How to deactivate input statement after some time?

Question

We know input function or operator (cin, scanf,gets….etc) wait to take input form user & this time has no limit.

Now, I will ask a question & user give the a

Answer 1

Another Method:
You can use POSIX select() function (and some macros FD_ZERO, FD_SET, FD_ISSET) to check which file descriptors (descriptor number 0 i.e. stdin, in this case) are ready to be read in a given time interval. When they are ready, use appropriate function to read the data (scanf() in this case).
This code might help you understand, what I want to say:

#include 
#include 
#include 

#define STDIN    0    // Standard Input File Descriptor
int main()
{
    fd_set input;       // declare a "file descriptor set" to hold all file descriptors you want to check
    int fds, ret_val, num;  // fds: Number of file descriptors;

    struct timeval tv;      // structure to store Timeout value in the format used by select() function
    unsigned int timeout = 5;   // Your timeout period in seconds

    tv.tv_sec = timeout;    
    tv.tv_usec = 0;

    fds = STDIN + 1;            // Set number of file decriptors to "1 more than the greatest file descriptor"
                // Here, we are using only stdin which is equal to 0

    FD_ZERO(&input);        // Initialize the set with 0
    FD_SET(STDIN, &input);      // Add STDIN to set

    printf("Enter a number within %d secs\n", timeout);
    ret_val = select(fds, &input, NULL, NULL, &tv); 
                // We need to call select only for monitoring the "input file descriptor set"
                // Pass rest of them as NULL

    if (ret_val == -1)          // Some error occured
        perror("select()");
    else if (ret_val > 0)       // At least one of the file descriptor is ready to be read
    {
//      printf("Data is available now.\n");
        if(FD_ISSET(0, &input))     // Check if stdin is set, here its not necessary as we are using STDIN only
                // So ret_val>0 means STDIN is raedy to read 
        {
            scanf("%d", &num);
        }
    }
    else
        printf("No data within five seconds.\n");   // select returns zero on timeout

    return 0;
}

More Help: select(2)

You can also try using poll() function available in (again a POSIX standard function) as an alternative to select(). See poll() & poll(2)