# `NervesUEvent`
[🔗](https://github.com/nerves-project/nerves_uevent/blob/v0.1.7/lib/nerves_uevent.ex#L5)

NervesUEvent listens for events from the Linux kernel, automatically loads
device drivers, and forwards them to your Elixir programs.

NervesUEvent is a very simple version of the Linux `udevd`. Just like `udevd`
does for desktop Linux, NervesUEvent registers to receive UEvents from the Linux
kernel. Unlike `udevd`, NervesUEvent only runs `modprobe` when needed and keeps
track of what hardware is in the system. For most Nerves use cases, `udevd`
isn't needed.

# `action_stats`

```elixir
@type action_stats() :: %{
  add: non_neg_integer(),
  change: non_neg_integer(),
  remove: non_neg_integer(),
  move: non_neg_integer(),
  bind: non_neg_integer(),
  unbind: non_neg_integer(),
  other: non_neg_integer()
}
```

Uevent action type counters

# `stats`

```elixir
@type stats() :: %{
  uevents_received: non_neg_integer(),
  uevents_dropped: non_neg_integer(),
  modprobes_called: non_neg_integer(),
  modaliases_queued: non_neg_integer(),
  modaliases_dropped: non_neg_integer(),
  modprobe_fork_failures: non_neg_integer(),
  peak_queue_n: non_neg_integer(),
  peak_queue_bytes: non_neg_integer(),
  actions: action_stats()
}
```

UEvent collection counters

* `:uevents_received` — netlink messages successfully read
* `:uevents_dropped` — ENOBUFS incidents (kernel dropped one or more messages)
* `:modprobes_called` — modprobe child processes launched
* `:modaliases_queued` — modaliases queued for modprobe
* `:modaliases_dropped` — modaliases dropped because the queue was full
  while a modprobe was already in flight
* `:modprobe_fork_failures` — `fork()` failures when launching modprobe
* `:peak_queue_n` — high-water mark for queued modalias count
* `:peak_queue_bytes` — high-water mark for the modalias byte buffer
* `:actions` — nested map of `add | change | remove | move | bind | unbind | other`
  counts, matching the uevent ACTION field

# `get`

```elixir
@spec get(PropertyTable.property(), PropertyTable.value()) :: PropertyTable.value()
```

Get the most recent  value of UEvent report

For example,

```elixir
> NervesUEvent.get(["devices", "platform", "leds", "leds", "red:indicator-1"])
%{
  "of_compatible_n" => "0",
  "of_fullname" => "/leds/rgb1-blue",
  "of_name" => "rgb1-blue",
  "subsystem" => "leds"
}}
```

# `get_all`

```elixir
@spec get_all() :: [{PropertyTable.property(), PropertyTable.value()}]
```

Get all reported UEvents

# `match`

```elixir
@spec match(PropertyTable.pattern()) :: [
  {PropertyTable.property(), PropertyTable.value()}
]
```

Run an arbitrary match against UEvents

Use `:_` in the path to accept any value in that position
Use `:"$"` at the end of the path to perform an exact match

# `stats`

```elixir
@spec stats() :: stats()
```

Return counters collected by the uevent port.

Counters are cumulative since the port started and updated after 5 seconds of
inactivity.  Values will lag bursts especially at boot.

# `subscribe`

```elixir
@spec subscribe(PropertyTable.pattern()) :: :ok
```

Subscribe to uevent notifications

Pass a pattern like one you'd pass to `match/1`. Instead of getting a
response, you'll receive a message when a matching UEvent happens.

---

*Consult [api-reference.md](api-reference.md) for complete listing*