C 接入该如何进行？

注意：
本文给出的实例均基于 Eclipse Paho C SDK 实现，该 SDK 下载请参见 MQTT 接入准备。如使用其他第三方的客户端，请适当修改。

1. 资源申请


使用 MQ 提供的 MQTT 服务，首先需要核实应用中使用的 Topic 资源是否已经申请，如果没有请先去控制台申请 Topic，Group ID 等资源。
申请资源时需要根据需求选择对应的 Region，例如，MQTT 需要使用华北 2 的接入点，那么 Topic 等资源就在华北 2 申请，资源申请具体请参见 申请 MQ 资源。
注意：MQTT 使用的多级子 Topic 不需要申请，代码里直接使用即可，没有限制。

2. C 客户端依赖


Paho MQTT C 客户端 SDK 从官网下载源码后,参考文档进行编译安装。可能需要安装以下开发库和工具。

1. openssl：用于签名计算。
2. cmake：用于编译示例程序，实际也可以根据习惯选用其他编译工具。

1. cmake_minimum_required(VERSION 3.7)
2. project(mqttdemo)
4. INCLUDE_DIRECTORIES(
5.         .
6.         ${CMAKE_SOURCE_DIR}/src
7.         ${CMAKE_BINARY_DIR}
8. )
9. SET(CMAKE_BUILD_TYPE   "Debug")
10. SET(CMAKE_CXX_FLAGS_DEBUG "$ENV{CXXFLAGS} -O0 -Wall -g2 -ggdb")
11. SET(CMAKE_CXX_FLAGS_RELEASE "$ENV{CXXFLAGS} -O3 -Wall")
13. set(CMAKE_C_STANDARD 99)
14. add_executable(mqttDemo mqttDemo.c)
15. TARGET_LINK_LIBRARIES(mqttDemo crypto)
16. TARGET_LINK_LIBRARIES(mqttDemo paho-mqtt3a)

1. #include "MQTTAsync.h"
3. #include <signal.h>
4. #include <memory.h>
5. #include <stdlib.h>
6. #include <unistd.h>
7. #include <openssl/hmac.h>
8. #include <openssl/bio.h>
10. volatile int connected = 0;
11. char *topic;
12. char *userName;
13. char *passWord;
15. int messageDeliveryComplete(void *context, MQTTAsync_token token) {
16.     /* not expecting any messages */
17.     printf("send message %d success\n", token);
18.     return 1;
19. }
21. int messageArrived(void *context, char *topicName, int topicLen, MQTTAsync_message *m) {
22.     /* not expecting any messages */
23.     printf("recv message from %s ,body is %s\n", topicName, (char *) m->payload);
24.     return 1;
25. }
27. void onConnectFailure(void *context, MQTTAsync_failureData *response) {
28.     connected = 0;
29.     printf("connect failed, rc %d\n", response ? response->code : -1);
30.     MQTTAsync client = (MQTTAsync) context;
31. }
33. void onSubcribe(void *context, MQTTAsync_successData *response) {
34.     printf("subscribe success \n");
35. }
37. void onConnect(void *context, MQTTAsync_successData *response) {
38.     connected = 1;
39.     printf("connect success \n");
40.     MQTTAsync client = (MQTTAsync) context;
41.     //do sub when connect success
42.     MQTTAsync_responseOptions sub_opts = MQTTAsync_responseOptions_initializer;
43.     sub_opts.onSuccess = onSubcribe;
44.     int rc = 0;
45.     if ((rc = MQTTAsync_subscribe(client, topic, 1, ⊂_opts)) != MQTTASYNC_SUCCESS) {
46.         printf("Failed to subscribe, return code %d\n", rc);
47.     }
48. }
50. void onDisconnect(void *context, MQTTAsync_successData *response) {
51.     connected = 0;
52.     printf("connect lost \n");
53. }
55. void onPublishFailure(void *context, MQTTAsync_failureData *response) {
56.     printf("Publish failed, rc %d\n", response ? -1 : response->code);
57. }
59. int success = 0;
61. void onPublish(void *context, MQTTAsync_successData *response) {
62.     printf("send success %d\n", ++success);
63. }
66. void connectionLost(void *context, char *cause) {
67.     connected = 0;
68.     MQTTAsync client = (MQTTAsync) context;
69.     MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
70.     int rc = 0;
72.     printf("Connecting\n");
73.     conn_opts.keepAliveInterval = 10;
74.     conn_opts.cleansession = 1;
75.     conn_opts.username = userName;
76.     conn_opts.password = passWord;
77.     conn_opts.onSuccess = onConnect;
78.     conn_opts.onFailure = onConnectFailure;
79.     conn_opts.context = client;
80.     conn_opts.ssl = NULL;
81.     if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS) {
82.         printf("Failed to start connect, return code %d\n", rc);
83.         exit(EXIT_FAILURE);
84.     }
85. }
88. int main(int argc, char **argv) {
89.     MQTTAsync_disconnectOptions disc_opts = MQTTAsync_disconnectOptions_initializer;
90.     MQTTAsync client;
91.     //MQTT使用的Topic，其中第一级父Topic需要在MQ控制台先申请
92.     topic = "XXXXX";
93.       //MQTT的接入域名，在MQ控制台购买付费实例后即分配该域名
94.     char *host = "XXX.mqtt.aliyuncs.com";
95.     //MQTT客户端分组Id，需要先在MQ控制台申请
96.     char *groupId = "GID_XXXXXX";
97.     //MQTT客户端设备Id，用户自行生成，需要保证对于所有TCP连接全局唯一
98.     char *deviceId = "XXXXXXX";
99.     //帐号AK
100.     char *accessKey = "XXXXXXX";
101.     //帐号SK
102.     char *secretKey = "XXXXXXX";
103.     //服务端口，使用MQTT协议时设置1883,其他协议参考文档选择合适端口
104.     int port = 1883;
105.     //QoS，消息传输级别，参考文档选择合适的值
106.     int qos = 1;
107.     //cleanSession，是否设置持久会话，如果需要离线消息则cleanSession必须是false，QoS必须是1
108.     int cleanSession = 0;
109.     int rc = 0;
110.     char tempData[100];
111.     int len = 0;
112.     HMAC(EVP_sha1(), secretKey, strlen(secretKey), groupId, strlen(groupId), tempData, &len);
113.     char resultData[100];
114.     int passWordLen = EVP_EncodeBlock((unsigned char *) resultData, tempData, len);
115.     resultData[passWordLen] = '\0';
116.     printf("passWord is %s", resultData);
117.     userName = accessKey;
118.     passWord = resultData;
119.     //1.create client
120.     MQTTAsync_createOptions create_opts = MQTTAsync_createOptions_initializer;
121.     create_opts.sendWhileDisconnected = 0;
122.     create_opts.maxBufferedMessages = 10;
123.     char url[100];
124.     sprintf(url, "%s:%d", host, port);
125.     char clientIdUrl[64];
126.     sprintf(clientIdUrl, "%s@@@%s", groupId, deviceId);
127.     rc = MQTTAsync_createWithOptions(&client, url, clientIdUrl, MQTTCLIENT_PERSISTENCE_NONE, NULL, &create_opts);
128.     rc = MQTTAsync_setCallbacks(client, client, connectionLost, messageArrived, NULL);
129.     //2.connect to server
130.     MQTTAsync_connectOptions conn_opts = MQTTAsync_connectOptions_initializer;
131.     conn_opts.keepAliveInterval = 90;
132.     conn_opts.cleansession = cleanSession;
133.     conn_opts.username = userName;
134.     conn_opts.password = passWord;
135.     conn_opts.onSuccess = onConnect;
136.     conn_opts.onFailure = onConnectFailure;
137.     conn_opts.context = client;
138.     conn_opts.ssl = NULL;
139.     conn_opts.automaticReconnect = 1;
140.     conn_opts.connectTimeout = 3;
141.     if ((rc = MQTTAsync_connect(client, &conn_opts)) != MQTTASYNC_SUCCESS) {
142.         printf("Failed to start connect, return code %d\n", rc);
143.         exit(EXIT_FAILURE);
144.     }
145.     //3.publish msg
146.     MQTTAsync_responseOptions pub_opts = MQTTAsync_responseOptions_initializer;
147.     pub_opts.onSuccess = onPublish;
148.     pub_opts.onFailure = onPublishFailure;
149.     for (int i = 0; i < 1000; i++) {
150.         do {
151.             char data[100];
152.             sprintf(data, "hello mqtt demo");
153.             rc = MQTTAsync_send(client, topic, strlen(data), data, qos, 0, &pub_opts);
154.             sleep(1);
155.         } while (rc != MQTTASYNC_SUCCESS);
156.     }
157.     sleep(1000);
158.     disc_opts.onSuccess = onDisconnect;
159.     if ((rc = MQTTAsync_disconnect(client, &disc_opts)) != MQTTASYNC_SUCCESS) {
160.         printf("Failed to start disconnect, return code %d\n", rc);
161.         exit(EXIT_FAILURE);
162.     }
163.     while (connected)
164.         sleep(1);
165.     MQTTAsync_destroy(&client);
166.     return EXIT_SUCCESS;
167. }