#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/select.h>
#include <fcntl.h>


#define PORT 12345

#define BUFFER_SIZE 1024
                                     

int main(void) {
  char buffer[BUFFER_SIZE];
  struct sockaddr_in server_addr;
  struct sockaddr_in client_addr;
  int server_fd, client_fd, client_len;
  fd_set read_fds, write_fds;
  int r, bytes = 0;
  int in = 1, out = 0; 
  int offset = 0;


  memset(&server_addr, 0, sizeof(server_addr));
  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(PORT);
  server_addr.sin_addr.s_addr = INADDR_ANY;

  if((server_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
  {
    printf("Can't create socket\n");
    exit(0);
  }

  if(bind(server_fd, (struct sockaddr *) &server_addr, sizeof(server_addr)) < 0)
  {
    printf("Can't bind socket\n");
    exit(0);
  }
  
  if(listen(server_fd, 1) < 0)
  {
    printf("Can't listen on socket\n");
    exit(0);
  }

  client_len = sizeof(client_addr);
  client_fd = accept(server_fd, (struct sockaddr *) &client_addr, &client_len);

  if(client_fd < 0)
  {
    printf("Can't accept connection\n");
    exit(0);
  }
 
  close(server_fd);

  fcntl(client_fd, F_SETFL, fcntl(client_fd, F_GETFL) | O_NONBLOCK);

  while(1) {
    FD_ZERO(&read_fds);
    FD_ZERO(&write_fds);


    if (in)
      FD_SET(client_fd, &read_fds);

    if (out)
      FD_SET(client_fd, &write_fds);

    r = select(client_fd + 1, &read_fds, &write_fds, NULL, NULL);

    if (r < 0) 
    {
      if (errno == EINTR) 
        continue;
      goto END;
    }

    if (FD_ISSET(client_fd, &read_fds))
    {
      bytes = read(client_fd, buffer, BUFFER_SIZE);
      offset = 0;

      if (bytes <= 0)
      {
        goto END;
      }
      else
        FD_SET(client_fd, &write_fds);
    }

    if (FD_ISSET(client_fd, &write_fds))
    {
      r = write(client_fd, buffer + offset, bytes);
      offset += r; 
      bytes -= r;

      if (bytes == 0)
      {
        in = 1;
        out = 0;
      }
      else
      {
        in = 0;
        out = 1;
      }
    }
  }

END:

  close(client_fd);

  printf("Finished...\n");

  return 0;
}