Reading stdin from another thread

Question

I am using p_threads in my code on unix. In my main program I have a thread Node which makes 2 threads one of which is doing a read from the standard input using getline. All of this working fine. Except at some point during my code I have to "restart my thread that reads from the standard input". And when i do that, my i am not able to read any thing from the stdin.

Any suggestions what i might be doing wrong ???/

Thanks.

This the part where i am reading from stdin

void* parseCmd(void* s) 
{

    sigset_t new2; 
    struct sigaction act; 
    sigemptyset(&new2); 
    sigaddset(&new2,SIGINT); 
    pthread_sigmask(SIG_UNBLOCK, &new2, NULL); 

    act.sa_handler = interrupt; 
    sigaction(SIGINT, &act, NULL); 
    signal(SIGUSR1, signal_Handler);

    std::string input("");
    while (1) 
    {

       std::cout << "SERVANT > ";
       std::getline(std::cin, input);

       doTheWork(input);
       cin.clear();

       std::cout << std::endl;

      if(global_shutdown==1 || auto_global_shutdown==1)
        break;
    }


    cout<<"cmd thread exit.Main\n"; 
    return 0;

}

You might have closed standard input, but it's hard to tell without seeing a code sample. — Fred Foo, Mar 24 '11 at 19:29
if the global shutdown flag is set then i exit in this function only and return 0 the last line .. Or if i catch a sigusr1 signal i am doing the pthread_exit(). — Teotia, Mar 24 '11 at 19:38
If the thread is in the middle o doing a getline() when a signal handler forces an exit then the state of stream may potentially be screwed up (especially any locks). The signal handler should set some state that allows the thread to exit after it returns from the signal handler. If the thread is stalling in getline() (waiting for user input) then you should probably not be using getline(). Lookup select(). This allows you to stall in a signal friendly way waiting for input on a stream (C stream but stdin is there). — Martin York, Mar 24 '11 at 19:47

score 4 · Accepted Answer · answered Mar 24 '11 at 19:41

You are probably aborting the thread that is still connected to standard input, remember that you aborted the thread while doing getline.

Now I don't know if there is a way to acquire the standard input pointer and free it to be able to get more lines but there probably is a method to continue reading the lines tha previous thread has been reading.

What you need to do is:

when entering the getline use a shared flag and set it to true
set variable into which you read as a static variable and never use getline on it in any thread except the one that needs restarting
when you reconstruct the new thread do not call getline if the flag entering getline is true
after you finished getting the line reset the boolean flag
use locking to prevent concurrent acess on the reading line flag

When you need the value simply use the static variable that you use to pass to getline.

Hey .. thanks man ... you were right i was exiting thread while still connected to stdin ... — Teotia, Mar 24 '11 at 19:54

Reading stdin from another thread

1 Answers1