https://github.com/maximilianfeldthusen/character-device-driver
character-device-driver written in c
https://github.com/maximilianfeldthusen/character-device-driver
c character device driver
Last synced: 3 months ago
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character-device-driver written in c
- Host: GitHub
- URL: https://github.com/maximilianfeldthusen/character-device-driver
- Owner: maximilianfeldthusen
- License: mit
- Created: 2025-07-08T03:03:46.000Z (3 months ago)
- Default Branch: TFD
- Last Pushed: 2025-07-08T03:33:40.000Z (3 months ago)
- Last Synced: 2025-07-08T05:22:44.640Z (3 months ago)
- Topics: c, character, device, driver
- Language: C
- Homepage:
- Size: 26.4 KB
- Stars: 0
- Watchers: 0
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# character-device-driver
## `char_driver.c` — What It Does
This kernel module:
- Registers a custom character device named `/dev/char_dev_demo`
- Allows `read()` and `write()` access to an internal buffer
- Supports two `ioctl()` commands:
- `MYDRV_RESET`: clears the internal buffer
- `MYDRV_GET_SIZE`: returns the number of bytes stored
---
### Key Sections Explained
#### Module Setup and Globals
```c
#define DEVICE_NAME "char_dev_demo"
#define CLASS_NAME "char_class"
```
Creates `/dev/char_dev_demo` and maps it in `/sys/class/char_class/`.
```c
static char buffer[256];
static size_t buffer_size = 0;
```
A 256-byte internal kernel buffer. `buffer_size` tracks how much valid data is present.
#### `dev_read()` and `dev_write()`
```c
copy_to_user(...) // kernel user
copy_from_user(...) // user kernel
```
The data transfer logic. Safe copying to and from user-space memory.
It also uses:
```c
if (*offset >= buffer_size) return 0;
```
Supports repeated reads with file offsets like a standard file.
#### `dev_ioctl()` — Custom Commands
```c
#define MYDRV_RESET _IO(...)
#define MYDRV_GET_SIZE _IOR(..., int)
```
- `MYDRV_RESET`: Sets all 256 bytes of the buffer to 0 with `memset()`
- `MYDRV_GET_SIZE`: Returns how much data has been written into `buffer`
#### `char_init()` — Driver Load
```c
alloc_chrdev_region() // reserves a major number
cdev_init() + cdev_add() // registers the device
class_create() + device_create() // creates the /dev entry
```
#### `char_exit()` — Driver Unload
```c
device_destroy(), class_destroy(), cdev_del(), unregister_chrdev_region()
```
Clean shutdown so no kernel resources are leaked.
---
## `Makefile` for Kernel Module
```makefile
obj-m := char_driver.o
KDIR := /lib/modules/$(shell uname -r)/build
```
This points the build to the Linux kernel source headers.
```bash
make
```
Invokes the kernel build system to create `char_driver.ko`.
---
## Debian Packaging (Inside `debian/`)
### control
Standard package metadata (package name, version, architecture, etc.).
### rules
```makefile
%:
dh $@
```
Tells `debhelper` to take over the build process.
### postinst / prerm
These are install/uninstall hooks:
- `postinst`:
- Inserts the kernel module (`insmod`)
- Creates `/dev/char_dev_demo` with correct major number and permissions
- `prerm`:
- Removes the module (`rmmod`)
- Deletes the device file
---
## Final Build & Test Workflow
```bash
make # Build the .ko module
dpkg-buildpackage -us -uc # Generate the .deb
sudo dpkg -i ../char-driver_1.0_amd64.deb # Install module via APT
```
After that:
```bash
echo "test" > /dev/char_dev_demo
cat /dev/char_dev_demo # See what you wrote
```