.dref_sender_index].value != drefRead.dref_flt_value)
+                        {
+                            valueCache[drefRead.dref_sender_index].value = drefRead.dref_flt_value;
                             printf("%d: %f\n", drefRead.dref_sender_index, drefRead.dref_flt_value);
-				} else {
+
+                            sprintf(mqttTopic, "/xplane/rref/%d", drefRead.dref_sender_index);
+                            sprintf(mqttPayload, "%f", drefRead.dref_flt_value);
+
+                            sendMqtt(mqtt, mqttTopic, mqttPayload, strlen(mqttPayload));
+                            valueCache[drefRead.dref_sender_index].force = false;
+                        }
+                        else
+                        {
+                            // neither changed nor forced
+                        }
+                    }
+                }
+                else
+                {
                     exit(EXIT_FAILURE);
                 }
-			} else {
+            }
+            else
+            {
                 // unknown to me
             }
-		} else {
+        }
+        else
+        {
             exit(EXIT_FAILURE);
         }
+
+        mosquitto_loop(mqtt, 0, 1);
     }
 
     // bye bye (and no, I don't care about error anymore; I'll exit anyway?!)
+    disconnectMqtt(mqtt);
     closeSocket(xPlaneSocket);
     exit(EXIT_SUCCESS);
 }

test/.gitignore

@@ -1,2 +1,5 @@
 airspeed
 heading
+mqtt
+mqtt.h
+strtoint

test/airspeed.c

@@ -15,11 +15,14 @@
 
 #define COMMAND_BUFFER 256
 
-char* printable(char *str) {
+char *printable(char *str)
+{
     char *ret = strdup(str);
 
-	for (int i = strlen(ret); i >= 0; i--) {
+    for (int i = strlen(ret); i >= 0; i--)
-		if (ret[i] == '\r' || ret[i] == '\n') {
+    {
+        if (ret[i] == '\r' || ret[i] == '\n')
+        {
             ret[i] = '\0';
         }
     }
@@ -27,7 +30,8 @@ char* printable(char *str) {
     return ret;
 }
 
-void sendCommand(int sock, char *cmd) {
+void sendCommand(int sock, char *cmd)
+{
     char bytesToSend[COMMAND_BUFFER];
     memset(bytesToSend, 0, COMMAND_BUFFER);
 
@@ -35,29 +39,35 @@ void sendCommand(int sock, char *cmd) {
     strcpy(&bytesToSend[5], cmd);
 
     int numBytesSend = send(sock, bytesToSend, 5 + strlen(cmd), 0);
-	if (numBytesSend >= 0) {
+    if (numBytesSend >= 0)
+    {
         // printf("Send %d bytes to %s:%d: %s\n", numBytesSend, SERVER, PORT, printable(bytesToSend));
-	} else {
+    }
+    else
+    {
         fprintf(stderr, "Failed to send message to %s:%d (%d): %s", SERVER, PORT, errno, strerror(errno));
         exit(EXIT_FAILURE);
     }
 }
 
-int main(int argc, char *argv[]) {
+int main(int argc, char *argv[])
+{
     int xplaneSocket;
     struct sockaddr_in6 xplaneAddress;
 
     int err;
 
     // check argument
-	if (argc != 2 || (strcmp(argv[1], "up") != 0 && strcmp(argv[1], "down") != 0)) {
+    if (argc != 2 || (strcmp(argv[1], "up") != 0 && strcmp(argv[1], "down") != 0))
+    {
         fprintf(stderr, "Usage: %s <up|down>\n", argv[0]);
         exit(EXIT_FAILURE);
     }
 
     // create socket
     xplaneSocket = socket(AF_INET6, SOCK_DGRAM, 0);
-	if (xplaneSocket == -1) {
+    if (xplaneSocket == -1)
+    {
         fprintf(stderr, "Error creating socket (%d): %s\n", errno, strerror(errno));
         exit(EXIT_FAILURE);
     }
@@ -69,17 +79,23 @@ int main(int argc, char *argv[]) {
 
     // connect to xplane
     err = connect(xplaneSocket, (struct sockaddr *)&xplaneAddress, sizeof(xplaneAddress));
-	if (err) {
+    if (err)
+    {
         fprintf(stderr, "Error connecting %s:%d (%d): %s\n", SERVER, PORT, err, gai_strerror(err));
         exit(EXIT_FAILURE);
     }
 
     // send payload
-	if (strcmp(argv[1], "up") == 0) {
+    if (strcmp(argv[1], "up") == 0)
+    {
         sendCommand(xplaneSocket, "sim/autopilot/airspeed_up");
-	} else if (strcmp(argv[1], "down") == 0) {
+    }
+    else if (strcmp(argv[1], "down") == 0)
+    {
         sendCommand(xplaneSocket, "sim/autopilot/airspeed_down");
-	} else {
+    }
+    else
+    {
         exit(EXIT_FAILURE);
     }
 

test/heading.c

@@ -15,11 +15,14 @@
 
 #define COMMAND_BUFFER 256
 
-char* printable(char *str) {
+char *printable(char *str)
+{
     char *ret = strdup(str);
 
-	for (int i = strlen(ret); i >= 0; i--) {
+    for (int i = strlen(ret); i >= 0; i--)
-		if (ret[i] == '\r' || ret[i] == '\n') {
+    {
+        if (ret[i] == '\r' || ret[i] == '\n')
+        {
             ret[i] = '\0';
         }
     }
@@ -27,7 +30,8 @@ char* printable(char *str) {
     return ret;
 }
 
-void sendCommand(int sock, char *cmd) {
+void sendCommand(int sock, char *cmd)
+{
     char bytesToSend[COMMAND_BUFFER];
     memset(bytesToSend, 0, COMMAND_BUFFER);
 
@@ -35,29 +39,35 @@ void sendCommand(int sock, char *cmd) {
     strcpy(&bytesToSend[5], cmd);
 
     int numBytesSend = send(sock, bytesToSend, 5 + strlen(cmd), 0);
-	if (numBytesSend >= 0) {
+    if (numBytesSend >= 0)
+    {
         // printf("Send %d bytes to %s:%d: %s\n", numBytesSend, SERVER, PORT, printable(bytesToSend));
-	} else {
+    }
+    else
+    {
         fprintf(stderr, "Failed to send message to %s:%d (%d): %s", SERVER, PORT, errno, strerror(errno));
         exit(EXIT_FAILURE);
     }
 }
 
-int main(int argc, char *argv[]) {
+int main(int argc, char *argv[])
+{
     int xplaneSocket;
     struct sockaddr_in6 xplaneAddress;
 
     int err;
 
     // check argument
-	if (argc != 2 || (strcmp(argv[1], "up") != 0 && strcmp(argv[1], "down") != 0)) {
+    if (argc != 2 || (strcmp(argv[1], "up") != 0 && strcmp(argv[1], "down") != 0))
+    {
         fprintf(stderr, "Usage: %s <up|down>\n", argv[0]);
         exit(EXIT_FAILURE);
     }
 
     // create socket
     xplaneSocket = socket(AF_INET6, SOCK_DGRAM, 0);
-	if (xplaneSocket == -1) {
+    if (xplaneSocket == -1)
+    {
         fprintf(stderr, "Error creating socket (%d): %s\n", errno, strerror(errno));
         exit(EXIT_FAILURE);
     }
@@ -69,17 +79,23 @@ int main(int argc, char *argv[]) {
 
     // connect to xplane
     err = connect(xplaneSocket, (struct sockaddr *)&xplaneAddress, sizeof(xplaneAddress));
-	if (err) {
+    if (err)
+    {
         fprintf(stderr, "Error connecting %s:%d (%d): %s\n", SERVER, PORT, err, gai_strerror(err));
         exit(EXIT_FAILURE);
     }
 
     // send payload
-	if (strcmp(argv[1], "up") == 0) {
+    if (strcmp(argv[1], "up") == 0)
+    {
         sendCommand(xplaneSocket, "sim/autopilot/heading_up");
-	} else if (strcmp(argv[1], "down") == 0) {
+    }
+    else if (strcmp(argv[1], "down") == 0)
+    {
         sendCommand(xplaneSocket, "sim/autopilot/heading_down");
-	} else {
+    }
+    else
+    {
         exit(EXIT_FAILURE);
     }
 

test/mqtt.c

@@ -0,0 +1,65 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <errno.h>
+#include <string.h>
+
+// https://mosquitto.org/api/files/mosquitto-h.html
+#include <mosquitto.h>
+
+#include "mqtt.h"
+
+int main(int argc, char *argv[])
+{
+    int err;
+    struct mosquitto *mqtt;
+
+    err = mosquitto_lib_init();
+    if (err)
+    {
+        fprintf(stderr, "Error initalizing mosquitto lib (%d): %s\n", err, mosquitto_strerror(err));
+        exit(EXIT_FAILURE);
+    }
+
+    mqtt = mosquitto_new(MQTT_USER, true, NULL);
+    if (mqtt == NULL)
+    {
+        fprintf(stderr, "Error creating mosquitto instance (%d): %s\n", errno, strerror(errno));
+        exit(EXIT_FAILURE);
+    }
+
+    err = mosquitto_username_pw_set(mqtt, MQTT_USER, MQTT_PASS);
+    if (err)
+    {
+        fprintf(stderr, "Error setting username & password (%d): %s\n", err, mosquitto_strerror(err));
+        exit(EXIT_FAILURE);
+    }
+
+    err = mosquitto_connect(mqtt, "openhab.sugarland.lan", 1883, 10);
+    if (err)
+    {
+        fprintf(stderr, "Error on connection of type ");
+        if (err == MOSQ_ERR_ERRNO)
+        {
+            fprintf(stderr, "system (%d): %s\n", errno, strerror(errno));
+        }
+        else
+        {
+            fprintf(stderr, "mosquitto (%d): %s\n", err, mosquitto_strerror(err));
+        }
+        exit(EXIT_FAILURE);
+    }
+
+    char *foo = "blah";
+    err = mosquitto_publish(mqtt, NULL, "/xplane/foo", strlen(foo), foo, 0, false);
+    if (err)
+    {
+        fprintf(stderr, "Error publishing message (%d): %s\n", err, mosquitto_strerror(err));
+        exit(EXIT_FAILURE);
+    }
+
+    err = mosquitto_disconnect(mqtt);
+    // ignore error
+
+    mosquitto_destroy(mqtt);
+}

test/strtoint.c

@@ -0,0 +1,44 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+
+int *strtoint(char *in, size_t start, size_t n)
+{
+    int *ret = malloc(sizeof(int));
+    *ret = 0;
+    int offset = start + n - 1;
+
+    if (offset < 0)
+    {
+        return NULL;
+    }
+
+    for (int i = 0; i < n; i++) {
+        int position = offset - i;
+        printf("foo: %c\n", in[position]);
+        if (in[position] >= '0' && in[position] <= '9')
+        {
+            *ret += (in[position] - '0') * pow(10, i);
+        }
+        else
+        {
+            return NULL;
+        }
+    }
+
+    return ret;
+}
+
+int main()
+{
+    char *in = "abcd123";
+    int *ret = strtoint(in, 4, 3);
+    if (ret != NULL)
+    {
+        printf("%s | %d\n", in, *ret);
+    }
+    else
+    {
+        printf("ret is NULL\n");
+    }
+}