Interview QA » Understanding Major and Minor Numbers in Linux

Understanding Major and Minor Numbers in Linux

linux device driver major minor number kernel device file

In Linux, major and minor numbers are used to identify and manage devices through device files located in the /dev directory. These numbers serve as identifiers for the kernel to communicate with the appropriate device driver.

Major Number

  • The major number identifies the device driver that handles a specific device.
  • It serves as a key for the kernel to map a device file to the corresponding device driver.
  • Example: A major number of “8” might be associated with the SCSI disk driver.

Minor Number

  • The minor number identifies a specific device or sub-device that the driver manages.
  • It is used in conjunction with the major number to differentiate between multiple devices handled by the same driver.
  • Example: For a hard disk with major number “8”, minor numbers 0, 1, 2, etc., may refer to partitions or different disks.

Examples in /dev Directory

ls -l /dev/sda


Output:

brw-rw---- 1 root disk 8, 0 Dec 9 10:30 /dev/sda


In the output:

- `8` is the major number, which indicates the SCSI disk driver.
- `0` is the minor number, which indicates the first disk or partition.

When a device file is accessed (for example, `/dev/sda`):

1.  The kernel looks at the major number to locate the corresponding device driver.
2.  The device driver then uses the minor number to interact with the specific device or sub-device.

## Allocation of Numbers: Static vs Dynamic

Linux represents devices as a pair of numbers `<major>:<minor>`.

Some functions used to assign and de-assign the major and minor numbers are as follows:

```c
int register_chrdev_region(dev_t first, unsigned int count, char *name); // Static allocation
int alloc_chrdev_region(dev_t *dev, unsigned int first_minor, unsigned int count, char *name); // Dynamic allocation
void unregister_chrdev_region(dev_t first, unsigned int count);  // Deregister major and minor numbers
MKDEV(major, minor) // function to initialize dev_t dev variable
MAJOR( dev_t dev) // to access major number
MINOR(dev_t dev) // to access minor number

Command to Create Device File

mknod –m <permissions> <name> <device type> <major> <minor>

// Manual creation

  • -m <permissions>: Sets the permission bits of the new device file to permissions.
  • <name>: Your device name with the full path (/dev/name).
  • <Major>: Major number of your device.
  • <Minor>: Minor number of your device.
  • <device type>: c - character device, b - block device.

Commands to View Major and Minor Numbers

  • ls -l: Shows the major and minor numbers for a device file.
  • cat /proc/devices: Lists major numbers associated with different drivers.
  • dmesg: Displays kernel messages, which may include major and minor numbers during device initialization.