#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>

int main(int argc, char *argv[])
{
    int sock = -1;
    unsigned short port = 9999u;
    struct sockaddr_in addr;

    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if(sock < 0)
    {
        perror("socket");
        return EXIT_FAILURE;
    }

    memset((void *) &addr, 0x00, sizeof addr);
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = INADDR_ANY;
    if (bind(sock, (struct sockaddr*) &addr, sizeof addr) != 0)
    {
        perror("bind");
        return EXIT_FAILURE;
    }

    int bytes = 0;
    char buffer[128] =    {0};
    while(bytes >= 0)
    {
        int addr_len = sizeof addr;

        bytes = recvfrom(sock, buffer, sizeof buffer, 0, (struct sockaddr*)&addr, &addr_len);
        if(bytes == 0)
        {
            printf("Transfer aborted by peer\n");
        }
        else if(bytes < 0)
        {
            perror("recvfrom");
        }
        else
        {
            if(strcmp(buffer, "Tell me something") == 0)
            {
                const char *ack = "Hello, world!";
                sendto(sock, ack, strlen(ack), 0, (struct sockaddr*)&addr,    sizeof addr);
            }
        }
                memset((void *)buffer, '\0', sizeof buffer);
    }
    close(sock);
    return EXIT_SUCCESS;
}