# Socketcan

There are two implementations of socketcan backends. One written with `ctypes` to be compatible with Python 2 and 3, and one written for future versions of Python3 which feature native support.

- SocketCAN (ctypes)
  - Bus
  - Broadcast-Manager
  - Internals
- SocketCAN (python)
  - Bus
  - Internals

Unless you’re running Python3.3 or lower the recommended backend is socketcan_native. For Python2.7 and Python3 <3.4, the available backend is socketcan_ctypes.

## Socketcan Quickstart

The full documentation for socketcan can be found in the kernel docs at networking/can.txt. The CAN network driver provides a generic interface to setup, configure and monitor CAN devices. To configure bit-timing parameters use the program `ip`.

### The virtual CAN driver (vcan)

The virtual CAN interfaces allow the transmission and reception of CAN frames without real CAN controller hardware. Virtual CAN network devices are usually named ‘vcanX’, like vcan0 vcan1 vcan2.

To create a virtual can interface using socketcan run the following:

    sudo modprobe vcan
    # Create a vcan network interface with a specific name
    sudo ip link add dev vcan0 type vcan
    sudo ip link set vcan0 up

### Real Device

`vcan` should be substituted for `can` and `vcan0` should be substituted for `can0` if you are using real hardware. Setting the bitrate can also be done at the same time, for example to enable an existing `can0` interface with a bitrate of 1MB:

    sudo ip link set can0 up type can bitrate 1000000

#### Send Test Message

The can-utils library for linux includes a script cansend which is useful to send known payloads. For example to send a message on vcan0:

    cansend vcan0 123#DEADBEEF

#### CAN Errors

A device may enter the “bus-off” state if too many errors occurred on the CAN bus. Then no more messages are received or sent. An automatic bus-off recovery can be enabled by setting the “restart-ms” to a non-zero value, e.g.:

    sudo ip link set canX type can restart-ms 100

Alternatively, the application may realize the “bus-off” condition by monitoring CAN error frames and do a restart when appropriate with the command:

    ip link set canX type can restart

Note that a restart will also create a CAN error frame.

### List network interfaces

To reveal the newly created `can0` or a `vcan0` interface:

    ifconfig

### Display CAN statistics

    ip -details -statistics link show vcan0

### Network Interface Removal

To remove the network interface:

    sudo ip link del vcan0

## Wireshark

Wireshark supports socketcan and can be used to debug python-can messages. Fire it up and watch your new interface.

To spam a bus:

    import time
    import can

    bustype = 'socketcan_native'
    channel = 'vcan0'

    def producer(id):
        """:param id: Spam the bus with messages including the data id."""
        bus = can.interface.Bus(channel=channel, bustype=bustype)
        for i in range(10):
            msg = can.Message(arbitration_id=0xc0ffee, data=[id, i, 0, 1, 3, 1, 4, 1], extended_id=False)
            bus.send(msg)
        # Issue #3: Need to keep running to ensure the writing threads stay alive. ?
        time.sleep(1)

    producer(10)

With debugging turned right up this looks something like this:

The process to follow bus traffic is even easier:

    for message in Bus(can_interface):
        print(message)

## Reading and Timeouts

Reading a single CAN message off of the bus is simple with the `bus.recv()` function:

    import can

    can_interface = 'vcan0'
    bus = can.interface.Bus(can_interface, bustype='socketcan_native')
    message = bus.recv()

By default, this performs a blocking read, which means `bus.recv()` won’t return until a CAN message shows up on the socket. You can optionally perform a blocking read with a timeout like this:

    message = bus.recv(1.0)  # Timeout in seconds.

    if message is None:
        print('Timeout occurred, no message.')

If you set the timeout to `0.0`, the read will be executed as non-blocking, which means `bus.recv(0.0)` will return immediately, either with a `Message` object or `None`, depending on whether data was available on the socket.