Thread: How to exit out of scanf after a certain amount of time

Hi all, I'm trying to make a very simple reaction game where a random number flickers on the screen for a fraction of a second and the user has to then enter the number before another comes on the screen after a random amount of time. However I dont know how i would make it so that the user cannot enter anything after a certain amount of time has passed, below is my code, could someone give me a hand? Also FYI, clock_start is at 5100 because by the time the program actually gets to scanning in the first number the time is at an absolute minimum of 5050 milliseconds however obviously this is an impossible number to reach due to processing, my machine clocks in at 5080.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <windows.h>


int main(void)
{
    int i;
    int random_number;
    int input, score = 0;
    int clock_start = 5100;
    
    system("cls");
    
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
    printf("\t\t\t       WELCOME TO REACTION\n");
    
    Sleep(2000);
    
    for(i=3;i>0;i--)
    {
        system("cls");
        printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
        printf("\t\t\t\t     %d", i);
    
        Sleep(1000);
    }
    
    srand(time(0));
    
    for(i=0;i<10;i++)
    {
        random_number = rand() % 1000;
        
        system("cls");
        printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
        printf("\t\t\t\t    %d\n", random_number);
        
        Sleep(50);


        system("cls");


        printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
        printf("\t\t\t\t    ");    


        clock();


        //this is the part im not getting
        ///////////////////
        if(clock() < clock_start + 3000)
        {    
            scanf("%d", &input);
        }
            
        else if(clock () > clock_start + 3000)
        {
            printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
            printf("\t\t\t\tTOO LATE!");
            
            Sleep(1000);
        }
        //////////////////////
        
        system("cls");
        
        Sleep(1000);
        
        if(input == random_number)
        {
            score++;
        }
    }
    
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
    printf("\t\t\t\tYour overall score is %d", score);
    printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
}

There is no way in standard C to force scanf() to time out, just as it is not possible in standard C to read keystrokes directly (without being forced to wait for the user to hit the enter key).

Given that you are using non-standard functions (Sleep() is win32 API specific, not standard C) I assume a windows-specific solution would suffice.

Have a look at this link. The windows-specific solution there is not a complete solution to your problem, but could provide a starting point.

You probably want to look into curses (cross platform terminal library) or win32 api (Windows only). As mentioned curses is not standard but it is pretty ubiquitous. For example on Linux and MacOS there is Ncurses (maybe on iOS as well?). Pdcurses works well on Windows. With this library you get to implement delays, non-blocking input and the kinds of things you seem to be trying like clearing the screen and printing text on certain positions on the terminal. If you want to learn curses you can go here

NCURSES Programming HOWTO

And then read this (it is directed at Python audience but notice how the concepts and library functions are identical to the C library): https://docs.python.org/2/howto/curses.html

You could use _dup2() to create a copy of the stdin stream, then fire up a second thread to try to read from it. In the main thread, sleep the required time and then close the duplicated stream handle. This will cause the call to fread() in the other thread to return with EOF.

Originally Posted by megafiddle

I should correct what I said above:

"Something like getch() could be used"

getch() does block and wouldn't work by itself.

kbhit() does not block, and would work. It only checks whether or not a character was entered and returns immediately. You then use getch() to extract it.

Both are not standard, but if you really need them, include conio.h

-

You could poll kbhit() say 1000000 times (which would take some time) - And if it does not detect an input in that time, it has timed out.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

int getInput(unsigned int timeout)
{
    for (; timeout != 0; timeout--)
    {
        if (kbhit())
        {
            return getch();
        }
    }

    return EOF;
}
int main(void)
{
    int ch;

    if ((ch = getInput(100000)) != EOF)
    {
        printf("Input Found: %c", ch);
    }
    else
    {
        printf("No input found\n");
    }

    return EXIT_SUCCESS;
}