Date Tags C

Using the methods I discussed previously I wrote a telnet-like client which can be used to communicate with any type of text based server such as mail and web servers. The following isn’t 100% perfect as it doesn’t handle early returns from send as I wasn’t able to properly test this behavior.

#include   // perror,printf
#include  // exit
#include  // strerror
#include   // gethostbyname
#include 
#include   // FD_ functions
#include  // sig functions
#include  // STDIN_FILENO

int client_socket;

void exit_error(char* msg) {
  perror(msg);
  exit(1);
}

void exit_close(char* msg) {
  close(client_socket);
  exit_error(msg);
}

void KBInterrupt(int sig) {
  close(client_socket);
  exit(0);
}

int main(int argc, char* argv[]) {
  /* Variables */
  struct hostent *server_host;
  struct sockaddr_in server_addr;
  fd_set fd;
  int charsRead, charsWritten;
  int char_buff_size = 1024;
  char char_buffer[char_buff_size];
  struct sigaction handler;

  /* Verify arguments */
  if (argc < 3) {
    fprintf(stderr,"usage %s hostname port\n",argv[0]);
    exit(0);
  }

  /* Setup Action Handler */
  handler.sa_handler = KBInterrupt;
  if (sigfillset(&handler.sa_mask) < 0)
    exit_error("sigfillset failed");
  handler.sa_flags = 0;
  if (sigaction(SIGINT,&handler,0) < 0)
    exit_error("sigaction failed");

  /* Create TCP socket */
  if((client_socket = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP)) < 0)
    exit_error("Could not create socket");

  /* Get host information. This takes a fqdn or IP address */
  if ((server_host = gethostbyname(argv[1])) == NULL) {
    herror(argv[1]);
    exit(1);
  }

  /* Setup and fill in values for server_addr */
  bzero((char*)&server_addr,sizeof(server_addr));
  server_addr.sin_family = server_host->h_addrtype;
  bcopy((char*)server_host->h_addr,
    (char*)&server_addr.sin_addr.s_addr,
    server_host->h_length);
  server_addr.sin_port = htons(atoi(argv[2]));

  /* Connect */
  if (connect(client_socket,(struct sockaddr*)&server_addr,sizeof(server_addr)) < 0)
    exit_error("ERROR connecting");

  /* The Main part of the program */
  for(;;) {
    /* Setup file descriptors */
    FD_ZERO(&fd);
    FD_SET(STDIN_FILENO,&fd);
    FD_SET(client_socket,&fd);

    /* Wait for input via keyboard or socket */
    select(client_socket+1,&fd,NULL,NULL,NULL);
    if(FD_ISSET(STDIN_FILENO,&fd)) { /* Keyboard input */
      if ((charsRead = read(STDIN_FILENO,char_buffer,char_buff_size)) < 0)
    exit_close("ERROR reading from STDIN");
      if ((charsWritten = send(client_socket,char_buffer,charsRead,0)) != charsRead)
      exit_close("Did not write enough\n");
    }
    if(FD_ISSET(client_socket,&fd)) { /* Socket input */
      if ((charsRead = recv(client_socket,char_buffer,char_buff_size,0)) < 0)
    exit_close("ERROR reading from socket");
      else if (charsRead == 0)
    exit_close("Connection closed");

      /* Output to screen */
      charsWritten = 0;
      int temp = 0;
      while ((temp = write(STDOUT_FILENO,&char_buffer[charsWritten],charsRead-charsWritten)) > 0)
    charsWritten += temp;
      if (temp < 0)
      exit_close("ERROR writing to screen");
    }
  }

  /* Unreachable */
  return 0;
}

Pretty simple.


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