How does printf() works as a safe cancel point? What is it depends on?

I have this little program that is supposed to create a thread and cancel it.

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

void *thread_function(void *arg)
{
    while(1)
    {
        printf("hello \n"); 
        //sleep(1);
    }
    return NULL;
}

int main()
{
    pthread_t thread;

    if(pthread_create(&thread, NULL, thread_function, NULL) != 0)
    {
        perror("Failed to create a thread");
        return -1;
    }

    printf("After sleep in main thread\n");
    if (pthread_cancel(thread) != 0) {
        perror("Failed to cancel thread");
        return EXIT_FAILURE;
    }


    pthread_join(thread, NULL);
    printf("Thread is cancelled\n");
}

If I use sleep() in thread function, created thread is canceled almost immediately, but if I use printf(), it takes some time to cancel it. In man pthreads I read that sleep() is a safe cancel point while printf() may be a safe cancel point.

How does it work? I mean in what period of time that programme works printf() turns into a cancel point and my thread is being canceled?

And also if I run the programm (also using printf() in thread function) with time command, it can show that programm worked for 2 sec but in reality it worked longer. why?

PS: sorry for the mistakes, English is not my first language.

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