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- [Instructor] Device files

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are another important way

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to talk to the kernel via user space.

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In particular, to talk to device drivers

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for block devices and character devices.

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So things like your keyboard, say

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or your disc,

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but not things like a network driver,

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usually, not the usual network sort of stuff you do

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with a network driver.

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Device files are special kind of files

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and they have a type, they're either a C

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or a B, a character or a block.

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And the LS minus L command will show that CRB

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and then instead of a size

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LS will put what's known as the major number

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and the minor number.

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And the kernel maintains a relationship

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between the major number and the minor number

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and the type

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to know which device driver to call.

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And classically,

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each device driver had its own major number,

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but there's way too many device drivers these days.

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So it's really the combination

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of the major number and the minor number

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that identifies which driver.

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And a driver can have multiple minor numbers.

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Oftentimes differing minor numbers would be used to

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distinguish between multiple instances of a device,

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for example.

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For a disc, the minor number can identify which partition.

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So a device file for a disc

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might have the same major number

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for all the discs and all the partitions,

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but the minor number would say which disc

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and which partition.

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Device files are in /dev.

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And /dev usually is

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a directory that gets created at boot time

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and then populated.

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The startup procedures for Linux

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probably create some of the classic device files,

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but for drivers you might not have a device file

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until the system discovers a device

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and then those device files get created automatically.

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Now, let's take a look at one example.

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I did a long list on dev/sda1.

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We see the first character is a B.

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It's a block device file.

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There's only one link to it.

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It's owned by root.

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The group is disc, but then we see eight, one.

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That's the major and minor number.

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So eight is the SCSI device driver

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and one is actually saying first disc, first partition.

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Now, we've got date and time information.

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Then we have the path dev/sda1.

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So that eight, one is what's important.

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If we create a device file with a different name,

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we called it, "Hello there, I'm a disc,"

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and we made it a block device file eight, one,

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it would act the same as dev/sda1.

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Device files don't have to be in dev, by the way,

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you could put one in your home directory

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as long as that's allowed for your file system.

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But that would be kind of weird.

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So device drivers are associated

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with these device files, right?

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The device file is saying which driver to call.

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So when you open up a device file,

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the kernel figures out what open function to call,

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and a device driver registers its sets of functions

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with the kernel and says, "I'm driver number 57.

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If somebody opens device file with a major number, 57,

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call my open," sort of thing.

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So there's a whole set of functions

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that drivers can implement.

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Character drivers have one, sort of, set of functions.

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Block drivers have a different, sort of, set of functions.

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And when we use a device file,

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like if we say echo high greater than dev, null,

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the shell's going to open up dev/null.

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Dev/null is a device file.

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There's a device driver associated with it.

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It's called the memory driver.

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So the open for that would be called.

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And then the gridder then,

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says, "We're going to do a right,"

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and then the right function for that driver would be called.

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Let's look at a few challenges

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having to do with device files.