paho的mqtt C语言库怎么设置订阅的qos

在paho的mqtt C语言库中，可以通过设置`struct mqtt_topic`结构体中的`qos`字段来设置订阅的qos。具体可以参考以下代码示例：

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

#include "MQTTClient.h"

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

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

    MQTTClient_subscribe(client, TOPIC, QOS);

    while (1)
    {
        MQTTClient_message* msg;
        MQTTClient_deliveryToken token;
        int rc;

        rc = MQTTClient_receive(client, &msg, TIMEOUT);
        if (rc != MQTTCLIENT_SUCCESS)
        {
            continue;
        }

        printf("Received message: %.*s\n", msg->payloadlen, (char*)msg->payload);
        MQTTClient_freeMessage(&msg);
        MQTTClient_free(topicName);
    }

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

在上述代码中，通过调用`MQTTClient_subscribe`函数来订阅指定的主题，并通过`QOS`参数来设置订阅的qos。在这里，我们将qos设置为1。