Paho MQTT C库提供的批量发布API是什么样的

Paho MQTT C库提供了批量发布API，可以一次性发布多条消息，以提高发布效率和性能。具体而言，Paho MQTT C库提供了以下两种批量发布API：

1. `MQTTClient_publishMany`：该API可以一次性发布多条不同主题的消息，使用方法如下：

   MQTTClient_message messages[3] = {
       MQTTClient_message_initializer, 
       MQTTClient_message_initializer, 
       MQTTClient_message_initializer
   };
   char* topics[] = { "topic1", "topic2", "topic3" };
   char* payloads[] = { "payload1", "payload2", "payload3" };
   int qos[] = { 0, 0, 0 };
   int i;

   for (i = 0; i < 3; ++i) {
       messages[i].payload = payloads[i];
       messages[i].payloadlen = strlen(payloads[i]);
       messages[i].qos = qos[i];
       messages[i].retained = 0;
   }

   MQTTClient_publishMany(client, 3, topics, messages);

2. `MQTTClient_publishMessageArr`：该API可以一次性发布多条相同主题的消息，使用方法如下：

   MQTTClient_message messages[3] = {
       MQTTClient_message_initializer, 
       MQTTClient_message_initializer, 
       MQTTClient_message_initializer
   };
   char* topic = "topic";
   char* payloads[] = { "payload1", "payload2", "payload3" };
   int qos[] = { 0, 0, 0 };
   int i;

   for (i = 0; i < 3; ++i) {
       messages[i].payload = payloads[i];
       messages[i].payloadlen = strlen(payloads[i]);
       messages[i].qos = qos[i];
       messages[i].retained = 0;
   }

   MQTTClient_publishMessageArr(client, topic, 3, messages);

需要注意的是，批量发布API只是提高了发布效率，但并不会影响MQTT的QoS（服务质量）功能，需要根据实际情况选择适当的QoS等级。