String & Char concatenations

Question

A newbie in Arduino- and I'm trying to concatenate string and chars ( well in python it is quite different ).

I'm trying to post a MQTT message- constructed following format to the client.publish(outTopic, "["+outTopic+"]"+ msg)

while outTopic is a char* variable, and msg is char- but I get an error for such concatenation.

please your help

#include <ESP8266WiFi.h>
#include <PubSubClient.h>
// GPIOs on SONOFF board
int LED_SONOFF = 13;
int REL_SONOFF = 12;
int PIN_SW = 14;
const char* ssid = "HomeNetwork_2.4G";
const char* password = "pswd";
const char* mqtt_server = "192.168.2.113";
WiFiClient espClient;
PubSubClient client(espClient);
char msg[50];
const char* outTopic = "HomePi/Dvir/Messages";
const char* inTopic = "HomePi/Dvir/Windows/SONOFF";
const char* inTopic2 = "HomePi/Dvir/Windows/All";
const char* clientID = "Sonoff#1";
String m1, m2;
void start_wifi() {
 WiFi.mode(WIFI_STA);
 WiFi.begin(ssid, password);
 while (WiFi.status() != WL_CONNECTED) {
 delay(500);
 Serial.print(".");
 }
 Serial.println("");
 Serial.println("WiFi connected"); 
 Serial.print("IP address: ");
 Serial.println(WiFi.localIP()); 
}
void setup() {
 start_wifi(); // start wifi service
 Serial.begin(115200); // start serial service
 delay(10);
 // define sonoff gpios
 pinMode(LED_SONOFF, OUTPUT);
 pinMode(REL_SONOFF, OUTPUT);
 pinMode(PIN_SW, INPUT_PULLUP); // GPIO14 has a pullup resistor
 digitalWrite(LED_SONOFF,HIGH); // LED high is OFF
 digitalWrite(REL_SONOFF,LOW); 
 // start mqtt
 client.setServer(mqtt_server, 1883);
 client.setCallback(callback); 
}
void switch_on() {
 digitalWrite(REL_SONOFF,HIGH);
 digitalWrite(LED_SONOFF, LOW); // LED ON
 pub_mqtt("Switch ON");
}
void switch_off() {
 digitalWrite(REL_SONOFF,LOW);
 digitalWrite(LED_SONOFF, HIGH); // LED OFF
 pub_mqtt("Switch OFF");
}
void callback(char* topic, byte* payload, unsigned int length) {
 Serial.print("Message arrived [");
 Serial.print(topic);
 Serial.print("] ");
 for (int i = 0; i < length; i++) {
 Serial.print((char)payload[i]);
 }
 Serial.println();
 if ((char)payload[0] == '1') {
 switch_on(); 
 } 
 if ((char)payload[0] == '0') {
 switch_off();
 Serial.print("payload: ");
 Serial.println(payload[0]);
 }
}
void reconnect() {
 while (!client.connected()) {
 Serial.print("Attempting MQTT connection...");
 if (client.connect("ESP8266Client")) {
 Serial.println("connected");
 client.publish(outTopic, "Connected");
 client.subscribe(inTopic);
 m1="Subscribed to: "+inTopic;
 client.publish(outTopic, m1);
 if (inTopic2 !=""){
 client.subscribe(inTopic2);
 m2="Subscribed to: ";//+ String(inTopic2);
 client.publish(outTopic, m2);
 }
 } else {
 Serial.print("failed, rc=");
 Serial.print(client.state());
 Serial.println(" try again in 5 seconds");
 delay(5000);
 }
 }
}
void pub_mqtt(char* msg) {
 client.publish(outTopic, msg);
}
void loop() {
 if (!client.connected()) {
 reconnect();
 }
 client.loop();
 if (digitalRead(PIN_SW)==1) {
 if (digitalRead(REL_SONOFF)!=LOW) {
 switch_off();
 }
 }
 else if (digitalRead(PIN_SW)==0) {
 if (digitalRead(REL_SONOFF)!=HIGH){
 switch_on();
 }
 }
 delay(100);
}

• st2000: Likely it would be best if you created a new string in which you place what you want to pass to the called method. Guessing, you have asked if the C/C++ compiler could implicitly do this by asking it to arrange for pulling out bits of the string from flash memory (the const outTopic and the "[" characters you added in the line of code) with bits from RAM (the string msg).

Answer 1

You can't simply concatenate C strings like that. Instead you have to build them up into a new string. For formatting like that the simplest way is with sprintf:

char outMessage[strlen(outTopic) + strlen(msg) + 3]; // Room for topic, message, [ and ] and terminating 0円
sprintf(outMessage, "[%s]%s", outTopic, msg);
client.publish(outTopic, outMessage);

Another method is with strcpy and strcat:

char outMessage[strlen(outTopic) + strlen(msg) + 3]; // Room for topic, message, [ and ] and terminating 0円
strcpy(outMessage, "[");
strcat(outMessage, outTopic);
strcat(outMessage, "]");
strcat(outMessage, msg);
client.publish(outTopic, outMessage);

You have to ensure that your "holding" string outMessage is big enough to hold everything you want to send, plus a terminating NULL character, otherwise your program will crash.

Another way of creating the "holding" variable is with alloca(). That allocates bytes on the stack for a variable - just like malloc() does on the heap, but doesn't require free()ing because it's on the stack and is reclaimed when the function exits:

char *outMessage = (char *)alloca(strlen(outTopic) + strlen(msg) + 3);

• guyd: thank you- i fund first method more short and sensable. does it matter that outTopic is char* ?
• Majenko: %s connects to a char * data type.
• guyd: I was referring to char outMessage[strlen(outTopic) + strlen(msg) + 3];
• Majenko: char *foo and char foo[x] are essentially the same. The second is a char * with memory allocation added in.
• guyd: @Mejnko - can you please explain why +3 is needed, since for purpose of string concatenation, it is not needed - does it relevent for sending ?