How To Use SocketCAN With C++ In Linux

Overview

The Linux C/C++ API allows you to control a SocketCAN interface via a C/C++ application.

SocketCAN supports standard frame format (SFF), extended frame format (EFF) and CAN FD frames.

Opening Interface

 

Reading And Writing

The data for reading and writing to the CAN bus is communicated through the can_frame struct which is declared and defined in <linux/can.h>.

The 32-bit can_id value has the following structure:

[0-28]: CAN identifier (11/29bit)
[29]: Error frame flag (0 = data frame, 1 = error frame)
[30]: Remote transmission request flag (1 = rtr frame)
[31]: Frame format flag (0 = standard 11bit, 1 = extended 29bit)

The padding in the above struct allows data to be aligned to a 64-bit boundary. This allows the user to define their own structs and unions to easily access the data (by casting). For example, you could access all 8-bytes of data as a single 64-bit value if desired.

Note that the can_id structure does not map directly to the bits present in the arbitration field of a CAN message.

There is also an extended frame struct, called canfd_frame.

The canfd_frame struct does not have a can_dlc  member to indicate the number of bytes, but rather a len member.

A read() system call transfers a can_frame or canfd_frame struct from the kernel CAN buffer to the user space.

Data is buffered internally, which means for virtual interfaces, you can do a write, pause, and then read the value back at a later time (you do not have to use separate threads/processes).

libsocketcan

libsocketcan is a Linux library that provides some userspace functionality to control a SocketCAN interface. It provides functions such as can_set_bitrate(), can_do_start() and can_do_stop().

WARNING: Although libsocketcan seems to work fine for physical CAN interfaces (e.g. can0), I have had issues when using it with a virtual CAN interface (e.g. vcan0). Specifically, functions such as can_get_state() do not seem to work correctly.

You can install libsocketcan on your Linux machine by following the below steps:

  1. Clone the libsocketcan git repository:
  2. Build/install (libsocketcan uses the autotools build system):

    You should now have libsocketcan.a installed to /usr/local/lib and the header file libsocketcan.h installed to /usr/local/include.

External Resources

https://www.kernel.org/doc/Documentation/networking/can.txt is a great page detailing with the Linux C API for SocketCAN, including code examples and use cases.

The can-utils package source code can be found at https://github.com/linux-can/can-utils. This has great C code examples on how to read and write messages to the SocketCAN interface.

Posted: July 22nd, 2017 at 9:46 am
Last Updated on: September 15th, 2017 at 4:12 am