MQTT 源码分析,搜索了一下发现网络上讲的很少,多是逍遥子的那几篇。
一、目录结构
首先我们还是来看一下 mosquitto-1.4.14 的源码目录结构
我们主要关注 client、lib、src 这三个目录。其中 src 和 lib 目录下主要放置 mosquitto 的实现代码以及部分底层与网络相关的操作,client 目录主要为两个客户端程序的实现源码。
我们主要就是来看看,这两个客户端的实现源码。
二、SUB 客户端源码
首先我们先看 sub_client.c
我们从 main 函数开始。
查看结构体:
结构体 struct mosq_config 主要为 MQTT 的配置信息
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struct mosq_config {
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char *id;
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char *id_prefix;
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int protocol_version;
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int keepalive;
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char *host;
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int port;
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int qos;
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bool retain;
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int pub_mode; /* pub */
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char *file_input; /* pub */
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char *message; /* pub */
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long msglen; /* pub */
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char *topic; /* pub */
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char *bind_address;
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bool use_srv;
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bool debug;
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bool quiet;
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unsigned int max_inflight;
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char *username;
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char *password;
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char *will_topic;
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char *will_payload;
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long will_payloadlen;
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int will_qos;
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bool will_retain;
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char *cafile;
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char *capath;
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char *certfile;
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char *keyfile;
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char *ciphers;
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bool insecure;
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char *tls_version;
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char *psk;
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char *psk_identity;
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bool clean_session; /* sub */
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char **topics; /* sub */
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int topic_count; /* sub */
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bool no_retain; /* sub */
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char **filter_outs; /* sub */
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int filter_out_count; /* sub */
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bool verbose; /* sub */
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bool eol; /* sub */
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int msg_count; /* sub */
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char *socks5_host;
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int socks5_port;
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char *socks5_username;
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char *socks5_password;
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};
结构体 struct mosquito 主要用于保存一个客户端连接的所有信息,例如用户名、密码、用户ID、向该客户端发送的消息等
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struct mosquitto {
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mosq_sock_t sock;
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mosq_sock_t sockpairR, sockpairW;
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struct gaicb *adns; /* For getaddrinfo_a */
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enum _mosquitto_protocol protocol;
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char *address;
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char *id;
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char *username;
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char *password;
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uint16_t keepalive;
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uint16_t last_mid;
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enum mosquitto_client_state state;
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time_t last_msg_in;
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time_t next_msg_out;
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time_t ping_t;
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struct _mosquitto_packet in_packet;
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struct _mosquitto_packet *current_out_packet;
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struct _mosquitto_packet *out_packet;
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struct mosquitto_message *will;
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SSL *ssl;
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SSL_CTX *ssl_ctx;
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char *tls_cafile;
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char *tls_capath;
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char *tls_certfile;
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char *tls_keyfile;
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int (*tls_pw_callback)( char *buf, int size, int rwflag, void *userdata);
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char *tls_version;
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char *tls_ciphers;
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char *tls_psk;
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char *tls_psk_identity;
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int tls_cert_reqs;
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bool tls_insecure;
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bool want_write;
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bool want_connect;
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pthread_mutex_t callback_mutex;
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pthread_mutex_t log_callback_mutex;
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pthread_mutex_t msgtime_mutex;
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pthread_mutex_t out_packet_mutex;
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pthread_mutex_t current_out_packet_mutex;
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pthread_mutex_t state_mutex;
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pthread_mutex_t in_message_mutex;
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pthread_mutex_t out_message_mutex;
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pthread_mutex_t mid_mutex;
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pthread_t thread_id;
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bool clean_session;
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bool is_dropping;
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bool is_bridge;
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struct _mqtt3_bridge *bridge;
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struct mosquitto_client_msg *msgs;
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struct mosquitto_client_msg *last_msg;
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int msg_count;
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int msg_count12;
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struct _mosquitto_acl_user *acl_list;
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struct _mqtt3_listener *listener;
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time_t disconnect_t;
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struct _mosquitto_packet *out_packet_last;
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struct _mosquitto_subhier **subs;
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int sub_count;
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int pollfd_index;
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struct lws *wsi;
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struct libwebsocket_context *ws_context;
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struct libwebsocket *wsi;
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bool ws_want_write;
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char *socks5_host;
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int socks5_port;
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char *socks5_username;
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char *socks5_password;
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void *userdata;
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bool in_callback;
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unsigned int message_retry;
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time_t last_retry_check;
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struct mosquitto_message_all *in_messages;
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struct mosquitto_message_all *in_messages_last;
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struct mosquitto_message_all *out_messages;
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struct mosquitto_message_all *out_messages_last;
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void (*on_connect)(struct mosquitto *, void *userdata, int rc);
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void (*on_disconnect)(struct mosquitto *, void *userdata, int rc);
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void (*on_publish)(struct mosquitto *, void *userdata, int mid);
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void (*on_message)(struct mosquitto *, void *userdata, const struct mosquitto_message *message);
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void (*on_subscribe)(struct mosquitto *, void *userdata, int mid, int qos_count, const int *granted_qos);
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void (*on_unsubscribe)(struct mosquitto *, void *userdata, int mid);
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void (*on_log)(struct mosquitto *, void *userdata, int level, const char *str);
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//void (*on_error)();
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char *host;
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int port;
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int in_queue_len;
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int out_queue_len;
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char *bind_address;
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unsigned int reconnect_delay;
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unsigned int reconnect_delay_max;
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bool reconnect_exponential_backoff;
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char threaded;
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struct _mosquitto_packet *out_packet_last;
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int inflight_messages;
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int max_inflight_messages;
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ares_channel achan;
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UT_hash_handle hh_id;
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UT_hash_handle hh_sock;
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struct mosquitto *for_free_next;
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};
client_config_load 客户端配置负载
第二个参数,可选择选择是 PUB 还是 SUB
然后看到 init_config 函数
可以看到一些初始化配置
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void init_config(struct mosq_config *cfg)
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{
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memset(cfg, 0, sizeof(*cfg));
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cfg->port = 1883;
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cfg->max_inflight = 20;
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cfg->keepalive = 60;
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cfg->clean_session = true;
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cfg->eol = true;
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cfg->protocol_version = MQTT_PROTOCOL_V31;
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}
mosquitto_lib_init 初始化 (重点)
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int mosquitto_lib_init(void)
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{
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#ifdef WIN32
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srand(GetTickCount());
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#else
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struct timeval tv;
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gettimeofday(&tv, NULL);
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srand(tv.tv_sec*1000 + tv.tv_usec/1000);
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#endif
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_mosquitto_net_init();
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return MOSQ_ERR_SUCCESS;
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}
所在文件 mosquitto-1.4.14/lib/mosquitto.c 所以说需要链接动态库 libmosquitto.so.1
client_id_generate 生成客户端 ID
其实就是我们讲MQTT服务器的时候,订阅主题然后在服务器上多出的那一行信息。
里面的 mosqsub|2431-ubuntu 就是客户端 ID。这个函数就是干这个。
1502159601: New client connected from 127.0.0.1 as mosqsub|2431-ubuntu (c1, k60)
mosquitto_new 新建一个 mosq。(重点)
看了一下这个函数里面就是一些初始化的东西
然后可以看到它也是在 lib 目录下定义的。所以说需要链接动态库 libmosquitto.so.1。其他不用改。
client_ipts_set 各种设置。懒得看...
一些调试信息
以及订阅回调设置 mosquitto_subscribe_callback_set (重要)
连接回调设置,和信息回调设置(重点)
这两个函数都是在lib目录下定义的。里面都是有互斥锁的。
client_connect 客户端连接
int client_connect(struct mosquitto *mosq, struct mosq_config *cfg)
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{
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char err[ 1024];
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int rc;
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if(cfg->use_srv){
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rc = mosquitto_connect_srv(mosq, cfg->host, cfg->keepalive, cfg->bind_address);
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} else{
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rc = mosquitto_connect_bind(mosq, cfg->host, cfg->port, cfg->keepalive, cfg->bind_address);
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}
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rc = mosquitto_connect_bind(mosq, cfg->host, cfg->port, cfg->keepalive, cfg->bind_address);
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if(rc> 0){
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if(!cfg->quiet){
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if(rc == MOSQ_ERR_ERRNO){
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strerror_r(errno, err, 1024);
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FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, NULL, errno, 0, (LPTSTR)&err, 1024, NULL);
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fprintf( stderr, "Error: %s\n", err);
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} else{
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fprintf( stderr, "Unable to connect (%s).\n", mosquitto_strerror(rc));
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}
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}
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mosquitto_lib_cleanup();
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return rc;
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}
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return MOSQ_ERR_SUCCESS;
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}
mosquitto_connect_srv
mosquitto_connect_bind --> _mosquitto_connect_init
mosquitto_connect_bind --> _mosquitto_connect_init
然后不断回环,里面的参数自己看。
最后是 mosq 的销毁和库的关闭。(重点)
到此结束!!
三、PUB 客户端源码
接下来来看 pub_client.c 有一些
相同部分我就不再重复了。
client_config_load 客户端配置负载
第二个参数,可选择选择是 PUB 还是 SUB
一些配置信息的比较
mosquitto_lib_init 初始化 (重点)
client_id_generate 生成客户端 ID
mosquitto_new 新建一个 mosq。(重点)
调试信息这里面就没有了订阅回调设置 mosquitto_subscribe_callback_set
然后这里看这里,是有区别的。(重点)
connect、disconnect、publish. 这些回调设置
client_ipts_set 各种设置。懒得看...
client_connect 客户端连接
回环开始、结束
最重要的来了,do while循环里的发布内容 (重点)
这里的 qos 就是消息发布服务质量级别。
然后还有 retain 用于区分新老订阅者
RETAIN标志位只用于 PUBLISH 消息,当服务器收到某个主题的 PUBLISH 消息时,如果RETAIN标志位为1,则表示服务在将该消息发送给所有的已订阅该主题的订阅者后(发送前服务器将RETAIN标志置为0),还需保持这条消息,当有新增的订阅者时,再将这条消息发给新增的订阅者;如果RETAIN标志位为0,则不保持消息,也不用发给新增的订阅者。
目的:
1.将RETAIN标志位置为1,可使新的订阅者收到之前保持的或上一个确定有效的消息。
2.区分新订阅者(RETAIN标志为1)和老订阅者(RETAIN标志为0)
1.将RETAIN标志位置为1,可使新的订阅者收到之前保持的或上一个确定有效的消息。
2.区分新订阅者(RETAIN标志为1)和老订阅者(RETAIN标志为0)
源码中这两个参数的设置都是 0
最后是 mosq 的销毁和库的关闭。(重点)