写一段程序，基于paho.mqtt.c的同步库，实现基本的MQTT客户端功能

好的，这是一个关于MQTT客户端功能的问题，我可以回答。您可以使用paho.mqtt.c同步库来创建一个MQTT客户端程序。以下是一个基本的示例程序：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "MQTTClient.h"

#define ADDRESS "tcp://localhost:1883"
#define CLIENTID "ExampleClientSub"
#define TOPIC "MQTT Examples"
#define QOS 1
#define TIMEOUT 10000L

int main(int argc, char* argv[])
{
MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_message msg = MQTTClient_message_initializer;
MQTTClient_deliveryToken token;
int rc;

MQTTClient_create(&client, ADDRESS, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL);

conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;

if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to connect, return code %d\n", rc);
exit(-1);
}

if ((rc = MQTTClient_subscribe(client, TOPIC, QOS)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to subscribe, return code %d\n", rc);
exit(-1);
}

printf("Subscribed to topic \"%s\"\nPress Q<Enter> to quit\n\n", TOPIC);

do
{
msg = MQTTClient_message_initializer;
if ((rc = MQTTClient_receive(client, &msg, TIMEOUT)) == MQTTCLIENT_SUCCESS)
{
printf("Received message on topic \"%s\": %s\n", msg.topicName, msg.payload);
MQTTClient_freeMessage(&msg);
MQTTClient_free(topicName);
}
} while (getchar()!='Q');

if ((rc = MQTTClient_unsubscribe(client, TOPIC)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to unsubscribe, return code %d\n", rc);
exit(-1);
}

MQTTClient_disconnect(client, 10000);
MQTTClient_destroy(&client);
return rc;
}

希望这个程序能够帮助您实现基本的MQTT客户端功能。