mqtt的c库怎么定时发布消息

要定时发布MQTT消息，您可以使用C语言中的定时器功能。您需要设置一个定时器，以指定您要发送消息的时间间隔。当定时器超时时，您就可以发布MQTT消息。

以下是一个基本的示例代码，用于使用MQTT C库和定时器定期发布消息：

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

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "test"
#define QOS         1
#define TIMEOUT     10000L

volatile MQTTClient_deliveryToken deliveredtoken;

void delivered(void *context, MQTTClient_deliveryToken dt) {
    printf("Message with token value %d delivery confirmed\n", dt);
    deliveredtoken = dt;
}

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

void connlost(void *context, char *cause) {
    printf("\nConnection lost\n");
    printf("     cause: %s\n", cause);
}

int main(int argc, char* argv[]) {
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = 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(EXIT_FAILURE);
    }

    while (1) {
        char payload[100];
        sprintf(payload, "%d", rand() % 100);

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

        MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
        printf("Waiting for publication of %s\n"
                "on topic %s for client with ClientID: %s\n",
                payload, TOPIC, CLIENTID);
        rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
        printf("Message with delivery token %d delivered\n", token);

        sleep(5); // 等待5秒后再次发布消息
    }

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

在上面的示例代码中，我们使用了`sleep(5)`来等待5秒后再次发布MQTT消息。您可以根据自己的需求修改这个时间间隔。