Questions about using forkpty to create pseudo terminal to ssh in C

Question

pid = forkpty (&pty,0,0,0);
if (pid == 0) {
    execl ("/usr/bin/ssh", "ssh", hostname, NULL);
    exit (0);
} else if (pid > 0) {
    ssh_pid = pid;
    ssh_pty = pty;
    if(child_ssh_success()) {
        get_user_input();
        send_user_input_to_child_ssh_and_child_forword_it_to_remote_server();
        get_remote_server_response_from_child();
        display_response_to_stdout();
    }
}

How can I tell whether ssh success or not?

How can parent know that child has successfully ssh-ed to remote server, so parent can send something to remote server?

Answer 1

Use the function waitpid in the parent ( ie inside your else if ( pid>0) condition ) to get the status of the child

#include   <sys/wait.h>
pid_t waitpid(pid_t pid, int *stat_loc, int options);

Quoting IBM DeveloperWorks

stat_loc

A pointer to an integer where the wait function will return the status of the child process. If the waitpid function returns because a process has exited, the return value is equal to the pid of the exiting process. For this, if the value of stat_loc is not NULL, information is stored in the location pointed to by stat_loc. If status is returned from a terminated child process that returned a value of 0, the value stored at the location pointed to by stat_loc is 0. If the return is greater than 0, you can evaluate this information using the following macros: WEXITSTATUS WIFEXITED WIFSIGNALED WTERMSIG.

So if stat_loc is zero, you may assume that SSH terminated normally

EDIT1

If you dont want the parent to be blocked, then you need to set up a signal handler for SIGCHLD and do the same waitpid there. This time as the child has already terminated, waitpid will return immediately

Something on these lines

pid_t pid;
int main ()
{
struct sigaction action;

memset (&action, 0, sizeof(action));
action.sa_handler = sigchld_handler;

if (sigaction(SIGCHLD, &action, 0)) 
    {
    perror ("sigaction");
    return 1;
}

pid = forkpty (&pty,0,0,0);
if (pid == 0) 
{
    execl ("/usr/bin/ssh", "ssh", hostname, NULL);
    exit (0);
} 
else if (pid > 0) 
{
    ssh_pid = pid;
    ssh_pty = pty;
}
}

/* SIGCHLD handler. */
static void sigchld_handler (int sig)
{
    int chld_state;

    while (waitpid(pid,&child_state,options) > 0) 
    {
       if (WIFEXITED(chld_state)) 
       {
           printf("Child exited with RC=%d\n",WEXITSTATUS(chld_state));
       }
    }

}

Make pid global so that you can access it from sigchld_handler too. Generally ssh returns 0 for SUCCESS and 255 ( or some other positive value ) for failure, though I am not cent percent sure on this.

EDIT2

From our discussion, I see you want to execute commands as well on the remote server. I suggest you run ssh this

ssh root@remoteserver.com 'ls -l'

You can pass these arguments of ssh via execl() and as explained in EDIT1 , you can check for its return value to verify whether everything went fine. Also, since you are doing ssh from code, you may not want to enter password manually. Here is how you do password less login