用C语言写一段MQTT协议服务器代码

以下是一个简单的MQTT协议服务器代码示例，使用了Eclipse Paho MQTT C库：

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

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

volatile MQTTAsync_token deliveredtoken;

int msgarrvd(void *context, char *topicName, int topicLen, MQTTAsync_message *message)
{
    printf("Message arrived\n");
    printf("  topic: %s\n", topicName);
    printf("  message: %.*s\n", message->payloadlen, (char*)message->payload);
    MQTTAsync_freeMessage(&message);
    MQTTAsync_free(topicName);
    return 1;
}

void onConnectFailure(void* context, MQTTAsync_failureData* response)
{
    printf("Connect failed, rc %d\n", response ? response->code : 0);
    exit(EXIT_FAILURE);
}

void onConnect(void* context, MQTTAsync_successData* response)
{
    MQTTAsync client = (MQTTAsync)context;
    MQTTAsync_responseOptions opts = MQTTAsync_responseOptions_initializer;
    MQTTAsync_message pubmsg = MQTTAsync_message_initializer;
    int rc;

    printf("Connected\n");

    opts.onSuccess = NULL;
    opts.onFailure = NULL;
    opts.context = NULL;

    pubmsg.payload = "Hello MQTT";
    pubmsg.payloadlen = strlen(pubmsg.payload);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    deliveredtoken = 0;

    if ((rc = MQTTAsync_sendMessage(client, TOPIC, &pubmsg, &opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start sendMessage, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }
}

void onDisconnect(void* context, MQTTAsync_successData* response)
{
    printf("Disconnected\n");
    exit(EXIT_SUCCESS);
}

int main(int argc, char* argv[])
{
    MQTTAsync client;
    MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
    MQTTAsync_disconnectOptions disc_opts = MQTTAsync_disconnectOptions_initializer;
    int rc;

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

    MQTTAsync_setCallbacks(client, NULL, NULL, msgarrvd, NULL);

    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;
    conn_opts.onSuccess = onConnect;
    conn_opts.onFailure = onConnectFailure;
    conn_opts.context = client;

    if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start connect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    while (!deliveredtoken) {}

    disc_opts.onSuccess = onDisconnect;
    if ((rc = MQTTAsync_disconnect(client, &disc_opts)) != MQTTASYNC_SUCCESS)
    {
        printf("Failed to start disconnect, return code %d\n", rc);
        exit(EXIT_FAILURE);
    }

    MQTTAsync_destroy(&client);

    return rc;
}

该代码示例连接到本地主机上的MQTT代理，并通过发布`example/topic`主题来发送一条消息。它还设置了一个回调函数，以接收从代理接收到的所有消息。当连接成功时，将发送一条消息，并在接收到消息后将其打印到控制台。最后，它断开连接并退出程序。