1 00:00:00,002 --> 00:00:05,002 (upbeat music begins) 2 00:00:05,002 --> 00:00:07,000 - [Instructor] Some fun with Dev Null, 3 00:00:07,000 --> 00:00:10,002 everybody's favorite device file, I think. 4 00:00:10,002 --> 00:00:14,006 Let's start out by looking at the file dev null 5 00:00:14,006 --> 00:00:18,006 and finding the major number, and it's number one. 6 00:00:18,006 --> 00:00:21,003 All righty, it's also minor number three, 7 00:00:21,003 --> 00:00:26,006 but let's see all the other device files, major number one. 8 00:00:26,006 --> 00:00:31,009 So let's look in Dev 9 00:00:31,009 --> 00:00:39,003 and we're going to grep space one comma space. 10 00:00:39,003 --> 00:00:41,004 And we check to make sure we're looking at 11 00:00:41,004 --> 00:00:44,003 only character device files. 12 00:00:44,003 --> 00:00:47,001 There's any B's in there, they could also be a one, 13 00:00:47,001 --> 00:00:48,009 but they would be a block driver, 14 00:00:48,009 --> 00:00:49,009 not a character driver. 15 00:00:49,009 --> 00:00:51,005 So that'd be a different device driver. 16 00:00:51,005 --> 00:00:54,003 But all these are C, they're all one. 17 00:00:54,003 --> 00:00:56,008 So these are all implemented, 18 00:00:56,008 --> 00:01:00,002 turns out, by the same driver. 19 00:01:00,002 --> 00:01:03,001 Now, for other sorts of major minor numbers, 20 00:01:03,001 --> 00:01:05,009 it might be possible that different minor numbers 21 00:01:05,009 --> 00:01:07,003 are implemented by different drivers, 22 00:01:07,003 --> 00:01:09,003 but for number one, it's the same driver. 23 00:01:09,003 --> 00:01:11,009 It's the memory driver. 24 00:01:11,009 --> 00:01:15,001 And notice that dev null 25 00:01:15,001 --> 00:01:16,009 everybody has write access to. 26 00:01:16,009 --> 00:01:18,009 Makes sense, right? 27 00:01:18,009 --> 00:01:20,003 Because you're always doing greater than dev null 28 00:01:20,003 --> 00:01:21,006 to throw away output. 29 00:01:21,006 --> 00:01:25,000 Some others have write access. 30 00:01:25,000 --> 00:01:28,006 Dev full, dev zero. 31 00:01:28,006 --> 00:01:32,005 Dev zero is very common to read binary zeros, 32 00:01:32,005 --> 00:01:34,002 as many as you want. 33 00:01:34,002 --> 00:01:35,009 It's interesting, you're going to write into dev zero. 34 00:01:35,009 --> 00:01:38,004 What happens when you write into dev zero? 35 00:01:38,004 --> 00:01:40,007 I'll let you figure that out. 36 00:01:40,007 --> 00:01:45,003 Now we want to look at what happens 37 00:01:45,003 --> 00:01:48,003 if we try to read from Dev Null. 38 00:01:48,003 --> 00:01:57,005 So let's strace catting Dev Null. 39 00:01:57,005 --> 00:02:00,009 And let's look at the trace file. 40 00:02:00,009 --> 00:02:08,003 Let's look for the open for Dev Null. 41 00:02:08,003 --> 00:02:11,001 So we see the open right there. 42 00:02:11,001 --> 00:02:12,005 That works. 43 00:02:12,005 --> 00:02:14,002 We get back file descriptor number three. 44 00:02:14,002 --> 00:02:15,005 That's what that three is. 45 00:02:15,005 --> 00:02:17,009 So when we read, we read from three, 46 00:02:17,009 --> 00:02:19,002 we go down just a few lines. 47 00:02:19,002 --> 00:02:22,003 There's where we try to read from three, 48 00:02:22,003 --> 00:02:24,008 that would be a buffer of where to put it. 49 00:02:24,008 --> 00:02:27,009 It actually tries to read 131,000 characters, 50 00:02:27,009 --> 00:02:31,000 says, "Gimme 131,000 characters if you have 'em." 51 00:02:31,000 --> 00:02:34,001 And the driver says you get zero, 52 00:02:34,001 --> 00:02:37,000 I have no characters for you. 53 00:02:37,000 --> 00:02:38,006 That means end of file. 54 00:02:38,006 --> 00:02:41,000 It's fine to try to read from Dev Null. 55 00:02:41,000 --> 00:02:42,008 You just get end of file. 56 00:02:42,008 --> 00:02:44,009 It never gives you any characters. 57 00:02:44,009 --> 00:02:47,000 Kind of interesting, I think.