Using SocketCAN Plugin

The SocketCAN plugin encapsulates the Linux sockets API for accessing the CAN devices. This API is a set of open source CAN drivers and a networking stack contributed by Volkswagen Research to the Linux kernel.

SocketCAN usage

To list all (including unconfigured) network interfaces, the command ifconfig -a can be used.

To use SocketCAN, the corresponding Linux Kernel modules must be loaded and the network interface must be configured.

Setting up real CAN hardware

This section assumes, that the device driver is already loaded (most likely automatically when connecting the CAN hardware).

To set the device can0 to a bitrate of 250 kBit/s:

sudo ip link set can0 type can bitrate 250000
sudo ip link set up can0

Setting up a virtual CAN bus

Note: For CAN FD usage, the MTU (Maximum Transmission Unit) has to be set to 72 byte.

sudo modprobe vcan
sudo ip link add dev vcan0 type vcan
sudo ip link set vcan0 mtu 72
sudo ip link set up vcan0

The command line test programs used in the following are from the can-utils package:

# Display received CAN messages
candump vcan0

# Send a CAN message
cansend vcan0 123##3112233445566778899aabbccddeeff

# Generate random CAN messages
cangen vcan0

Creating CAN Bus Devices

At first it is necessary to check that QCanBus provides the desired plugin:

if (QCanBus::instance()->plugins().contains(QStringLiteral("socketcan"))) {
    // plugin available

Where socketcan is the plugin name.

Next, a connection to a specific interface can be established:

QCanBusDevice *device = QCanBus::instance()->createDevice(
    QStringLiteral("socketcan"), QStringLiteral("can0"));

Where can0 is the active CAN interface name. CAN interfaces act like regular network interfaces on Linux systems and can be discovered using ifconfig.

The device is now open for writing and reading CAN frames:

QCanBusFrame frame;
QByteArray payload("A36E");

The reading can be done using the readFrame() method. The framesReceived() signal is emitted when at least one new frame is available for reading:

QCanBusFrame frame = device->readFrame();

SocketCAN supports the following configurations that can be controlled through setConfigurationParameter():

Configuration parameter keyDescription
QCanBusDevice::LoopbackKeyTo meet the multiple-user needs, the local loopback is enabled by default. This means, whenever a CAN frame is transmitted on the CAN bus, a local echo of this frame is sent to all applications connected to this CAN device.
QCanBusDevice::ReceiveOwnKeyThe reception of the CAN frames on the same socket that was sending the CAN frame is disabled by default. When enabling this option, all CAN frames sent to the CAN bus immediately appear in the receive buffer. This can be used to check if sending was successful.
QCanBusDevice::ErrorFilterKeyA CAN interface driver can generate so called Error Message Frames that can optionally be passed to the user application in the same way as other CAN frames. The possible errors are divided into different error classes that may be filtered using the appropriate error mask. The values for the error mask are defined in linux/can/error.h.
QCanBusDevice::RawFilterKeyThis configuration can contain multiple filters of type QCanBusDevice::Filter. By default, the connection is configured to accept any CAN bus message.
QCanBusDevice::BitRateKeyThis configuration is not supported by the socketcan plugin. However it is possible to set the rate when configuring the CAN network interface using the ip link command.
QCanBusDevice::CanFdKeyThis configuration option determines whether CANFD frames may be sent or received. By default, this option is disabled. It controls the CAN_RAW_FD_FRAMES option of the CAN socket.

For example:

QList<QCanBusDevice::Filter> list;
QCanBusDevice::Filter f;

// only accept odd numbered frame id of type remote request
// frame can utilize extended or base format
f.frameId = 0x1;
f.frameIdMask = 0x1;
f.format = QCanBusDevice::Filter::MatchBaseAndExtendedFormat;
f.type = QCanBusFrame::RemoteRequestFrame;

device->setConfigurationParameter(QCanBusDevice::RawFilterKey, QVariant::fromValue(list));

Extended frame format and flexible data-rate are supported in SocketCAN.

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