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Device number

Learn device numbers and registration.

dev_t: device number

On Linux every character or block device has a device number (dev_t). The number has two parts: major and minor.

The major number identifies the driver; the minor identifies a device under that driver. The dev_t type is defined in <linux/types.h>. MKDEV(major, minor) combines them; MAJOR(dev) and MINOR(dev) split them. ls -l /dev shows major and minor numbers. Driver authors request numbers with alloc_chrdev_region.

Number allocation

Driver authors get device numbers two ways: static register_chrdev_region or dynamic alloc_chrdev_region. The dynamic path lets the kernel pick a suitable major.

Dynamic allocation is preferred to avoid clashes. On success, the dev_t variable holds the major and minor. When the driver is removed, unregister_chrdev_region gives the number back. On the init error path the number must be released, or it leaks. The device cannot be registered before the number is allocated.

Practice Print device numbers

Define void print_dev(int major, int minor) with body printf("dev=%d:%d\n", major, minor);. In main call it for three devices: (10, 0), (1, 5), (254, 1).

Starter code

main.c
#include <stdio.h>

// Define print_dev function
// Call it for three (major, minor) pairs

int main(void) {
    return 0;
}

Sample solution

main.c
#include <stdio.h>

void print_dev(int major, int minor) {
    printf("dev=%d:%d\n", major, minor);
}

int main(void) {
    print_dev(10, 0);
    print_dev(1, 5);
    print_dev(254, 1);
    return 0;
}

Expected output

dev=10:0
dev=1:5
dev=254:1

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What does the dev_t type hold?

  • Only major
  • Major and minor Correct answer
  • The device name
  • Only minor

Explanation: dev_t stores both major and minor together.

2 Match device number macros

Term Meaning
dev_t Device number
MKDEV Combine major and minor
MAJOR Major number
MINOR Minor number

3 Complete the MKDEV call

Question 
dev_t dev = MKDEV(____, minor);
Answer 
dev_t dev = MKDEV(major, minor);

Explanation: MKDEV takes major first, then minor.

All lessons in Expert

1 Kernel basics

  1. What is the kernel?
  2. Syscall
  3. ioctl
  4. /proc
  5. dmesg and logs
  6. Summary - Kernel basics

2 Driver basics

  1. Module loading
  2. Module structure
  3. printk
  4. Device number
  5. fops
  6. Summary - Driver basics

3 Compiler basics

  1. Lexer
  2. Parser
  3. AST
  4. Code generation
  5. Symbol table
  6. Summary - Compiler basics

4 Security

  1. Buffer overflow
  2. Stack canary
  3. ASLR
  4. Format string
  5. Safe functions
  6. Summary - Security

5 Modern C

  1. C11 and C17
  2. _Generic
  3. _Static_assert
  4. Atomics
  5. threads.h
  6. Summary - Modern C

6 Linker and loader

  1. Object files
  2. Symbol resolution
  3. Relocation
  4. Static and dynamic
  5. ELF format
  6. Summary - Linker

7 Embedded C

  1. What is embedded?
  2. Memory-mapped I/O
  3. Interrupts
  4. Bit manipulation
  5. Hardware timing
  6. Summary - Embedded C

8 Expert final

  1. Project design
  2. Kernel module
  3. Compiler component
  4. Security layer
  5. Putting it together