用C++将从mqtt接收到的字节数组写入图像

Question

您好，我正在尝试从 mqtt 读取字节数组（图像二进制）并将其写回 C++ 中的 png/jpeg（基于原始文件扩展名）文件，下面是我的代码，它读取二进制数据并写入图像文件但是当我尝试打开图像时出现错误：

“解释 JPEG 图像文件时出错（状态 201 中对 JPEG 库的调用不当）”

下面是我用于测试的图像，为了测试，我使用下面的 python 代码发布图像

更新：按照@hardillb的建议，我尝试发布单个但相同的结果。

import paho.mqtt.client as mqtt

f=open("/home/anup/Documents/img_2.jpg", "rb") #3.7kiB in same folder
fileContent = f.read()
byteArr = bytearray(fileContent)
import paho.mqtt.client as mqtt #import the client1
broker_address="0.0.0.0" 
client = mqtt.Client("P1") #create new instance
client.connect(broker_address, "1883") #connect to broker
res = client.publish("in",byteArr)#publish
client.loop()
print(res)

import paho.mqtt.publish as publish
publish.single("in", payload=byteArr, hostname="localhost", port=1883)

JPG源图片：

我正在使用此存储库中的 MQTT 二进制文件 aschiffler/cqq-mqtt 并使用 Ninja 对其进行编译。

下面是我的 C++ 订阅者代码：

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.
#include <mqtt/client.h>  // Mosquitto client.
#include <ostream>  // std::cout.
#include <string>
#include <cstring>
#include <cstdlib>
#include <unistd.h>
#include <fstream>


// With the library header files included, continue by defining a main function.
int main()
{
    // In order to connect the mqtt client to a broker, 
    // Define an Ip address pointing to a broker. In this case, the localhost on port 1883.
    std::string ip = "localhost:1883";
    // Then, define an ID to be used by the client when communicating with the broker.
    std::string id = "consumer";

    // Construct a client using the Ip and Id, specifying usage of MQTT V5.
    mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));
    // Use the connect method of the client to establish a connection to the broker.
    client.connect();
    // In order to receive messages from the broker, specify a topic to subscribe to.
    client.subscribe("in");
    // Begin the client's message processing loop, filling a queue with messages.
    client.start_consuming();

    bool running = true;
    while (running)
    {
        // Construct a message pointer to hold an incoming message.
        mqtt::const_message_ptr messagePointer;

        // Try to consume a message, passing messagePointer by reference.
        // If a message is consumed, the function will return `true`, 
        // allowing control to enter the if-statement body.
        if (client.try_consume_message(&messagePointer))
        {
            // Construct a string from the message payload.
            std::string messageString = messagePointer -> get_payload_str();
            // Print payload string to console (debugging).
            std::cout << messageString << std::endl;

            std::string filename = "/home/anup/Documents/mqtt/cpp-mqtt/image.jpg";
            std:: ofstream outfile(filename, std::ios::binary);
            outfile.write((char*)messageString.c_str(), sizeof(messageString));
            outfile.close();


            // Perform processing on the string.
            // This is where message processing can be passed onto different
            // functions for parsing. 
            // Here, we break the loop and exit the program if a `quit` is received.
            if (messageString == "quit")
            {
                running = false;
            } 
        }
    }

    return 0;
}

我在订阅者控制台上看到的输出（被截断以适合此处）

�Y,���àyPà�U���ٳ�F��!*�G9�6dD������xr��é�ì�Tm塦d��˨�TKGj��W�5��6'��nvK��W�D��۩(ځf՛��6��˲p�
                                                                                          �V�+°��?)°27M���̴I��R�������8��/;�޽���<�ì�DB�����5�u^�M�j�ò��K���U��gg��QF�/3%����MЮ��W��8_���d�0j£�"�OX�R���mm�K��K�E4H���L��
                                           ���w��F;�'���8�9�O ��
                                                                        �S)M��é���-ĜBxcP���§-� ֆ����9��+><$?�g�Rf��+a6��1R���.�
e��d��O�ù����� ��Y�_�h�.X�i�mù����W��x�9eG�v3é��T:�����1Z�
�n�����7�_�$G7���ù
W)Wn�=�R���z�è��uè�XZ*(ø�>F���$�B&��h�>�e�;Ҩ�x�d�aò�?��y��՛�è�nҕ�����5���jI�^ç��/e���a�
                                                                                        ��a�òm�N*�߀U��5.<���8z:A��ӍZ��&V§��.�IEND�Bù�

Answer 1

经过 Mark Setchell 和 @hardillb 的一番努力和宝贵指导后，我终于能够让消费者工作了。下面是工作代码，我将保留原始问题和工作解决方案，以便对其他人有用。

// Start by `#include`-ing the Mosquitto MQTT Library and other standard libraries.
#include <mqtt/client.h>  // Mosquitto client.
#include <ostream>  // std::cout.
#include <string>
#include <cstring>
#include <cstdlib>
#include <unistd.h>
#include <fstream>
#include <cstddef>
#include <sstream>
#include <vector>


// With the library header files included, continue by defining a main function.
int main()
{
    // In order to connect the mqtt client to a broker, 
    // Define an Ip address pointing to a broker. In this case, the localhost on port 1883.
    std::string ip = "localhost:1883";
    // Then, define an ID to be used by the client when communicating with the broker.
    std::string id = "consumer";

    // Construct a client using the Ip and Id, specifying usage of MQTT V5.
    mqtt::client client(ip, id, mqtt::create_options(MQTTVERSION_5));
    // Use the connect method of the client to establish a connection to the broker.
    client.connect();
    // In order to receive messages from the broker, specify a topic to subscribe to.
    client.subscribe("in");
    // Begin the client's message processing loop, filling a queue with messages.
    client.start_consuming();

    bool running = true;
    while (running)
    {
        // Construct a message pointer to hold an incoming message.
        mqtt::const_message_ptr messagePointer;
        if (client.try_consume_message(&messagePointer))
        {
            // Construct a string from the message payload.
            const auto& byteArry  = messagePointer -> get_payload();
            if (byteArry[0] == '\xFF' && byteArry[1] == '\xD8' && byteArry[2] == '\xFF' && byteArry.back() == '\xD9'){
                std::cout << "The byte array is a JPEG" << std::endl;
                std::string filename = "/home/user/Documents/mqtt/cpp-mqtt/image.jpg";
                std:: ofstream outfile(filename, std::ios::binary);
                outfile.write(reinterpret_cast<const char*>(byteArry.data()), byteArry.size());
                outfile.close();

            }
            else {
                std::cout << "The byte array is a png" << std::endl;
                std::string filename = "/home/user/Documents/mqtt/cpp-mqtt/image.png";
                std:: ofstream outfile(filename, std::ios::binary);
                outfile.write(reinterpret_cast<const char*>(byteArry.data()), byteArry.size());
                outfile.close();
            }
        }
    }

    return 0;
}