////server.c#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>int main(int argc, char **argv){    if (argc != 2)    {        printf("Usage: %s port\n", argv[0]);        exit(1);    }    printf("Welcome! This is a UDP server, I can only received message from client and reply with same message\n");        struct sockaddr_in addr;    addr.sin_family = AF_INET;    addr.sin_port = htons(atoi(argv[1]));    addr.sin_addr.s_addr = htonl(INADDR_ANY);    int sock;    if ( (sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0)    {        perror("socket");        exit(1);    }    if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < 0)    {        perror("bind");        exit(1);    }    char buff[512];    struct sockaddr_in clientAddr;    int n;    int len = sizeof(clientAddr);    while (1)    {        n = recvfrom(sock, buff, 511, 0, (struct sockaddr*)&clientAddr, &len);        if (n>0)        {            buff[n] = 0;            printf("%s %u says: %s\n", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port), buff);            n = sendto(sock, buff, n, 0, (struct sockaddr *)&clientAddr, sizeof(clientAddr));            if (n < 0)            {                perror("sendto");                break;            }        }        else        {            perror("recv");            break;        }    }    return 0;}

///client.c#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>int main(int argc, char **argv){    if (argc != 3)    {        printf("Usage: %s ip port", argv[0]);        exit(1);    }    printf("This is a UDP client\n");    struct sockaddr_in addr;    int sock;    if ( (sock=socket(AF_INET, SOCK_DGRAM, 0)) <0)    {        perror("socket");        exit(1);    }    addr.sin_family = AF_INET;    addr.sin_port = htons(atoi(argv[2]));    addr.sin_addr.s_addr = inet_addr(argv[1]);    if (addr.sin_addr.s_addr == INADDR_NONE)    {        printf("Incorrect ip address!");        close(sock);        exit(1);    }    char buff[512];    int len = sizeof(addr);    while (1)    {        gets(buff);        int n;        n = sendto(sock, buff, strlen(buff), 0, (struct sockaddr *)&addr, sizeof(addr));        if (n < 0)        {            perror("sendto");            close(sock);            break;        }        n = recvfrom(sock, buff, 512, 0, (struct sockaddr *)&addr, &len);        if (n>0)        {            buff[n] = 0;            printf("received:");            puts(buff);        }        else if (n==0)        {            printf("server closed\n");            close(sock);            break;        }        else if (n == -1)        {            perror("recvfrom");            close(sock);            break;        }    }        return 0;}