스레드 프로그래밍 관련해서 질문입니다

#include <pthread.h>
#include <stdio.h>
 
struct char_print_parms
{
        char    character;
        int     count;
}
 
void *char_print(void *parameters)
{
        struct char_print_parms *p = (struct char_print_parms*)parameters;
        int i;
        for(i=0;i<p->count;++i)
                fputc(p->character, stderr);
        return NULL;
}
 
int main()
{
        pthread_t thread1_id;
        pthread_t thread2_id;
 
        struct char_print_parms thread1_args;
        struct char_print_parms thread2_args;
 
        thread1_args.character  = 'x';
        thread1_args.count      = 30000;
        pthread_create(&thread1_id, NULL, &char_print, &thread1_args);
 
        thread2_args.character  = 'o';
        thread2_args.count      = 20000;
        pthread_create(&thread2_id, NULL, &char_print, &thread2_args);
 
        pthread_join(thread1_id, NULL);
        pthread_join(thread2_id, NULL);
 
        return 0;
}