time.sleep() is busy wait?

[techie1991]

I was looking around the time module. The time.sleep(no_of_seconds) sleeps the program for no_of_seconds seconds.
I don't think this is busy wait. It must be scheduling the process at a later time. Just curious of how this is working.! :)

[Gribouillis]

You can look in the source code of timemodule.c in the python source. In linux, this is done by calling select() with NULL arguments.

[techie1991]

It would be great if someone already knowing about it could give me a better answer. I always try to get my answers from the source code and am left dazzled by the code in C. (last time I was trying to see how long was implemented)

[Gribouillis]

It would be great if someone already knowing about it could give me a better answer. I always try to get my answers from the source code and am left dazzled by the code in C. (last time I was trying to see how long was implemented)

Well, here is the C function that does the job (from timemodule.c in python 3.1.1)

static int
floatsleep(double secs)
{
/* XXX Should test for MS_WINDOWS first! */
#if defined(HAVE_SELECT) && !defined(__EMX__)
	struct timeval t;
	double frac;
	frac = fmod(secs, 1.0);
	secs = floor(secs);
	t.tv_sec = (long)secs;
	t.tv_usec = (long)(frac*1000000.0);
	Py_BEGIN_ALLOW_THREADS
	if (select(0, (fd_set *)0, (fd_set *)0, (fd_set *)0, &t) != 0) {
#ifdef EINTR
		if (errno != EINTR) {
#else
		if (1) {
#endif
			Py_BLOCK_THREADS
			PyErr_SetFromErrno(PyExc_IOError);
			return -1;
		}
	}
	Py_END_ALLOW_THREADS
#elif defined(__WATCOMC__) && !defined(__QNX__)
	/* XXX Can't interrupt this sleep */
	Py_BEGIN_ALLOW_THREADS
	delay((int)(secs * 1000 + 0.5));  /* delay() uses milliseconds */
	Py_END_ALLOW_THREADS
#elif defined(MS_WINDOWS)
	{
		double millisecs = secs * 1000.0;
		unsigned long ul_millis;

		if (millisecs > (double)ULONG_MAX) {
			PyErr_SetString(PyExc_OverflowError,
					"sleep length is too large");
			return -1;
		}
		Py_BEGIN_ALLOW_THREADS
		/* Allow sleep(0) to maintain win32 semantics, and as decreed
		 * by Guido, only the main thread can be interrupted.
		 */
		ul_millis = (unsigned long)millisecs;
		if (ul_millis == 0 ||
		    main_thread != PyThread_get_thread_ident())
			Sleep(ul_millis);
		else {
			DWORD rc;
			ResetEvent(hInterruptEvent);
			rc = WaitForSingleObject(hInterruptEvent, ul_millis);
			if (rc == WAIT_OBJECT_0) {
				/* Yield to make sure real Python signal
				 * handler called.
				 */
				Sleep(1);
				Py_BLOCK_THREADS
				errno = EINTR;
				PyErr_SetFromErrno(PyExc_IOError);
				return -1;
			}
		}
		Py_END_ALLOW_THREADS
	}
#elif defined(PYOS_OS2)
	/* This Sleep *IS* Interruptable by Exceptions */
	Py_BEGIN_ALLOW_THREADS
	if (DosSleep(secs * 1000) != NO_ERROR) {
		Py_BLOCK_THREADS
		PyErr_SetFromErrno(PyExc_IOError);
		return -1;
	}
	Py_END_ALLOW_THREADS
#elif defined(PLAN9)
	{
		double millisecs = secs * 1000.0;
		if (millisecs > (double)LONG_MAX) {
			PyErr_SetString(PyExc_OverflowError, "sleep length is too large");
			return -1;
		}
		/* This sleep *CAN BE* interrupted. */
		Py_BEGIN_ALLOW_THREADS
		if(sleep((long)millisecs) < 0){
			Py_BLOCK_THREADS
			PyErr_SetFromErrno(PyExc_IOError);
			return -1;
		}
		Py_END_ALLOW_THREADS
	}
#else
	/* XXX Can't interrupt this sleep */
	Py_BEGIN_ALLOW_THREADS
	sleep((int)secs);
	Py_END_ALLOW_THREADS
#endif

	return 0;
}

As you can see, the algorithm is to call a 'waiting' function in the underlying OS. If the system has a select function (GNU/linux for example), the main job is done by a call to C's select function select(0,0,0,0,&t); where t is a timeval structure defining a time interval. On the watcom compiler, a C function delay(milliseconds); does the job, in MS windows, a call WaitForSingleObject(hInterruptEvent, ul_millis); waits for a given number of milliseconds. In OS2 it is DosSleep(milliseconds) . In a plan9 and other systems, it is sleep(milliseconds or seconds) .

Now you only need to find the documentation for these low level C functions !

[techie1991]

Oh thanks, :)
Will take some time to see and understand the whole code. (Will need to remember the C things).