forkpty - socket

Question

I'm trying to develop a simple "telnet/server" daemon which have to run a program on a new socket connection. This part working fine.

But I have to associate my new process to a pty, because this process have some terminal capabilities (like a readline).

The code I've developped is (where socketfd is the new socket file descriptor for the new input connection) :

int masterfd, pid;
const char *prgName = "...";
char *arguments[10] = ....;

if ((pid = forkpty(&masterfd, NULL, NULL, NULL)) < 0)
    perror("FORK");
else if (pid)
    return pid;
else
{
    close(STDOUT_FILENO);
    dup2(socketfd, STDOUT_FILENO);

    close(STDIN_FILENO);
    dup2(socketfd, STDIN_FILENO);

    close(STDERR_FILENO);
    dup2(socketfd, STDERR_FILENO);

    if (execvp(prgName, arguments) < 0)
    {
        perror("execvp");
        exit(2);
    }
}

With that code, the stdin / stdout / stderr file descriptor of my "prgName" are associated to the socket (when looking with ls -la /proc/PID/fd), and so, the terminal capabilities of this process doesn't work.

A test with a connection via ssh/sshd on the remote device, and executing "localy" (under the ssh connection) prgName, show that the stdin/stdout/stderr fd of this process "prgName" are associated to a pty (and so the terminal capabilities of this process are working fine).

What I am doing wrong? How to associate my socketfd with the pty (created by forkpty) ?

Thank

Alex

Answer 1

You must write some code to transfer data from the socket to the master pty and vice versa. It's usually a parent process' job. Note that the data transfer must be bidirectional. There are many options: a select()-driven cycle to track both the masterfd and the socketfd

(just as hint, very bad code, not for production!!! Missing error and eof checks!!!)

for (;;) {
  FD_ZERO(&set);
  FD_SET(masterfd,&set);
  FD_SET(socketfd,&set);
  select(...,&set,...);
  if (FD_ISSET(masterfd,&set)) {
       read(masterfd,&c,1);
       write(socketfd,&c,1);
  }
  if (FD_ISSET(sockerfd,&set)) {
       read(sochetfd,&c,1);
       write(masterfd,&c,1);
  }

or a pair of threads, one for socketfd->masterfd and one for masterfd->sockefd transfers.

(just as hint, very bad code, not for production!!!)

/*thread 1 */
      while (read(masterfd,&c,1) > 0)
           write(socketfd,&c,1);


/*thread 2 */
      while (read(socketfd,&c,1) > 0)
           write(masterfdfd,&c,1);

Anyway you must add some code in the parent side of the branch.

Regards

---EDIT--- Of course, you must not redirect fd 0,1 and 2 to socketfd in the child process.