pthread_attr_setscope 함수 관련 질문

#include <pthread.h>
#include <string.h>
#include <unistd.h>

pthread_mutex_t mutex_lock   = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t   thread_cond  = PTHREAD_COND_INITIALIZER;
pthread_cond_t   thread_cond2  = PTHREAD_COND_INITIALIZER;

struct com_data
{
    int a;
    int b;
};

struct com_data mydata;
int sig_num;

void *do_write(void *data)
{
    int loop = 0;
    mydata.a = 0;
    mydata.b = 0;
    while(loop < 10)
    {
        pthread_mutex_lock(&mutex_lock);
        mydata.a = random() % 6000;
        mydata.b = random() % 6000;
        pthread_cond_signal(&thread_cond);
        pthread_mutex_unlock(&mutex_lock);
        loop++;
        sleep(1);
    }
    printf("sig is %d
", sig_num);
    exit(0);
}

void *do_read(void *data)
{
    sig_num = 0;
    while(1)
    {
        pthread_mutex_lock(&mutex_lock);
        pthread_cond_wait(&thread_cond, &mutex_lock);
        printf("%4d + %4d = %4d
", mydata.a, mydata.b, mydata.a + mydata.b); 
        sig_num++;    
        pthread_mutex_unlock(&mutex_lock);
    }
}

int main()
{
    pthread_t p_thread[2];
    pthread_attr_t myattr;
    pthread_attr_t myattr2;

    int thr_id;
    int status;
    int a = 1;
    int b = 2;

    int scope;

    pthread_attr_init(&myattr);
    pthread_attr_init(&myattr2);
    pthread_attr_setscope(&myattr, PTHREAD_SCOPE_PROCESS);
    pthread_attr_setscope(&myattr2, PTHREAD_SCOPE_SYSTEM);

    status = pthread_attr_getscope(&myattr, &scope);
    printf("PROCESS : %d, %d, %d
", scope, PTHREAD_SCOPE_PROCESS, status);

    status = pthread_attr_getscope(&myattr2, &scope);
    printf("SYSTEM : %d, %d, %d
", scope, PTHREAD_SCOPE_SYSTEM, status);

    thr_id = pthread_create(&p_thread[0], &myattr, do_write, (void *)&a); 
    thr_id = pthread_create(&p_thread[1], &myattr2, do_read, (void *)&b);

    pthread_join(p_thread[0], (void **) status);
    pthread_join(p_thread[1], (void **) status);

    return 0;
}

리눅스는 clone 시스템 콜을 통한 kernel 쓰레드로 알고 있습니다.

헌데...

pthread_attr_setscope(&myattr, PTHREAD_SCOPE_PROCESS);

이렇게.. User_level_thread 로 설정되도 돌아가는 이유는.. 멀까요? ㅎ