UDP是无连接的。

UDP Server网络编程基本步骤

• 创建socket，指定使用UDP协议
• 将socket与地址和端口绑定
• 使用recv/send接收/发送数据
  由于UDP是无连接的，直接侦听就行
• 使用close关闭连接

这个UDP接收数据的时候用的API是recvfrom,发送数据是sendto

客户端

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

int main(int argc, char * *argv)
{
    struct sockaddr_in addr;

    socklen_t addrlen = sizeof(addr);

    int sock;

    addr.sin_family     = AF_INET;
    addr.sin_port       = htons(9876);
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    char buff_send[512] = "Hello";
    char buff_recv[512] = {0};
    int len = sizeof(addr);

    int n = 0;

    printf("This is a UDP client\n");

    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        printf("socket error.\n");
        exit(1);
    }

    if (addr.sin_addr.s_addr == INADDR_NONE)
    {
        printf("Incorrect ip address!");
        close(sock);
        exit(1);
    }

    n = sendto(sock, buff_send, strlen(buff_send), 0, (struct sockaddr *) &addr, sizeof(addr));
    if (n < 0)
    {
        printf("sendto error.\n");
        close(sock);
    }

    n = recvfrom(sock, buff_recv, 512, 0, (struct sockaddr *) &addr, &addrlen);
    if (n > 0)
    {
        buff_recv[n] = 0;
        printf("received from sever:");
        puts(buff_recv);
    }
    else if (n == 0)
        printf("server closed.\n");
    else if (n == -1)
        printf("recvfrom error.\n");

    close(sock);

    return 0;
}

服务器端

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

int main(int argc, char * *argv)
{
    struct sockaddr_in addr;
    addr.sin_family     = AF_INET;
    addr.sin_port       = htons(9876);
    addr.sin_addr.s_addr = INADDR_ANY;

    char buff_recv[512] = {0};
    char buff_send[512] = "world";

    struct sockaddr_in clientAddr;

    socklen_t addrlen = sizeof(clientAddr);

    int n;
    int len = sizeof(clientAddr);

    int sock;

    printf("Welcome! This is a UDP server.\n");

    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        printf("socket error.\n");
        exit(1);
    }

    if (bind(sock, (struct sockaddr *) &addr, sizeof(addr)) < 0)
    {
        printf("bind error.\n");
        exit(1);
    }

    while (1)
    {
        //ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,struct sockaddr *src_addr, socklen_t *addrlen);

        n = recvfrom(sock, (void *)buff_recv, 511, 0, (struct sockaddr *) &clientAddr, &addrlen);
        if (n > 0)
        {
            buff_recv[n] = 0;
            printf("recv data from client:%s %u says: %s\n", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port), buff_recv);

            n = sendto(sock, (void *)buff_send, n, 0, (struct sockaddr *) &clientAddr, sizeof(clientAddr));
            if (n < 0)
            {

                printf("sendto error.\n");
                break;

            }else {
                printf("recv error.\n");
                break;
            }
        }

    }
    return 0;
}