udpclient.c

#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>

#include <arpa/inet.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#define SERV_PORT 20001
#define BUFSIZE 1024
#define SADDR struct sockaddr
#define SLEN sizeof(struct sockaddr_in)

int main(int argc, char **argv) {
  int sockfd, n;
  char sendline[BUFSIZE], recvline[BUFSIZE + 1];
  struct sockaddr_in servaddr;
  struct sockaddr_in cliaddr;

  if (argc != 2) {
    printf("usage: client <IPaddress of server>\n");
    exit(1);
  }

  memset(&servaddr, 0, sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  servaddr.sin_port = htons(SERV_PORT);

  if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) < 0) {
    perror("inet_pton problem");
    exit(1);
  }
  if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
    perror("socket problem");
    exit(1);
  }

  write(1, "Enter string\n", 13);

  while ((n = read(0, sendline, BUFSIZE)) > 0) {
    if (sendto(sockfd, sendline, n, 0, (SADDR *)&servaddr, SLEN) == -1) {
      perror("sendto problem");
      exit(1);
    }

    if (recvfrom(sockfd, recvline, BUFSIZE, 0, NULL, NULL) == -1) {
      perror("recvfrom problem");
      exit(1);
    }

    printf("REPLY FROM SERVER= %s\n", recvline);
  }
  close(sockfd);
}