Mosquitto C++ Client on Raspberry PI not recieving messages - c++

Question

I'm trying to use mqtt on my raspberrpi4.
I installed mqtt and the c++ headers.
I start my broker with :
mosquitto -d
Then for a validation I start a client via the command line:
mosquitto_sub -d -t "test"
Then I also start the c++ client (following code):
#include <string.h>
#include <stdio.h>
#include <mosquitto.h>
char mqMsg[30];
void my_message_callback(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *message)
{
if(message->payloadlen){
printf("BANANA %s %s\n", message->topic, message->payload);
}else{
printf("BONONO %s (null)\n", message->topic);
}
fflush(stdout);
}
void my_connect_callback(struct mosquitto *mosq, void *userdata, int result)
{
int i;
if(!result){
mosquitto_publish(mosq, NULL, "hello/world", strlen(mqMsg), mqMsg, 2, 0);
}else{
fprintf(stderr, "Connect failed\n");
}
}
void my_log_callback(struct mosquitto *mosq, void *userdata, int level, const char *str)
{
/* Pring all log messages regardless of level. */
printf("%s\n", str);
}
void my_publish_callback(struct mosquitto *mosq, void *userdata, int usernumber)
{
/* We've published so lets exit nicely */
mosquitto_disconnect(mosq);
}
int main(int argc, char *argv[])
{
int i;
char *host = "localhost";
int port = 1883;
int keepalive = 60;
bool clean_session = true;
struct mosquitto *mosq = NULL;
sprintf(mqMsg,"%s",argv[1]);
printf("And the word is >> %s <<\n", mqMsg);
mosquitto_lib_init();
mosq = mosquitto_new(NULL, clean_session, NULL);
if(!mosq){
fprintf(stderr, "Error: Out of memory.\n");
return 1;
}
mosquitto_log_callback_set(mosq, my_log_callback);
mosquitto_connect_callback_set(mosq, my_connect_callback);
mosquitto_message_callback_set(mosq, my_message_callback);
mosquitto_publish_callback_set(mosq, my_publish_callback);
if(mosquitto_connect(mosq, host, port, keepalive)){
fprintf(stderr, "Unable to connect.\n");
return 1;
}
mosquitto_loop_forever(mosq, -1, 1);
mosquitto_subscribe(mosq, NULL, "test", 2);
while(1){
}
mosquitto_destroy(mosq);
mosquitto_lib_cleanup();
return 0;
}
After that I publish something on the topic
mosquitto_pub -d -t "test" -m "test message"
I can see the message on the command line client but not on the c++ client?
Any idea why?
B.t.w. publishing via the c++ client works.
Thanks!

Answer 1

You call mosquitto_subscribe(mosq, NULL, "test", 2); after mosquitto_loop_forever(), therefore when your client actually runs, it's not subscribed to your topic. This also means that you don't need the while(1) in your code. See https://mosquitto.org/api/files/mosquitto-h.html#mosquitto_loop_forever for the description of the `mosquitto_loop_forever()' function.