Matlab 中的客户端如何将二进制数据发送到 C 中的服务器？

Question

我想从一个用Matlab写的客户端发送数据到一个用C写的服务器。当在服务器中读取时，并不是所有的数据都在一次读取操作中被接收到，它被分成两次连续的读取。

Matlab 代码包含 TCP 服务器的代码，该服务器接收一些数据，然后进行一些处理，然后通过 TCP 套接字将处理的输出作为客户端写入（发送）到 C 中的服务器。

这里是用Matlab写的客户端代码。

% A TCP server object defined for getting raw data and processing
% it. The object is called 'TCPServer'. It is not related to the 
% problem. 

% TCP client for sending the data to the C server
TCPClient = tcpip('192.168.1.2', 8080, 'NetworkRole', 'client');
set(TCPClient,'OutputBufferSize', 1464); 
% 1464 is total number of bytes of the struct to be sent
% to the C server.

set(TCPClient,'Timeout', 10);
fopen(TCPClient);

 while (1)
     while(1)       % Waits for incoming CSI data
         nBytes = get(TCPServer,'BytesAvailable');
         if nBytes >= bufferLen
             disp('Data received');
             break;
         end
     end

     data = fread(TCPServer,nBytes,'uint8');
     flushinput(TCPServer);

     % Does some processing and generate 'spec' and 'doas'
     % arrays to be sent to the C server

     message = [typecast(spec, 'uint8') typecast(doas, 'uint8')];
     fwrite(TCPClient, message); 
 end

这里是用C语言编写的代码，用于在Matlab中从客户端接收数据的服务器。

#define SA struct sockaddr

struct doa_struct {
    double spec[181], doa[2];
};

// Function that reads received data
void func(int sockfd) 
{ 
    struct doa_struct *doa_data;
    unsigned char buff[1464];
    int num_bytes;
    // infinite loop receiving data
    for (;;) { 
        bzero(buff, 1464); 

        // read the data from client and copy it in buffer 
        num_bytes = read(sockfd, buff, 1464);
        // Get the buffer which contains the client contents 
        doa_data = (struct doa_struct *) buff; 
        printf("doa: %f\t%f\t%d\n", doa_data->doa[0], doa_data->doa[1], num_bytes);
    }
} 

// Driver function 
int main()
{ 
    int sockfd, connfd, len;
    struct sockaddr_in servaddr, cli; 

    // socket create and verification
    sockfd = socket(AF_INET, SOCK_STREAM, 0); 
    if (sockfd == -1) { 
        printf("socket creation failed...\n"); 
        exit(0); 
    } 
    else
        printf("Socket successfully created..\n"); 
    bzero(&servaddr, sizeof(servaddr)); 

    // assign IP, PORT 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(8080); 

    // Binding newly created socket to given IP and verification
    if ((bind(sockfd, (SA*)&servaddr, sizeof(servaddr))) != 0) {
        printf("socket bind failed...\n");
        exit(0);
    }
    else
        printf("Socket successfully binded..\n");

    // Now server is ready to listen and verification
    if ((listen(sockfd, 5)) != 0) {
        printf("Listen failed...\n");
        exit(0);
    }
    else
        printf("Server listening..\n");
    len = sizeof(cli);

    // Accept the data packet from client and verification
    connfd = accept(sockfd, (SA*)&cli, &len);
    if (connfd < 0) {
        printf("server acccept failed...\n");
        exit(0); 
    } 
    else
        printf("server acccept the client...\n");

    // Function for chatting between client and server
    func(connfd);

    // After chatting close the socket
    close(sockfd); 
}

我在服务器上收到以下结果。

doa: 0.000000   0.000000    1408
doa: 0.000000   0.000000    56

打印的每一行都有三个值。前两个是接收到的不应该为零的数据。最后一个数字是服务器中 read() 函数接收的字节数。服务器在两行（两次 read() 操作）中接收到的字节总和为 1464，这是客户端在一次 fwrite() 操作中发送的字节数。

正确的输出应该是一行，如下所示：

doa: 25.000000  45.000000   1464

通过 1464 字节的传输接收到两个非零数据值。我还检查了客户端代码。客户端在Matlab中通过fwrite()操作发送（写入）1464字节。

Answer 1

read() 调用仅 return 您期望的一些数据是完全正常的。发生这种情况是因为在低级别，网络堆栈将整个数据流分解为多个固定大小的数据包，以便通过线路传输。来自 read(2) 手册页：

RETURN VALUE ... It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was interrupted by a signal.

接收网络数据时，您需要不断调用 read() 直到接收到预期的字节数。例如，像这样的东西（未经测试的代码）：

void func(int sockfd) 
{ 
    struct doa_struct *doa_data;
    int expected_bytes = 1464;
    unsigned char buff[expected_bytes];
    int num_bytes;

    // Read a message
    bzero(buff, 1464);
    int bytes_read = 0;
    while (bytes_read < expected_bytes) {

        // read data into buff until we've read all the expected bytes
        // NOTE: if the Matlab side sends a malformed message, this could
        // hang in this loop forever...for real-world use, you'd need to
        // account for those types of scenarios.
        num_bytes = read(sockfd, buff + bytes_read, expected_bytes - bytes_read);
        if (num_bytes <= 0) {
            // handle error cases...
            return;
        }
        bytes_read += num_bytes;
    }

    // Get the buffer which contains the client contents 
    doa_data = (struct doa_struct *) buff; 
    printf("doa: %f\t%f\t%d\n", doa_data->doa[0], doa_data->doa[1], num_bytes);
}

至于为什么您的输出打印的是 0 而不是预期值：这可能是因为您的 printf 格式说明符。你的编译器可能需要 "%lf" 来打印双打，而不是 "%f"。传统上，双精度值需要“%lf”，这是 64 位值而不是 32 位浮点值。但是，C99 和更高版本的编译器可以模糊这条线——参见 this related stack overflow question.

Matlab 中的客户端如何将二进制数据发送到 C 中的服务器？

How a client in Matlab can send binary data to server in C?

c

matlab

tcp-ip