使用std :: cin语句自动超时 [英] making auto timeout with std::cin statement

问题描述

我写的程序

#include<iostream>
using namespace std;
int n;
int main(int argc, char *argv[])
{
  std::cout << "Before reading from cin" << std::endl;

   // Below reading from cin should be executed within stipulated time
   bool b=std::cin >> n;
   if (b)
      std::cout << "input is integer for n and it's correct" << std::endl;
   else
      std::cout << "Either n is not integer or no input for n" << std::endl;
  return 0;
}

这里std :: cin语句会等待控制台输入，

Here std::cin statement would wait for the console for input and will go for sleep mode until we provide some input and press Enter.

我希望std :: cin语句在10秒钟后超时（如果用户没有输入任何数据10秒之后，编译器将开始执行std :: cin语句下面的程序的下一条语句。

I want std::cin statement to go for timeout after 10 seconds(if user doesn't enter any data between 10 seconds then compiler would start executing the next statement of the program present below std::cin statement.

我可以使用多线程机制来解决它，下面是我的代码：

I am able to solve it using multithreading mechanism. Below is my code:

#include<unistd.h>
#include<stdlib.h>
#include<pthread.h>

#include<iostream>
using namespace std;
void *thread_function(void *arg);
int input_value;

int main(int argc, char *argv[])
{
    int res;
    pthread_t a_thread;
    void *thread_result;
    res=pthread_create(&a_thread,NULL,thread_function,NULL);
    if(res!=0){
            perror("Thread creation error");
            exit(EXIT_FAILURE);
    }
    //sleep(10);
    cout<<"cancelling thread"<<endl;
    res=pthread_cancel(a_thread);

    cout<<"input value="<<input_value<<endl;
    exit(EXIT_SUCCESS);
 }
 void *thread_function(void *arg)
 {
    int res;
    res=pthread_setcancelstate(PTHREAD_CANCEL_ENABLE,NULL);
    if(res!=0){
            perror("Unable to set pthread to cancel enbable state");
            exit(EXIT_FAILURE);
    }
    cin>>input_value;
    pthread_exit(&input_value);
 }

但这里我面临一个问题。由于睡眠功能，用户输入值或不睡眠功能默认睡眠10秒。这是我滞后的地方。

But here I am facing a problem. Due to the sleep function either user inputs value or not the sleep function by default sleeps for 10 seconds. This is where I am lagging.

如何解决这个问题，如使用（信号，二进制信号量等）。

How do I solve this problem like using(signals,binary semaphores etc..). Please relate your answer w.r.t my solution(i.e multithreading).

任何信息最受欢迎...

Any information is most welcome...

推荐答案

由于你使用POSIX机器，你可以使用eg 选择 ，以检查是否有任何内容对标准输入：

Since you're on a POSIX machine, you can use e.g. select to check if there's anything on the standard input:

fd_set fds;
FD_ZERO(&fds);
FD_SET(STDIN_FILENO, &fds)

timeval timeout;
timeout.tv_sec = 5;   // A five-second timeout
timeout.tv_usec = 0;

int rc = select(STDIN_FILENO + 1, &fds, nullptr, nullptr, &timeout);
if (rc < 0)
    perror("select");
else if (rc == 0)
{
    // Timeout
}
else
{
    // There is input to be read on standard input
}

---

poll （根据Basile Starynkevitch的建议），可以这样做：

---

Using poll (as suggested by Basile Starynkevitch) it could be done something like this:

struct pollfd poller;
poller.fd = STDIN_FILENO;
poller.events = POLLIN;
poller.revents = 0;

int rc = poll(&poller, 1, 5);  // Poll one descriptor with a five second timeout
if (rc < 0)
    perror("select");
else if (rc == 0)
{
    // Timeout
}
else
{
    // There is input to be read on standard input
}

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