Possible to retrieve input from user and running another process?

Question

Is it possible to use getline(cin,buffer); at the top of my program, then have a "animated menu" still running below it? For example (very basic):

#include <iostream> 
#include <string> 

using namespace std; 
int stringLen=0;
string buffer; 

getline(cin, buffer);

for (int i = 0; i < kMaxWait;i++)
{
     printf("counter waiting for user input %d",i);
     if (1 >= buffer.length())
     break; 
}

Would I have to fork that loop somehow so it would keep counting and display the counter until the user enters something??

Why did you tag this with fork? You seem to know it's possible using e.g. fork(). In general you would use a std::thread waiting for changes in the buffer. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Mar 15, 2016 at 17:28
I don't understand fully how fork works, so I wasn't sure if this is a scenario where I would use it — Robolisk
– Robolisk, Commented Mar 15, 2016 at 17:30
@Robolisk If you definitely want/need anoother process, you can use fork() to achieve such behavior, access to buffer.length() needs to be synchronized in any case. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Mar 15, 2016 at 17:33

Sam Varshavchik · Accepted Answer · 2016-03-15 17:26:55Z

3

One possible answer, given in the comments, is to use threads. But it's not necessary, there's a way to do this without threads.

Make stdin a non-blocking file descriptor.
Wait for stdin to become readable, via poll()/select(), in the meantime do your animation, etc...
Make stdin a blocking file descriptor, again.
Use std::getline().

There are also some ancillary issues to consider, such as the buffering that comes from std::streambuf, so before doing all that, check if there's already something to read from std::cin, first.

answered Mar 15, 2016 at 17:26

Sam Varshavchik

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6 Comments

πάντα ῥεῖ Over a year ago

So that basically proposes to use the stdin fd in a select()/poll() loop, right? That's another way.

Robolisk Over a year ago

I see what you mean, sorta. In a way, the animation block example I gave is very basic. This would actually be a fairly large file doing ascii art type animation in the command window until the user enter data ( Logs in)

Sam Varshavchik Over a year ago

Doesn't matter how extensive. The principle's the same.

Lightness Races in Orbit Over a year ago

This is a pretty elegant solution actually. Took me a couple of reads before I got there but finally a +1 from me.

Robolisk Over a year ago

Is this route, or using fork() more efficient?

|

Ramon · Accepted Answer · 2016-03-15 18:22:14Z

This is something I used sometime ago. It's quite rudimentary, but you can get the gist of the process - using poll. It returns true if there is input, and puts it in str, false otherwise. So, you can put this in your loop somewhere, and take action when there is input.

bool polled_input(std::string& str)
{
    struct pollfd fd_user_in;
    fd_user_in.fd = STDIN_FILENO;
    fd_user_in.events = POLLIN;
    fd_user_in.revents = 0;

    int rv = poll(&fd_user_in, 1, 0);

    if (rv == -1) {/* error */}
    else if (rv == 0) return false;
    else if (fd_user_in.revents & POLLIN)
    {         
        char buffer[MAX_BUFF_SIZE];
        int rc = read(STDIN_FILENO, buffer, MAX_BUFF_SIZE-1);
        if (rc >= 0) 
        {
            buffer[rc]='\0';
            str = std::string(buffer);
            return true;       
        }
        else {/* error */}
    }        
    else {/* error */}
}

Charles Pehlivanian · Accepted Answer · 2016-03-19 02:02:01Z

select is meant for this, multiplexed, blocking I/O. It can be done without a poll I think:

#include <iostream>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char **arg)
{
   const int time_in_secs = 10;
   const int buffer_size = 1024;
   fd_set readfds;

   FD_ZERO(&readfds);
   FD_SET(STDIN_FILENO, &readfds);

   struct timeval tv;
   tv.tv_sec = time_in_secs;
   tv.tv_usec = 0;

   int ret = select(STDIN_FILENO + 1, &readfds, NULL, NULL, &tv);

   if (!ret)
   {
       std::cout << "Timeout\n";
       exit(1);
   }

   char buf[buffer_size];

   if (FD_ISSET(STDIN_FILENO, &readfds))
   {
        int len = read(STDIN_FILENO, buf, buffer_size);
        buf[len] = '\0';
   }

    std::cout << "You typed: " << buf << "\n";

    return 0;
}

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