udpEchoClient.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[]) {
    struct sockaddr_in server;
    socklen_t len = sizeof(struct sockaddr_in);
    char buf[BUF_SIZE];
    struct hostent *host;
    int n, s, port;

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

    if (argc < 4) {
        fprintf(stderr, "Usage: %s <host> <port> <message>\n", argv[0]);
        return 1;
    }

    host = gethostbyname(argv[1]);
    if (host == NULL) {
        perror("gethostbyname");
        return 1;
    }

    port = atoi(argv[2]);

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

    /* initialize server addr */
    memset((char *)&server, 0, sizeof(struct sockaddr_in));
    server.sin_family = AF_INET;
    server.sin_port = htons(port);
    server.sin_addr = *((struct in_addr*) host->h_addr);

    /* send message */
    if (sendto(s, argv[3], strlen(argv[3]), 0, (struct sockaddr *) &server, len) == -1) {
        perror("sendto()");
        return 1;
    }

    /* receive echo.
    ** for single message, "while" is not necessary. But it allows the client
    ** to stay in listen mode and thus function as a "server" - allowing it to
    ** receive message sent from any endpoint.
    */
    (n = recvfrom(s, buf, BUF_SIZE, 0, (struct sockaddr *) &server, &len));
    if (n > 0) {
        printf("Received from %s:%d: ",
            inet_ntoa(server.sin_addr),
            ntohs(server.sin_port));
        fflush(stdout);
        buf[n] = 0;
        printf("%s\n", buf);
    }

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