Interrupt cin while loop without the user entering input

5
votes

In main, I give the user the ability to enter a command to stop the application:

while( run_processes && cin >> command_line_input ){

I'd also like to stop the application elsewhere by setting run_processes = false;.

However, when I set run_processes to false, the above loop doesn't stop without the user entering input.

How can I properly interrupt the loop without the user entering input?

3
c++linuxwhile-loopinterruptcin
show more code : where and how exactly do you set it to false ? – quantdev
@quantdev In another boost thread. I apologize for leaving out a key detail, that I'd like the loop to stop without user input. I've confirmed that run_processes is properly being set. Does cin not necessarily depend upon user input for the while to proceed? Thank you so much in advance! – user1382306
If you create another thread which calls cin >> command_line_input (and then send the string to the main thread), then you can decide in the main thread not to wait for thread reading from cin. I don't think it's possible to abort the cin >> command_line_input call itself in a portable way. – pts

3 Answers

7
votes

It is not possible to interrupt std::cin in a portable way.

You could however resolve to non-portable solution, e.g. manually poll() the standard input on a UNIX system and check run_processes while polling.

0
votes

Does cin not necessarily depend upon user input for the while to proceed?

Yes, the user would have to enter input each loop.

bool stop = false;
std::string command;
int count = 0;
while(stop==false && std::cin>>command)
{
    ++count;
    std::cout<<count<<std::endl;
}

The code above only counts +1 after I enter something. Unless you are changing the value of run_process based on the user input I don't see why it shouldn't work...?

0
votes

I don't know if the following solution is standard enough, but it works on both my Fedora Linux (GCC 10.3) & Windows 10 (MSVC 16.11)

#include <iostream>
#include <csignal>

int main()
{
    std::signal(SIGINT, [] (int s)
    {
        std::signal(s, SIG_DFL);
        std::fclose(stdin);
        std::cin.setstate(std::ios::badbit); // To differenciate from EOF input
    });

    std::string input;
    while (std::getline(std::cin, input));

    std::cout << (std::cin.bad() ? "interrupted" : "eof") << std::endl;
}

Don't ask me how to reuse cin from that state now.