Char devices are accessed through names in the filesystem. Those names are called special files or device files or simply nodes of the filesystem tree; they are conventionally located in the /dev directory. Special files for char drivers are identified by a “c” in the first column of the output of ls -l. Block devices appear in /dev as well, but they are identified by a “b.” The focus of this chapter is on char devices, but much of the following information applies to block devices as well.

If you issue the ls -l command, you’ll see two numbers (separated by a comma) in the device file entries before the date of last modification, where the file length normally appears. These numbers are the major device number and minor device number for the particular device. The following listing shows a few devices as they appear on a typical system. Their major numbers are 1, 4, 7, and 10, while the minors are 1, 3, 5, 64, 65, and 129.

 crw-rw-rw- 1 root   root    1, 3   Feb 23 1999  null
 crw------- 1 root   root   10, 1   Feb 23 1999  psaux
 crw------- 1 rubini tty     4, 1   Aug 16 22:22 tty1
 crw-rw-rw- 1 root   dialout 4, 64  Jun 30 11:19 ttyS0
 crw-rw-rw- 1 root   dialout 4, 65  Aug 16 00:00 ttyS1
 crw------- 1 root   sys     7, 1   Feb 23 1999  vcs1
 crw------- 1 root   sys     7, 129 Feb 23 1999  vcsa1
 crw-rw-rw- 1 root   root    1, 5   Feb 23 1999  zero

The major number identifies the driver associated with the device. For example, /dev/null and /dev/zero are both managed by driver 1, whereas virtual consoles and serial terminals are managed by driver 4; similarly, both vcs1 and vcsa1 devices are managed by driver 7. The kernel uses the major number at open time to dispatch execution to the appropriate driver.

The minor number is used only by the driver specified by the major number; other parts of the kernel don’t use it, and merely pass it along to the driver. It is common for a driver to control several devices (as shown in the listing); the minor number provides a way for the driver to differentiate among them.

Version 2.4 of the kernel, though, introduced a new (optional) feature, the device file system or devfs. If this file system is used, management of device files is simplified and quite different; on the other hand, the new filesystem brings several user-visible incompatibilities, and as we are writing it has not yet been chosen as a default feature by system distributors. The previous description and the following instructions about adding a new driver and special file assume that devfs is not present. The gap is filled later in this chapter, in “The Device Filesystem”.

When devfs is not being used, adding a new driver to the system means assigning a major number to it. The assignment should be made at driver (module) initialization by calling the following function, defined in <linux/fs.h>:

 int register_chrdev(unsigned int major, const char *name,
      struct file_operations *fops);

The return value indicates success or failure of the operation. A negative return code signals an error; a 0 or positive return code reports successful completion. The major argument is the major number being requested, name is the name of your device, which will appear in /proc/devices, and fops is the pointer to an array of function pointers, used to invoke your driver’s entry points, as explained in “File Operations”, later in this chapter.

The major number is a small integer that serves as the index into a static array of char drivers; “Dynamic Allocation of Major Numbers” later in this chapter explains how to select a major number. The 2.0 kernel supported 128 devices; 2.2 and 2.4 increased that number to 256 (while reserving the values 0 and 255 for future uses). Minor numbers, too, are eight-bit quantities; they aren’t passed to register_chrdev because, as stated, they are only used by the driver itself. There is tremendous pressure from the developer community to increase the number of possible devices supported by the kernel; increasing device numbers to at least 16 bits is a stated goal for the 2.5 development series.

Once the driver has been registered in the kernel table, its operations are associated with the given major number. Whenever an operation is performed on a character device file associated with that major number, the kernel finds and invokes the proper function from the file_operations structure. For this reason, the pointer passed to register_chrdev should point to a global structure within the driver, not to one local to the module’s initialization function.

The next question is how to give programs a name by which they can request your driver. A name must be inserted into the /dev directory and associated with your driver’s major and minor numbers.

The command to create a device node on a filesystem is mknod; superuser privileges are required for this operation. The command takes three arguments in addition to the name of the file being created. For example, the command

 mknod /dev/scull0 c 254 0

creates a char device (c) whose major number is 254 and whose minor number is 0. Minor numbers should be in the range 0 to 255 because, for historical reasons, they are sometimes stored in a single byte. There are sound reasons to extend the range of available minor numbers, but for the time being, the eight-bit limit is still in force.

Please note that once created by mknod, the special device file remains unless it is explicitly deleted, like any information stored on disk. You may want to remove the device created in this example by issuing rm /dev/scull0.