udpEchoServer.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#if defined(_WIN32)
#include <WinSock2.h>
#include <ws2tcpip.h>
#pragma comment(lib, "Ws2_32.lib")
#else
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#endif

#define BUF_SIZE 1024

int main(int argc, char* argv[]) {
    char buf[BUF_SIZE];
    struct sockaddr_in self, other;
    int len = sizeof(struct sockaddr_in);
    int n, s, port;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <port>\n", argv[0]);
        return 1;
    }

    {
#if defined(_WIN32)
        WSADATA wsaData;
        if (WSAStartup(MAKEWORD(2, 2), &wsaData) != NO_ERROR) {
            printf("Error at WSAStartup()\n");
            return 1;
        }
#endif
    }

    /* initialize socket */
    if ((s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) {
        perror("socket");
        return 1;
    }

    /* bind to server port */
    port = atoi(argv[1]);
    memset((char *)&self, 0, sizeof(struct sockaddr_in));
    self.sin_family = AF_INET;
    self.sin_port = htons(port);
    self.sin_addr.s_addr = htonl(INADDR_ANY);
    if (bind(s, (struct sockaddr *) &self, sizeof(self)) == -1) {
        perror("bind");
        return 1;
    }

    while ((n = recvfrom(s, buf, BUF_SIZE, 0, (struct sockaddr *) &other, &len)) != -1) {
        printf("Received from %s:%d: ",
            inet_ntoa(other.sin_addr),
            ntohs(other.sin_port));
        fflush(stdout);
        write(1, buf, n);
        write(1, "\n", 1);

        /* echo back to client */
        sendto(s, buf, n, 0, (struct sockaddr *) &other, len);
    }

#if defined(_WIN32)
    closesocket(s);
#else
    close(s);
#endif
    return 0;
}