리눅스에서 쓰레드 시작한지 1시간 된 초보입니다.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
 
void *thread_function(void *arg);
 
char message[]="Hello World";
int run_now =1;
int main(){
        int res;
        pthread_t a_thread;
        void *thread_result;
 
        res= pthread_create(&a_thread, NULL, thread_function, (void *)message);
 
        if(res != 0) {
                perror("Thread creation failed");
                exit(EXIT_FAILURE);
        }
 
        int print_count1 = 0;
 
        while(print_count1++ <20) {
                if(run_now == 1){
                        printf("1");
                        run_now=2;
                }
                else{
                        sleep(1);
                }
        }
        printf("Waiting for thread to finish...\n");
        res = pthread_join(a_thread, &thread_result);
        if(res != 0){
                perror("Thread join failed");
                exit(EXIT_FAILURE);
        }
 
        printf("Thread joined\n");
        exit(EXIT_SUCCESS);
}
 
void *thread_function(void *arg){
        int print_count2 = 0;
        while (print_count2++ <20){
                if(run_now ==2){
                        printf("2");
                        run_now =1;
                }
                else{
                        sleep(1);
                }
 
        }
}