Character Device Driver Notes¶

🧠 Core Concept¶

A Linux character device driver provides a file interface to user space.

struct file_operations {
    .open
    .read
    .write
    .release
};

open() → create file instance
read/write → operate with offset
close() → release() when ref count = 0

release is not called on every close()

Each file instance has its own offset:

if (*offset >= data_size)
    return 0;

EOF is per-open

Global variables must be protected:

mutex_lock()

critical section

mutex_unlock()

Used for cleanup:

mutex_lock()

if (error)
    goto out;

...

out:
mutex_unlock()

Wrong:

Correct:

Wrong:

Correct:

Wrong:

Used for safe memory access:

copy_to_user()
copy_from_user()

Never directly access user pointer