【胡杨林大讲堂】地理空间大数据的尺度分析和...

我的手指在终端上颤抖，汗水混着血渍模糊了屏幕。迷雾已经吞没了大半条山路，再晚一点，就彻底找不到回村的方向了。

身后传来窸窣声——是夜行兽，它们嗅到了我的血腥味。我咬牙加快脚步，爬上最后的石坡，信号格终于跳动了一下。快啊……

我输入指令，按下发送。MQTT的数据包像一道微弱的求救电波，在迷雾中挣扎着飞向村庄。

“轰——” 远处，篝火台爆出炽烈的光焰，驱散黑暗。夜行兽嘶叫着退却，而村庄的轮廓，终于清晰。

我瘫倒在地，笑了。

下面用mosquitto实现MQTT，点燃希望的LED

Mosquitto 是一个轻量级的开源 MQTT 代理服务器（MQTT Broker），广泛用于物联网（IoT）通信。
C （使用 libmosquitto）编程可以实现MQTT的各种功能。
创龙资料中以及有相关的MQTT例程，可以很好地帮助我们学习如何开发。

虚拟机如果未安装libmosquitto，可以执行如下：

sudo apt install libmosquitto-dev

下面是一个登录巴法云的实验

#include <stdio.h>
#include <stdbool.h>
#include <getopt.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <libgen.h>
#include <stdint.h>
#include <signal.h>
#include <math.h>
#include <mosquitto.h>


enum Mode{
    MQTT_PUBLISH,
    MQTT_SUBSCRIBE,
    MQTT_INVALID
};

struct _Params {
    char host[64];
    char topic[64];
    char message[128];
    int  port;
    int  quality;
    enum Mode mode;
};

static const int keepalive = 600;
static int is_connect = false;

static char* g_led="/sys/class/leds/user-led0";
/* Exit flag */
volatile bool g_quit = false;

struct mosquitto *mosq = NULL;

void mqtt_connect_callback(struct mosquitto *mosq, void *obj, int rc)
{
    struct _Params *userdata = (struct _Params *)obj;

    if (rc == 0) {
        is_connect = true;
        printf("Connect mqtt broker success (%s:%d).\n", userdata->host, userdata->port);
        
        /* Subscribe topic */
        if (userdata->mode == MQTT_SUBSCRIBE) {
            mosquitto_subscribe(mosq, NULL, userdata->topic, userdata->quality);
        }
    }
}

void mqtt_disconnect_callback(struct mosquitto *mosq, void *obj, int result)
{
    struct _Params *userdata = (struct _Params *)obj;

    if (userdata->mode == MQTT_SUBSCRIBE) {
        printf("disconnect mqtt broker server\n");
    }
}

void mqtt_message_callback(struct mosquitto *mosq, void *obj, const struct mosquitto_message *message)
{
    char cmd[64] = {0};
    printf("recv message: %s\n", (char *)message->payload);
    if(strstr(message->payload,"on"))
    {
        snprintf(cmd, 64, "echo 1 > %s/brightness", g_led);
        if (system(cmd) != 0) {
            fprintf(stderr, "Error: Failed to turn on %s\n", g_led);
            exit(EXIT_FAILURE);
        }
    }else if(strstr(message->payload,"off"))
    {
        snprintf(cmd, 64, "echo 0 > %s/brightness", g_led);
        if (system(cmd) != 0) {
            fprintf(stderr, "Error: Failed to turn on %s\n", g_led);
            exit(EXIT_FAILURE);
        }
    }
}

void sig_handle(int arg)
{
    g_quit = true;
    mosquitto_disconnect(mosq);
}

int main(int argc, char **argv)
{
    struct _Params params;
    int ret = -1;
    int status = 0;
    /* Ctrl+c handler */
    signal(SIGINT, sig_handle);

    /* Must be called before any other mosquitto functions. */
    ret = mosquitto_lib_init();
    if (ret != MOSQ_ERR_SUCCESS) {
        printf("mosquitto lib init failed!\n");
        exit(-1);
    }

    strcpy(params.host, "bemfa.com");
    params.port=9501;
    params.mode=MQTT_SUBSCRIBE;
    strcpy(params.topic,"LED002");
    
    mosq = mosquitto_new("这里是CLIENTID", true, &params);
    if (mosq == NULL) {
        printf("Create new mosquitto client instance failed!\n");
        exit(-1);
    }

    /* Set the connect callback. */
	mosquitto_connect_callback_set(mosq, mqtt_connect_callback);
    /* Set the disconnect callback. */
	mosquitto_disconnect_callback_set(mosq, mqtt_disconnect_callback);
    /* Set the message callback.  This is called when a message is received from the broker. */
    mosquitto_message_callback_set(mosq, mqtt_message_callback);

    /* Connect to an MQTT broker. */
    ret = mosquitto_connect(mosq, params.host, params.port, keepalive);
    if (ret != MOSQ_ERR_SUCCESS) {
        printf("mosquitto connect failed!\n");
        status = -1;
        goto error;
    }

    /* subscribe */
    mosquitto_loop_forever(mosq, -1, 1);
error:
    mosquitto_destroy(mosq);
    mosquitto_lib_cleanup();
    
    return status;
}

没有自己搭建MQTT服务器，利用巴法云

建立一个主题(topic)LED002

1、ctrl+c处理

signal(SIGINT, sig_handle);

void sig_handle(int arg)
{
g_quit = true;//退出标识
mosquitto_disconnect(mosq);//断开mqtt链接
}

2、mosquitto初始化

ret = mosquitto_lib_init();
if (ret != MOSQ_ERR_SUCCESS) {
printf("mosquitto lib init failed!\n");
exit(-1);
}

3、参数设置

strcpy(params.host, "bemfa.com");//host设置巴法云地址
params.port=9501;//端口
params.mode=MQTT_SUBSCRIBE;//MQTT_SUBSCRIBE=1，订阅
strcpy(params.topic,"LED002");//主题：LED002

4、新建mosquitto
mosquitto_new的第一个参数是clientid，一般云平台会提供

mosq = mosquitto_new("这里是CLIENTID", true, &params);
if (mosq == NULL) {
    printf("Create new mosquitto client instance failed!\n");
    exit(-1);
}

5、设置回调处理函数
其中，mqtt_message_callback是收到订阅消息处理，这个用于实现点LED

/* Set the connect callback. */
mosquitto_connect_callback_set(mosq, mqtt_connect_callback);
/* Set the disconnect callback. */
mosquitto_disconnect_callback_set(mosq, mqtt_disconnect_callback);
/* Set the message callback.  This is called when a message is received from the broker. */
mosquitto_message_callback_set(mosq, mqtt_message_callback);

6、mqtt_message_callback订阅消息回调点LED
收到MQTT消息on点LED,收到off熄灭LED

static char* g_led="/sys/class/leds/user-led0";
void mqtt_message_callback(struct mosquitto *mosq, void *obj, const struct mosquitto_message *message)
{
char cmd[64] = {0};
printf("recv message: %s\n", (char *)message->payload);
if(strstr(message->payload,"on"))
{
snprintf(cmd, 64, "echo 1 > %s/brightness", g_led);
if (system(cmd) != 0) {
fprintf(stderr, "Error: Failed to turn on %s\n", g_led);
exit(EXIT_FAILURE);
}
}else if(strstr(message->payload,"off"))
{
snprintf(cmd, 64, "echo 0 > %s/brightness", g_led);
if (system(cmd) != 0) {
fprintf(stderr, "Error: Failed to turn on %s\n", g_led);
exit(EXIT_FAILURE);
}
}
}

7、链接MQTT服务器等待订阅消息

/* Connect to an MQTT broker. */
ret = mosquitto_connect(mosq, params.host, params.port, keepalive);
if (ret != MOSQ_ERR_SUCCESS) {
    printf("mosquitto connect failed!\n");
    status = -1;
    goto error;
}

/* subscribe */
mosquitto_loop_forever(mosq, -1, 1);