Mosquitto MQTT Client调用loop_start()返回错误10求助

问题现象 #

编写的MQTT客户端调用loop_forever()时可正常连接Broker并收到连接确认，但改用loop_start()启动线程处理回调时，返回错误MOSQ_ERR_NOT_SUPPORTED（错误码10）。开发环境为Visual Studio 2022，已添加所有Mosquitto依赖项。

客户端代码 #

myMosq.h

#include <mosquittopp.h>
#include <mosquitto.h>

class myMosq : public mosqpp::mosquittopp
{
private:
    const char* host;
    const char* id;
    const char* topic;
    int         port;
    int         keepalive;

    void on_connect(int rc);
    void on_disconnect(int rc);
    void on_publish(int mid);
    void on_message(const struct mosquitto_message* message);

public:
    myMosq(const char* id, const char* _topic, const char* host, int port);
    ~myMosq();
    bool send_message(const char* _message);
};

myMosq.cpp

#include <iostream>

myMosq::myMosq(const char* _id, const char* _topic, const char* _host, int _port) : mosquittopp(_id)
{
    mosqpp::lib_init() ;        // Mandatory initialization for mosquitto library

    this->keepalive = 60;    // Basic configuration setup for myMosq class
    this->id = _id;
    this->port = _port;
    this->host = _host;
    this->topic = _topic;

    int result;
    result = connect_async(host, port, keepalive);     // non blocking connection to broker request
    if (result != MOSQ_ERR_SUCCESS)
    {
        std::cerr << "Error in connect_async" << std::endl;
    }
    result = loop_start();            // Start thread managing connection / publish/subscribe
    //result = loop_forever();            // Start thread managing connection / publish / subscribe
    if (result != MOSQ_ERR_SUCCESS)
    {
        std::cerr << "Error in loop_start: " << (mosq_err_t)result << std::endl;
    }
};

myMosq::~myMosq() 
{
    loop_stop();            // Kill the thread
    mosqpp::lib_cleanup();    // Mosquitto library cleanup
}

bool myMosq::send_message(const  char* _message)
{
    int ret = publish(NULL, this->topic, strlen(_message), _message, 1, false);
    return (ret == MOSQ_ERR_SUCCESS);
}

void myMosq::on_disconnect(int rc) 
{
    std::cout << ">>> myMosq - disconnection(" << rc << ")" << std::endl;
}

void myMosq::on_connect(int rc)
{
    if (rc == 0){
        std::cout << ">>> myMosq - connected with server" << std::endl;
    }
    else{
        std::cout << ">>> myMosq - Impossible to connect with server(" << rc << ")" << std::endl;
    }
}

void myMosq::on_publish(int mid)
{
    std::cout << ">>> myMosq - Message (" << mid << ") succeed to be published " << std::endl;
}

void myMosq::on_message(const struct mosquitto_message* message)
{
    std::cout << "Topic: " << message->topic << " Payload: " << message->payload << " Payload_Length: " << message->payloadlen << std::endl;
}

main.cpp

#include <windows.h>
#include <string>

#include "myMosq.h"

void MQTT_Subscriber(myMosq& i_client)
{
    int result = i_client.subscribe(nullptr, "logrequest", 1);
    if (result != MOSQ_ERR_SUCCESS)
    {
        std::cout << "mosquitto : subscriber error: " << mosqpp::strerror(result) << std::endl;
    }
    else
    {
        std::cout << "mosquitto : subscriber : " << result << std::endl;
    }
}

void MQTT_Publisher(myMosq& i_client)
{
    int i = 1;
    std::string message = "I am mosquitto VS pub";
    while (1)
    {
        std::string message1 = message + std::to_string(i);
        if (!i_client.send_message(message1.c_str()))
        {
            break;
        }
        Sleep(2000);
        i++;
    }
}

int main()
{
    std::cout << "In main" << std::endl;

    std::shared_ptr<myMosq> m_client = std::make_shared<myMosq>("dev3_", "logrequest", "127.0.0.1", 1883);

    //MQTT_Publisher(m_client);
    //MQTT_Subscriber(m_client);

    while (1);
    return 0;
}

问题原因 #

loop_start()是Mosquitto的线程模式接口，需要库在编译时启用线程支持（开启WITH_THREADING编译选项）。如果当前使用的Mosquitto库没有启用该选项，调用loop_start()就会返回MOSQ_ERR_NOT_SUPPORTED错误。而loop_forever()是单线程阻塞模式，不需要线程支持，因此可以正常运行。

解决方法 #

1. 重新编译带线程支持的Mosquitto库 #

• 下载Mosquitto官方源码包
• 使用CMake配置编译参数：
  • 勾选WITH_THREADING选项（默认可能未启用）
  • 选择与VS2022匹配的编译架构（x86或x64）
  • 配置生成动态库或静态库，完成编译

2. 更新项目依赖 #

• 将编译好的带线程支持的mosquitto.lib/mosquitto.dll替换到项目的依赖目录中
• 在VS2022项目属性中，更新包含目录、库目录和链接器输入，指向新编译的库文件

3. 验证修复 #

重新编译客户端项目，运行后loop_start()应能正常启动线程，不再返回错误码10。

额外提示 #

• 若使用预编译Mosquitto库，需确认其是否明确支持线程；多数预编译包默认不包含线程支持，建议自行编译
• 确保mosqpp::lib_init()在所有Mosquitto操作前调用（代码中已正确实现）
• 线程模式下，避免在主线程和回调线程中同时修改客户端对象状态，防止竞态问题

内容的提问来源于stack exchange，提问作者Piyush Malaviya