MQTTClient_willOptions定义客户端的MQTT “Last Will and Testament” （LWT）设置。 如果客户端意外失去与服务器的连接，服务器将代表客户端将LWT消息发布到LWT主题。 这使其他客户端（已订阅LWT主题）可以知道该客户端已断开连接。 为了为特定客户端启用LWT功能，在将客户端连接到服务器的MQTTClient_connect（）调用中使用的MQTTClient_connectOptions结构中传递了指向MQTTClient_willOptions结构的有效指针。 如果不需要LWT函数，则可以将指向MQTTClient_willOptions的指针设置为NULL。
LWT payload格式：
struct{
int len;
const void* data;
}payload;
Data Fields：
| Type | Data | Description |
|---|---|---|
| char | struct_id [4] | The eyecatcher for this structure. must be MQTW. |
| int | struct_version | The version number of this structure. Must be 0 or 1 0 means there is no binary payload option |
| const char * | topicName | The LWT topic to which the LWT message will be published. |
| const char * | message | The LWT payload in string form. |
| int | retained | The retained flag for the LWT message |
| int | qos | The quality of service setting for the LWT message. |
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include "MQTTClient.h"
#define ADDRESS "tcp://192.168.1.101:1883"
#define CLIENTID "ExampleClientPub"
#define TOPIC "MQTT_Examples"
#define PAYLOAD "Hello MQTT!"
#define QOS 1
#define TIMEOUT 10000L
#define LAST_WILL_TOPIC "Hello_Will"
#define LAST_WILL_MSG "take care yourself"
int main(int argc, char* argv[])
{
MQTTClient client;
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_message pubmsg = MQTTClient_message_initializer;
MQTTClient_willOptions will_opts = MQTTClient_willOptions_initializer;
MQTTClient_deliveryToken token;
int rc;
MQTTClient_create(&client, ADDRESS, CLIENTID,
MQTTCLIENT_PERSISTENCE_NONE, NULL);
conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;
will_opts.topicName = LAST_WILL_TOPIC;
will_opts.message = LAST_WILL_MSG;
conn_opts.will = &will_opts;
if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
printf("Failed to connect, return code %d\n", rc);
exit(EXIT_FAILURE);
}
pubmsg.payload = PAYLOAD;
pubmsg.payloadlen = strlen(PAYLOAD);
pubmsg.qos = QOS;
pubmsg.retained = 0;
while (1)
{
MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
printf("Waiting for up to %d seconds for publication of %s\n"
"on topic %s for client with ClientID: %s\n",
(int)(TIMEOUT/1000), PAYLOAD, TOPIC, CLIENTID);
rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
printf("Message with delivery token %d delivered\n", token);
sleep(1);
}
MQTTClient_disconnect(client, 10000);
MQTTClient_destroy(&client);
return rc;
}
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