Concurrency Between Parent & Child code in operating system?

Concurrency Between Parent & Child
 

#include <unistd.h>

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

int main() {
  int ret, i, max = 10, status;
  printf("welcome..pid=%d\n", getpid());
  ret = fork();
  if (ret == 0) {
    printf("child--welcome,pid=%d,ppid=%d\n", getpid(), getppid());
    for (i = 1; i <= max; i++) {
      printf("child--%d\n", i);
      sleep(1);
    }
    exit(0);
  } else // ret>0
  {
    printf("parent--hello,pid=%d,ppid=%d\n", getpid(), getppid());
    for (i = 1; i <= max; i++) {
      printf("parent--%d\n", i);
      sleep(1);
    }
    waitpid(-1, &status, 0);
  }
  // printf("thank you,pid=%d,ppid=%d\n",
  //		getpid(),getppid());
  return 0;
}