counter.cpp

#include <cstdio>   // for printf()
#include <unistd.h> // for sleep()

int number = 0;

void setNumber() {
  printf("%d\n", number);
}

void loop()  {
   setNumber();
   sleep(1);
   number = (number + 1) % 10;
};

int main(void) {
  while (1) {
    loop();
  }
}