Take input in an infinte loop in c

Question

I am writing a C program to blink LED for Raspberry Pi. Its like (i) Blink LED (2) Stop blinking it.

Now while the LED is blinking, if I press 2 it should stop blinking. How to do it ?

If I include scanf inside the code will stop blinking.

while(1)
{
 printf("Enter a command \n 1.Blink Led\n 2. Stop blinking\n");
 scanf("%d",&choice);
if(choice==1)
    for (;;)
    {
        digitalWrite (LED, HIGH) ;  // On
        delay (500) ;               // mS
        digitalWrite (LED, LOW) ;   // Off
        delay (500) ;
        // If i press 2 the led should stop blinking
  }

else if(choice==2){
 digitalWrite (LED, LOW) ;
}

Two things: You need to check for the `2` key *in* the blinking loop. And you need to find a way to poll for keyboard input *without* blocking (which the `scanf` does by default). — Some programmer dude, Jul 06 '17 at 05:30
Unclear question. You might want to use [poll(2)](http://man7.org/linux/man-pages/man2/poll.2.html) in some [*event loop*](https://en.wikipedia.org/wiki/Event_loop) — Basile Starynkevitch, Jul 06 '17 at 05:32
@Someprogrammerdude ya that is what I m looking for. I was unaware of that polling. Thanks I will find out. — Aditya Jha, Jul 06 '17 at 05:35
Here are some links you can check: https://stackoverflow.com/questions/717572/how-do-you-do-non-blocking-console-i-o-on-linux-in-c or https://stackoverflow.com/questions/37292474/how-to-do-nonblocking-input-from-stdin-in-c or https://stackoverflow.com/questions/448944/c-non-blocking-keyboard-input — Support Ukraine, Jul 06 '17 at 05:36

I.AMIR · Accepted Answer · 2017-07-06T06:19:53.110

0

if you don't want to interrupt the Blinking you should try to run the blinking in a parallel process using pthread_t. i really want to give you the code but i'm not on my PC at the moment.

edited Jul 06 '17 at 06:19

answered Jul 06 '17 at 06:07

I.AMIR

31
1

i forgot NOT . thnx – I.AMIR Jul 06 '17 at 06:20

score 0 · Answer 2 · answered Jul 06 '17 at 09:58

0

Use poll:

#include <stdio.h>
#include <unistd.h>
#include <poll.h>
int main()
{
    struct pollfd mypoll = { STDIN_FILENO, POLLIN|POLLPRI };
    unsigned char c, blink=1, on =1, q=1;
    while(q) {
      if( poll(&mypoll, 1, 500) ) {
        c=getchar();
        switch(c) {
          case '1':
              blink = 1;
              break;
          case '2':
              blink =0;
              break;
          case 'q':
              q= 0;
              break;
       }
   }
   else{
    if (blink) {
       if (on)
          on =0;
       else 
         on =1;
     }
     printf("%u\n",on);
    }
  } 
  return 0;
}

replace the on parameter with the led functions.

answered Jul 06 '17 at 09:58

You can replace `if(on)on=0; else on=1;}` with `on=1-on` :-) – Mark Setchell Jul 07 '17 at 10:03
@Mark Setchell or with on = 1^on , but will any of these make the code easier to read ? – Jul 07 '17 at 17:28
It was just a fun observation reminding me of a real-time system I once worked on which was double-buffered between buffer `0` and `1`, and the timing was so critical that avoiding the CPU-stall caused by the `if` test made all the difference :-) – Mark Setchell Jul 07 '17 at 17:59

Take input in an infinte loop in c

2 Answers2