Skip to content

Embedded Linux Learning

file_operations

`struct file_operations`¶

Purpose¶

struct file_operations defines how a character device responds to user-space file operations.

Common Callbacks¶

static const struct file_operations mydev_fops = {
    .owner = THIS_MODULE,
    .open = mydev_open,
    .read = mydev_read,
    .write = mydev_write,
    .poll = mydev_poll,
    .unlocked_ioctl = mydev_ioctl,
    .fasync = mydev_fasync,
    .release = mydev_release,
};

Common Fields¶

Field	Purpose
`.open`	Initialize per-open state
`.read`	Copy data or events to user space
`.write`	Accept user-space commands or data
`.poll`	Report readiness to `poll` / `epoll`
`.unlocked_ioctl`	Handle control commands
`.fasync`	Register or unregister asynchronous listeners for `SIGIO` notification
`.release`	Cleanup per-open state

`.poll()` vs `.fasync()`¶

Drivers commonly implement both .poll() and .fasync() to support different user-space I/O models.

Callback	Purpose
`.poll()`	Reports the current device readiness for `poll()` and `epoll()`
`.fasync()`	Registers asynchronous listeners for `SIGIO` notification

A typical driver event updates the device state first, then notifies all supported mechanisms:

Device Event
        │
data_ready = true
        │
 ┌───────────────┬────────────────┐
 ▼               ▼                ▼
wake_up()   kill_fasync()     .poll()

The driver does not choose the notification model. Applications decide whether to use blocking I/O, poll(), epoll(), or SIGIO.