Is it possible to set timeout for std::cin?

Question

Is it possible to set timeout for std::cin? For example, std::cin doesn't receive any data during 10 seconds - it throws an exception or returns an error.

Edited:

And what about timer from Boost library? As far as I know, it is portable library. Is it possible to ask timer of Boost library to throw exceptions after predefined period of time? I guess it can solve this problem.

Answer 1

It isn't possible to set a time out for std::cin in a portable way. Even when resorting to non-portable techniques, it isn't entirely trivial to do so: you will need to replace std::cin's stream buffer.

On a UNIX system I would replace the default stream buffer used by std::cin by a custom one which uses file descriptor 0 to read the input. To actually read the input I would use poll() to detect presence of input and set a timeout on this function. Depending on the result of poll() I would either read the available input or fail. To possibly cope with typed characters which aren't forwarded to the file descriptor, yet, it may be reasonable to also turn off the buffering done until a newline is entered.

When using multiple threads you can create a portable filtering stream buffer which uses on thread to read the actual data and another thread to use a timed condition variable waiting either for the first thread to signal that it received data or for the time out to expire. Note that you need to guard against spurious wake-ups to make sure that the timeout is indeed reached when there is no input. This would avoid having to tinker with the actual way data is read from std::cin although it still replaces the stream buffer used by std::cin to make the functionality accessible via this name.

Answer 2

I just figured out how to do that, polling the std::cin file descriptor.

poll function returns 0 if timeout occurs and no event happened, 1 if something happened, and -1 if error happened.

#include <iostream>

#include <signal.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <poll.h>


bool stop = false;

void intHandler(int dummy)
{
    stop = true;
}

std::string readStdIn()
{
    struct pollfd pfd = { STDIN_FILENO, POLLIN, 0 };

    std::string line;
    int ret = 0;
    while(ret == 0)
    {
        ret = poll(&pfd, 1, 1000);  // timeout of 1000ms
        if(ret == 1) // there is something to read
        {
            std::getline(std::cin, line);
        }
        else if(ret == -1)
        {
            std::cout << "Error: " << strerror(errno) << std::endl;
        }
    }
    return line;
}

int main(int argc, char * argv[])
{
    signal(SIGINT, intHandler);
    signal(SIGKILL, intHandler);

    while(!stop)
    {
        std::string line = readStdIn();
        std::cout << "Read: " << line << std::endl;
    }
    std::cout << "gracefully shutdown" << std::endl;
}