python - 通过套接字从 C++ 服务器向 Python 客户端发送数据时出现问题

Question

我创建了一个简单的 C++ 服务器，它试图发送 vector<uint8_t> 的字节。通过套接字发送到 Python 客户端。我已经让所有服务器和客户端连接正常，但是数据在 python 中输出不正确。我首先发送一个描述预期字节数的整数，然后发送字节。但是，当我运行我的 python 脚本并调用 recv 时在套接字上，我有时会得到不正确的预期字节数值。

我已经调试了 C++ 和 Python 代码，它显示出一些奇怪的行为。首先我意识到当我调试 Python 代码时它工作正常。事实证明，在我的 Python 脚本中，如果我调用 sleep(1)在我读取整数 header 的前 4 个字节和读取它工作的其余字节之间。但是，当我执行多个 recv 时没有 sleep 就打电话，然后事情就出错了。我还检查了字节字节序的内容是否正确。我什至在服务器和客户端之间添加了一个握手程序，以确保它们能够很好地协同工作。

对于 C++:设置套接字:

int Server::setup_socket(int port){
    int server_fd, new_socket; 
    struct sockaddr_in address; 
    int opt = 1; 
    int addrlen = sizeof(address); 

    // Creating socket file descriptor 
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0){ 
        perror("socket failed"); 
        exit(EXIT_FAILURE); 
    } 

    // Forcefully attaching socket to the port 
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))){ 
        perror("setsockopt"); 
        exit(EXIT_FAILURE); 
    } 
    address.sin_family = AF_INET; 
    address.sin_addr.s_addr = INADDR_ANY; 
    address.sin_port = htons(port); 

    // Forcefully attaching socket to the port 
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address))<0){ 
        perror("bind failed"); 
        exit(EXIT_FAILURE); 
    } 
    if (listen(server_fd, 3) < 0){ 
        perror("listen"); 
        exit(EXIT_FAILURE); 
    } 
    printf("Successfully connected to port %d\n", port);
    if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen))<0){ 
        perror("accept"); 
        exit(EXIT_FAILURE); 
    } 

    return new_socket;
}

发送数据和握手方式:

void Server::send_image(cv::Mat &image) {
    std::vector<uint8_t> buf;
    std::vector<int> param(2);
    param[0] = cv::IMWRITE_JPEG_QUALITY;
    param[1] = 80; //default(95) 0-100
    cv::imencode(".jpg", image, buf, param);

    int length = buf.size();
    printf("Sending image of size: %d\n", length);
    write(data_socket, &length, sizeof(length));
    write(data_socket, buf.data(), length);
}

void Server::confirm_sent(){
    uint8_t confirmation[1];
    write(conf_socket, confirmation, 1);
}

void Server::confirm_received(){
    uint8_t confirmation[1];
    read(conf_socket, confirmation, 1);
}

Python代码:

data_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)  # For sending data
conf_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)  # For hand shaking

# Connect the socket to the port where the server is listening
data_address = ('192.168.1.146', 2323)
conf_address = ('192.168.1.146', 2324)
print('connecting to %s port %s' % data_address)
data_sock.connect(data_address)
time.sleep(1)
print('connecting to %s port %s' % conf_address)
conf_sock.connect(conf_address)

while True:
    conf_sock.recv(1)  # Confirm sent
    size1 = int.from_bytes(data_sock.recv(4), byteorder="big") 
    size2 = socket.ntohl(size1) # Fixes endian problems
    # time.sleep(1) # Inserting this fixes things, but I don't want the delay
    data = np.frombuffer(data_sock.recv(size2), dtype=np.uint8)
    print(f"{size1}, {size2}, {data.shape}")
    conf_sock.send(bytes(1))

C++ 输出和预期大小:

Max speed spi is 8000000
OV5642 detected.
Successfully connected to port 2323
Successfully connected to port 2324
Sending image of size: 134966
Sending image of size: 135072
Sending image of size: 134628
Sending image of size: 134846
Sending image of size: 134704
Sending image of size: 134885
Sending image of size: 133942

Python 接收到的尺寸:

connecting to 192.168.1.146 port 2323
connecting to 192.168.1.146 port 2324
906953216, 134966, (95568,)
1224436735, 4285266760, (45190,)
2585803520, 3874970, (137968,)
939478527, 4283301687, (137524,)
103119361, 24782086, (136294,)
1526714366, 4275044186, (127464,)
469746175, 4290903835, (136333,)

Answer 1

网络数据到达的速度可能很慢，socket.recv() 会给你最多请求的字节数，但如果没有足够的数据则更少缓冲区还没有。

您需要不断调用 recv() 直到所有字节都到达。这很常见，您需要一个函数来为您处理重复调用:

def socket_read(sock, expected):
    """Read expected number of bytes from sock

    Will repeatedly call recv until all expected data is received

    """
    buffer = b''
    while len(buffer) < expected:
        buffer += sock.recv(expected - len(buffer))
    return buffer

并使用该函数接收您的数据:

message_size = int.from_bytes(socket_read(data_sock, 4), byteorder="little")
data = np.frombuffer(socket_read(data_sock, message_size), dtype=np.uint8)

关于字节顺序的注意事项:由服务器以特定的字节顺序发送数据。 TCP/IP 连接上的网络顺序是 big-endian，因此您的 C++ 代码需要使用该顺序，并使用 int.from_bytes(..., byteorder="big") 而不使用 使用 socket.ntohl()。 byteorder 参数与平台无关，使用此方法将字节解释为整数不受平台差异的影响。

现在，您的 C++ 代码根本不处理字节顺序；使用 htonl() function在您的服务器代码中确保您写出正确的字节顺序:

write(data_socket, &(htonl(length)), sizeof(length));

然后在 Python 中使用 int.from_bytes(..., byteorder="big")。