"Linux network development must learn course" 15_UDP data sending and receiving actual combat

Question: How to use UDP to send and receive data?

On UDP Protocol Again

• UDP is connectionless ( unreliable, no reply messages, no sequence number identification of packets )
• UDP is packet-oriented, 对应用层数据既不合并也不拆分 (packet boundaries are preserved)
• UDP has no congestion control, network congestion will not reduce the sending rate of the source host
• UDP supports one-to-one, one-to-many, many-to-one and many-to-many interactive communication
• UDP message header overhead is small, only 8 bytes (TCP message header has a total of 20 bytes)

Difference between UDP and IP

UDP is a data transfer protocol built on top of IP

• IP is responsible for transferring UDP packets from source host to destination host
• UDP delivers application layer data to the target socket (port number)

UDP almost completely "inherits" the characteristics of IP transmission

• There is no interaction between the two ends of the communication, no flow control, no timeout retransmission, and no reliability.

UDP send and receive data

 #include "sys/socket.h"

ssize_t send(int sock,                // socket 文件描述符
             void *buf,               // 需要发送的数据
             size_t nbytes,           // 需要发送的数据量
             int flags,               // 发送选项
             struct sockaddr *to,     // 接收地址信息
             socklen_t addrlen);      // to 参数长度

ssize_t recvfrom(int sock,            // socket 文件描述符
                 void *buf,            // 保存接收数据的地址
                 size_t nbytes,        // 可接收的最大数据量
                 int flags,            // 接收选项
                 struct sockaddr *from, // 接收地址信息
                 socklen_t *addrlen);    // 指向保存 from 参数长度的变量地址

UDP programming mode

Programming experiment: UDP data transmission and reception

client.c

 #include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main()
{
    int sock = 0;
    struct sockaddr_in addr = {0};
    struct sockaddr_in remote = {0};
    socklen_t len = 0;
    char buf[128] = {0};
    char input[32] = {0};
    int r = 0;

    sock = socket(PF_INET, SOCK_DGRAM, 0);

    if (sock == -1) {
        printf("socket error\n");
        return -1;
    }

    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(7777);

    if (bind(sock, (struct sockaddr*)&addr, sizeof(addr)) == -1) {  // 特别说明, 当使用 udp socket 进行数据发送时，不指定 bind (即，ip 和 port) 也能将数据成功发送
        printf("udp bind error\n");                                 // 但 udp 是无连接的，因此不像 TCP 有非常明显的服务端等待客户端发起连接的过程 
        return -1;                                                  // udp 的通信端点是对等的，为了方便接收其他端点发送的数据，因此指定了 ip 和 端口号 （尤其端口号）
    }

    remote.sin_family = AF_INET;
    remote.sin_addr.s_addr = inet_addr("127.0.0.1");
    remote.sin_port = htons(8888);

    while (1) {
        printf("Input: ");

        scanf("%s", input);

        len = sizeof(remote);

        sendto(sock, input, strlen(input), 0, (struct sockaddr*)&remote, len);

        r = recvfrom(sock, buf, sizeof(buf), 0, (struct sockaddr*)&remote, &len);

        if (r > 0) {
            buf[r] = 0;
            printf("Recvfrom %s\n", buf);
        } else {
            break;
        }
    }

    close(sock);

    return 0;
}

server.c

 #include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main()
{
    int server = 0;
    struct sockaddr_in saddr = {0};
    int client = 0;
    struct sockaddr_in remote = {0};
    socklen_t asize = 0;
    int len = 0;
    char buf[32] = {0};
    int r = 0;

    server = socket(PF_INET, SOCK_DGRAM, 0);

    if (server == -1) {
        printf("server socket error");
        return -1;
    }

    saddr.sin_family = AF_INET;
    saddr.sin_addr.s_addr = htonl(INADDR_ANY);
    saddr.sin_port = htons(8888);

    if (bind(server, (struct sockaddr*)&saddr, sizeof(saddr)) == -1) {
        printf("udp server bind error\n");
        return -1;
    }

    printf("udp server start sucess\n");

    while (1) {
        len = sizeof(remote);

        r = recvfrom(server, buf, sizeof(buf), 0, (struct sockaddr*)&remote, &len);

        buf[r] = 0;

        printf("r = %d\n", r);
        printf("buf = %s\n", buf);
        printf("remote ip = %s\n", inet_ntoa(remote.sin_addr));
        printf("remote port = %d\n", ntohs(remote.sin_port));

        sendto(server, buf, r, 0, (struct sockaddr*)&remote, len);
    }

    close(server);

    return 0;
}

output:

 server:
udp server start sucess
r = 5
buf = hello
remote ip = 127.0.0.1
remote port = 7777

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client:
Input: hello
Recvfrom hello

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Thinking: How to send and receive UDP data one-to-many?