udp-client.c

/* 
 * udp-client.c - A simple UDP client
 * usage: udp-client <host> <port>
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#if defined(_WIN32)
#include <WinSock2.h>
#pragma comment(lib, "Ws2_32.lib")
#else
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 
#include <unistd.h>
#endif

#define BUFSIZE 1024

/* 
 * error - wrapper for perror
 */
void error(char *msg) {
    perror(msg);
    exit(0);
}

int main(int argc, char **argv) {
    int sockfd, portno, n;
    int serverlen;
    struct sockaddr_in serveraddr;
    struct hostent *server;
    char *hostname;
    char buf[BUFSIZE];

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

    /* check command line arguments */
    if (argc != 3) {
       fprintf(stderr,"usage: %s <hostname> <port>\n", argv[0]);
       exit(0);
    }
    hostname = argv[1];
    portno = atoi(argv[2]);

    /* socket: create the socket */
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) 
        error("ERROR opening socket");

    /* gethostbyname: get the server's DNS entry */
    server = gethostbyname(hostname);
    if (server == NULL) {
        fprintf(stderr,"ERROR, no such host as %s\n", hostname);
        exit(0);
    }

    /* build the server's Internet address */
    memset(&serveraddr, 0, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET;
    memcpy(&serveraddr.sin_addr.s_addr, (char *)server->h_addr, server->h_length);
    serveraddr.sin_port = htons(portno);

    /* get a message from the user */
    memset(buf, 0, BUFSIZE);
    printf("Please enter msg: ");
    fgets(buf, BUFSIZE, stdin);

    /* send the message to the server */
    serverlen = sizeof(serveraddr);
    n = sendto(sockfd, buf, strlen(buf), 0, (struct sockaddr *)&serveraddr, serverlen);
    if (n < 0) {
        error("ERROR in sendto");
    }
    /* print the server's reply */
    n = recvfrom(sockfd, buf, BUFSIZE, 0, (struct sockaddr *)&serveraddr, &serverlen);
    if (n < 0) {
        error("ERROR in recvfrom");
    }
    printf("Echo from server: %s", buf);
    return 0;
}