Книга: Embedded Linux Primer: A Practical, Real-World Approach

8.3.2. Device Nodes and mknod

8.3.2. Device Nodes and mknod

To understand how an application binds its requests to a specific device represented by our device driver, we must understand the concept of a device node. A device node is a special file type in Linux that represents a device. Virtually all Linux distributions keep device nodes in a common location (specified by the Filesystem Hierarchy Standard[66]), in a directory called /dev. A dedicated utility is used to create a device node on a file system. This utility is called mknod.

An example of node creation is the best way to illustrate its functionality and the information it conveys. In keeping with our simple device driver example, let's create the proper device node to exercise it:

$ mknod /dev/hello1 c 234 0

After executing this command on our target embedded system, we end up with a new file called /dev/hello1 that represents our device driver module. If we list this file to the console, it looks like this:

$ ls -l /dev/hello1
crw-r--r-- 1 root root 234, 0 Jul 14 2005 /dev/hello1

The parameters we passed to mknod include the name, type, and major and minor numbers for our device driver. The name we chose, of course, was hello1. Because we are demonstrating the use of a character driver, we use c to indicate that. The major number is 234, the number we chose for this example, and the minor number is 0.

By itself, the device node is just another file on our file system. However, because of its special status as a device node, we use it to bind to an installed device driver. If an application process issues an open() system call with our device node as the path parameter, the kernel searches for a valid device driver registered with a major number that matches the device nodein our case, 234. This is the mechanism by which the kernel associates our particular device to the device node.

As most C programmers know, the open() system call, or any of its variants, returns a reference (file descriptor) that our applications use to issue subsequent file system operations, such as read, write, and close. This reference is then passed to the various file system operations, such as read, write, or their variants.

For those curious about the purpose of the minor number, it is a mechanism for handling multiple devices or subdevices with a single device driver. It is not used by the operating system; it is simply passed to the device driver. The device driver can use the minor number in any way it sees fit. As an example, with a multiport serial card, the major number would specify the driver. The minor number might specify one of the multiple ports handled by the same driver on the multiport card. Interested readers are encouraged to consult one of the excellent texts on device drivers for further details.

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