// Source: http://shirleyanengineer.blogspot.com/2019/11/socket-socket-programming-in-linux-os.html
// Server side implementation of UDP client-server model

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

#define PORT    8080
#define MAXLINE 1024

int main() {
    int sockfd, len, n;
    char buffer[MAXLINE];
    char *hello = "Hello from server";
    struct sockaddr_in servaddr, cliaddr;
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE); }
    memset(&servaddr, 0, sizeof(servaddr));
    memset(&cliaddr, 0, sizeof(cliaddr));
    servaddr.sin_family    = AF_INET; // IPv4
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);
    if (bind(sockfd, (const struct sockaddr *)&servaddr,
             sizeof(servaddr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE); }
    n = recvfrom (sockfd, (char *)buffer, MAXLINE, MSG_WAITALL, 
                          (struct sockaddr *) &cliaddr, &len);
    buffer[n] = '\0';
    printf("Client : %s\n", buffer);
    sendto(sockfd, (const char *)hello, strlen(hello), MSG_CONFIRM, 
                   (const struct sockaddr *) &cliaddr, len);
    printf("Hello message sent.\n");
    return 0; }