Get User Input without Blocking Endless loop

Question

I have written a simple C program that basically consists of an endless loop that counts upwards. During the loop, the user is asked for input- and here comes the tricky part: the loop should NOT be blocked while waiting on the user, but display his input as soon as he entered it:

int main(void){
  int i;
  char dec;

  for(;;i++){
    printf("%d\n", i);
    sleep(5);
    if(i==4 || i==8){
        printf("Please enter Y or N\n");
        dec = fgetc(stdin);
        printf("%c\n", dec);
    }
  }

return 0;
}

I found a similiar question for Python here Python. So do I need to push the user interaction into a new thread with pthread or is there an easier option?

Thanks!

EDIT

int main(void){
int i=0;
char dec;

fd_set input_set;

for(;;i++){
    printf("%d\n", i);
    sleep(2);
    if(i==4 || i==8){

        FD_ZERO(&input_set );   /* Empty the FD Set */
        FD_SET(0, &input_set);  /* Listen to the input descriptor */
        dec = select(1, &input_set, NULL, NULL, 0);

    }
}

return 0;
}

What operating system do you use? – nrofis Oct 14 '14 at 21:03 — nrofis, Oct 14 '14 at 21:03

score 1 · Answer 1 · answered Oct 14 '14 at 20:16

What you want to do is only possible with system dependent libraries. For instance on Unix you would typically use ncurses to get from the user if they have pressed a button.

The reason it is system dependent is that asynchronous IO is not available for all file system streams. In particular User I/O blocks and that block is unavoidable.

If you are committed to having a multi-threaded program that still uses read/write system calls you would need to have two threads, one for I/O and one for everything else. On the everything else thread you could query some shared memory area and see if the I/O thread has written the correct type of data to this shared memory area.

On linux see curs_getch man page. This is the one line modification to make your program work the way your example works. Sorry for not making that clearer with the answer above. — Clarus, Oct 14 '14 at 22:55

score 1 · Answer 2 · edited May 23 '17 at 11:49

1

If you are on linux only, check out this SO post : What are the differences between poll and select?

If you are on both and/or you already have pthreads, then use a separate thread.

edited May 23 '17 at 11:49

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answered Oct 14 '14 at 20:26

Thomas

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+1 Thanks for mentioning select! I looked into select and updated my question. Did you ever work with select? It also blocks the loop while waiting for the input... – user4122787 Oct 14 '14 at 21:00

score 0 · Answer 3 · edited May 23 '17 at 12:11

0

If you are using Windows, maybe you can try to use keyboard hooks. See SetWindowsHookEx. It will capture all the keyboard clicks with callback.

If you are usingLinux, maybe you can use this: Non-blocking keyboard read - C/C++

edited May 23 '17 at 12:11

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answered Oct 14 '14 at 20:27

nrofis

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Get User Input without Blocking Endless loop

3 Answers3