未在客户端获取输出（C 中的 UDP 套接字）

Question

这是我的 UDP 套接字服务器和客户端的代码。

问题是，当我在终端中运行它时，一旦客户端到达服务器，服务器执行就会停止，但在客户端我没有收到任何数据。

客户端.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int network_socket;

    network_socket = socket(AF_INET, SOCK_DGRAM, 0);

    struct sockaddr_in server_address;

    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = INADDR_ANY;

    sendto(network_socket, 0, 0, MSG_CONFIRM, (const struct sockaddr *) &server_address, sizeof(server_address));
    char server_response[256];
    int len = sizeof(network_socket);
    int = recvfrom(network_socket, &server_response, sizeof(server_response), 0,(struct sockaddr*) &network_socket, (socklen_t*)&len);

    printf("The server sent the data : %s\n", server_response);

    close(network_socket);
    return 0;
}

服务器.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    char server_message[256] = "you have reached the UDP server";

    int server_socket,client_address;
    server_socket = socket(AF_INET, SOCK_DGRAM,0);

    struct sockaddr_in server_address;
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = INADDR_ANY;

    bind(server_socket, (struct sockaddr *) &server_address, sizeof(server_address));

    int n,len;
    len = sizeof(client_address);
    n = recvfrom(server_socket, server_message, sizeof(server_message), 0, (struct sockaddr *) &client_address, (socklen_t*)&len);

    sendto(server_socket, server_message, sizeof(server_message), 0, (struct sockaddr *) &client_address, len);
    close(server_socket);
    return 0;
}

Answer 1

sendto(network_socket, 0, 0, MSG_CONFIRM, (const struct sockaddr *) &server_address, sizeof(server_address));

您没有发送任何数据。

int = recvfrom(network_socket, &server_response, sizeof(server_response), 0,(struct sockaddr*) &network_socket, (socklen_t*)&len);

printf("The server sent the data : %s\n", server_response);

您没有保留收到的字节数，这就是recvfrom 返回的内容。所以你不知道你收到了多少数据。然后您尝试通过%s 打印此数据，这仅 用于C 样式字符串。您不能使用它来打印任意数据。至少，您必须告诉它要打印多少数据（否则，它怎么可能知道？）然后您将这些信息扔掉了。

Answer 2

在双方，您根本没有进行任何错误处理。

在客户端，你不能发送数据报到INADDR_ANY，你需要发送到一个真实的IP地址。而当调用recvfrom()时，你使用network_socket文件描述符作为sockaddr_in结构来接收发送者的地址/端口，这是错误的。

在服务器端，当 client_address 需要成为 sockaddr_in 结构时，您将其声明为 int。而且您正在使用相同的char[] 缓冲区来接收用于存储服务器问候消息的客户端数据，因此您实际上是在回显客户端发送的内容（这没什么），但您并没有占用客户的数据考虑在内。在recvfrom() 退出后，您在滥用printf()，因为客户端没有发送以null 结尾的字符串，但%s 需要一个。

试试这个：

客户端.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main()
{
    int network_socket = socket(AF_INET, SOCK_DGRAM, 0);
    if (network_socket < 0)
    {
        perror("socket() failed");
        return -1;
    }

    struct sockaddr_in server_address;

    bzero(&server_address, sizeof(server_address));
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = inet_addr("Server IP Address Here"); // or "127.0.0.1" for localhost

    if (sendto(network_socket, 0, 0, MSG_CONFIRM, (struct sockaddr *) &server_address, sizeof(server_address)))
    {
        perror("sendto() failed");
        close(network_socket);
        return -1;
    }

    char server_response[256];

    struct sockaddr_in sender_address;
    socklen_t addrlen = sizeof(sender_address);

    int n = recvfrom(network_socket, server_response, sizeof(server_response), 0, (struct sockaddr*) &sender_address, &addrlen);
    // or, since you are not using the sender's sockaddr_in for anything:
    // int n = recvfrom(network_socket, server_response, sizeof(server_response), 0, NULL, NULL);
    if (n < 0)
    {
        perror("recvfrom() failed");
        close(network_socket);
        return -1;
    }

    printf("The server sent the data : %.*s\n", n, server_response);

    close(network_socket);
    return 0;
}

服务器.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main()
{
    char server_message[] = "you have reached the UDP server";

    int server_socket = socket(AF_INET, SOCK_DGRAM, 0);
    if (server_socket < 0)
    {
        perror("socket() failed");
        return -1;
    }

    struct sockaddr_in server_address;

    bzero(&server_address, sizeof(server_address));
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(9002);
    server_address.sin_addr.s_addr = INADDR_ANY;

    if (bind(server_socket, (struct sockaddr *) &server_address, sizeof(server_address)) < 0)
    {
        perror("bind() failed");
        close(server_socket);
        return -1;
    }

    char client_message[256];

    struct sockaddr_in client_address;
    socklen_t addrlen = sizeof(client_address);

    int n = recvfrom(server_socket, client_message, sizeof(client_message), 0, (struct sockaddr *) &client_address, &addrlen);
    if (n < 0)
    {
        perror("recvfrom() failed");
        close(server_socket);
        return -1;
    }

    printf("The client sent the data : %.*s\n", n, client_message);

    if (sendto(server_socket, /*client_message*/server_message, /*n*/strlen(server_message), 0, (struct sockaddr *) &client_address, addrlen) < 0)
    {
        perror("sendto() failed");
        close(server_socket);
        return -1;
    }

    close(server_socket);
    return 0;
}