一、sleep 和 usleep
1、不属于系统调用，是glibc 库函数实现的；
2、glibc函数库中通过调用内核的nanosleep实现的；
3、内核nanosleep通过调用 hrtimer_nanosleep 实现（/kernel/hrtime）；
 
二、源码
2.1 usleep  （glibc2.11）

int usleep (useconds_t useconds)
{
  struct timespec ts = { .tv_sec = (long int) (useconds / 1000000),
		.tv_nsec = (long int) (useconds % 1000000) * 1000ul };
  /* Note the usleep() is a cancellation point.  But since we call
     nanosleep() which itself is a cancellation point we do not have
     to do anything here.  */
  return __nanosleep (&ts, NULL);
}

2.2 sleep源码

/* We are going to use the `nanosleep' syscall of the kernel.  But the
   kernel does not implement the stupid SysV SIGCHLD vs. SIG_IGN
   behaviour for this syscall.  Therefore we have to emulate it here.  */
unsigned int
__sleep (unsigned int seconds)
{
  const unsigned int max
    = (unsigned int) (((unsigned long int) (~((time_t) 0))) >> 1);
  struct timespec ts;
  sigset_t set, oset;
  unsigned int result;

  /* This is not necessary but some buggy programs depend on this.  */
  if (__builtin_expect (seconds == 0, 0))
    {
#ifdef CANCELLATION_P
      CANCELLATION_P (THREAD_SELF);
#endif
      return 0;
    }

  ts.tv_sec = 0;
  ts.tv_nsec = 0;
 again:
  if (sizeof (ts.tv_sec) <= sizeof (seconds))
    {
      /* Since SECONDS is unsigned assigning the value to .tv_sec can
	 overflow it.  In this case we have to wait in steps.  */
      ts.tv_sec += MIN (seconds, max);
      seconds -= (unsigned int) ts.tv_sec;
    }
  else
    {
      ts.tv_sec = (time_t) seconds;
      seconds = 0;
    }

  /* Linux will wake up the system call, nanosleep, when SIGCHLD
     arrives even if SIGCHLD is ignored.  We have to deal with it
     in libc.  We block SIGCHLD first.  */
  __sigemptyset (&set);
  __sigaddset (&set, SIGCHLD);
  if (__sigprocmask (SIG_BLOCK, &set, &oset))
    return -1;

  /* If SIGCHLD is already blocked, we don't have to do anything.  */
  if (!__sigismember (&oset, SIGCHLD))
    {
      int saved_errno;
      struct sigaction oact;

      __sigemptyset (&set);
      __sigaddset (&set, SIGCHLD);

      /* We get the signal handler for SIGCHLD.  */
      if (__sigaction (SIGCHLD, (struct sigaction *) NULL, &oact) < 0)
	{
	  saved_errno = errno;
	  /* Restore the original signal mask.  */
	  (void) __sigprocmask (SIG_SETMASK, &oset, (sigset_t *) NULL);
	  __set_errno (saved_errno);
	  return -1;
	}

      /* Note the sleep() is a cancellation point.  But since we call
	 nanosleep() which itself is a cancellation point we do not
	 have to do anything here.  */
      if (oact.sa_handler == SIG_IGN)
	{
	  //__libc_cleanup_push (cl, &oset);

	  /* We should leave SIGCHLD blocked.  */
	  while (1)
	    {
	      result = __nanosleep (&ts, &ts);

	      if (result != 0 || seconds == 0)
		break;

	      if (sizeof (ts.tv_sec) <= sizeof (seconds))
		{
		  ts.tv_sec = MIN (seconds, max);
		  seconds -= (unsigned int) ts.tv_nsec;
		}
	    }

	  //__libc_cleanup_pop (0);

	  saved_errno = errno;
	  /* Restore the original signal mask.  */
	  (void) __sigprocmask (SIG_SETMASK, &oset, (sigset_t *) NULL);
	  __set_errno (saved_errno);

	  goto out;
	}

      /* We should unblock SIGCHLD.  Restore the original signal mask.  */
      (void) __sigprocmask (SIG_SETMASK, &oset, (sigset_t *) NULL);
    }

  result = __nanosleep (&ts, &ts);
  if (result == 0 && seconds != 0)
    goto again;

 out:
  if (result != 0)
    /* Round remaining time.  */
    result = seconds + (unsigned int) ts.tv_sec + (ts.tv_nsec >= 500000000L);

  return result;
}

2.3 内核nanosleep源码

long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,const enum hrtimer_mode mode, const clockid_t clockid)
{
	struct restart_block *restart;
	struct hrtimer_sleeper t;
	int ret = 0;
	unsigned long slack;
	slack = current->timer_slack_ns;
	if (rt_task(current))
		slack = 0;
	hrtimer_init_on_stack(&t.timer, clockid, mode);
	hrtimer_set_expires_range_ns(&t.timer, timespec_to_ktime(*rqtp), slack);
	if (do_nanosleep(&t, mode))
		goto out;
	/* Absolute timers do not update the rmtp value and restart: */
	if (mode == HRTIMER_MODE_ABS) {
		ret = -ERESTARTNOHAND;
		goto out;
	}
	if (rmtp) {
		ret = update_rmtp(&t.timer, rmtp);
		if (ret <= 0)
			goto out;
	}
	restart = &current_thread_info()->restart_block;
	restart->fn = hrtimer_nanosleep_restart;
	restart->nanosleep.clockid = t.timer.base->clockid;
	restart->nanosleep.rmtp = rmtp;
	restart->nanosleep.expires = hrtimer_get_expires_tv64(&t.timer);
	ret = -ERESTART_RESTARTBLOCK;
out:
	destroy_hrtimer_on_stack(&t.timer);
	return ret;
}
SYSCALL_DEFINE2(nanosleep, struct timespec __user *, rqtp,
		struct timespec __user *, rmtp)
{
	struct timespec tu;
	if (copy_from_user(&tu, rqtp, sizeof(tu)))
		return -EFAULT;
	if (!timespec_valid(&tu))
		return -EINVAL;

	return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);