////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 != )

    {

        printf("Usage: %s port\n", argv[]);

        exit();

    }

    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[]));

    addr.sin_addr.s_addr = htonl(INADDR_ANY);

    int sock;

    if ( (sock = socket(AF_INET, SOCK_DGRAM, )) < )

    {

        perror("socket");

        exit();

    }

    if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < )

    {

        perror("bind");

        exit();

    }

    char buff[];

    struct sockaddr_in clientAddr;

    int n;

    int len = sizeof(clientAddr);

    while ()

    {

        n = recvfrom(sock, buff, , , (struct sockaddr*)&clientAddr, &len);

        if (n>)

        {

            buff[n] = ;

            printf("%s %u says: %s\n", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port), buff);

            n = sendto(sock, buff, n, , (struct sockaddr *)&clientAddr, sizeof(clientAddr));

            if (n < )

            {

                perror("sendto");

                break;

            }

        }

        else

        {

            perror("recv");

            break;

        }

    }

    return ;

}

///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 != )

    {

        printf("Usage: %s ip port", argv[]);

        exit();

    }

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

    struct sockaddr_in addr;

    int sock;

    if ( (sock=socket(AF_INET, SOCK_DGRAM, )) <)

    {

        perror("socket");

        exit();

    }

    addr.sin_family = AF_INET;

    addr.sin_port = htons(atoi(argv[]));

    addr.sin_addr.s_addr = inet_addr(argv[]);

    if (addr.sin_addr.s_addr == INADDR_NONE)

    {

        printf("Incorrect ip address!");

        close(sock);

        exit();

    }

    char buff[];

    int len = sizeof(addr);

    while ()

    {

        gets(buff);

        int n;

        n = sendto(sock, buff, strlen(buff), , (struct sockaddr *)&addr, sizeof(addr));

        if (n < )

        {

            perror("sendto");

            close(sock);

            break;

        }

        n = recvfrom(sock, buff, , , (struct sockaddr *)&addr, &len);

        if (n>)

        {

            buff[n] = ;

            printf("received:");

            puts(buff);

        }

        else if (n==)

        {

            printf("server closed\n");

            close(sock);

            break;

        }

        else if (n == -)

        {

            perror("recvfrom");

            close(sock);

            break;

        }

    }

    return ;

}