Programming

CAN Programming

CAN Sockets …

  • Packet switching ⟶ much like UDP on the Internet

  • New protocol family: PF_CAN

  • No addresses ⟶ binding by interface index

  • Fixed size packets (frames)

CAN in C - Socket, Interface Index

From Documentation/networking/can.txt

int s;
struct sockaddr_can addr;
struct ifreq ifr;

s = socket(PF_CAN, SOCK_RAW, CAN_RAW);

strcpy(ifr.ifr_name, "can0");
ioctl(s, SIOCGIFINDEX, &ifr);

addr.can_family = AF_CAN;
addr.can_ifindex = ifr.ifr_ifindex;

bind(s, (struct sockaddr *)&addr, sizeof(addr));

CAN in Python - Socket, Interface Index

import socket

can_socket = socket.socket(socket.PF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
can_socket.bind(('can0',))

CAN in C - Frames (1)

Kernel accepts (write()/send()) and delivers (read()/recv()) fixed size network packtes ⟶ CAN frames

#include <linux/can.h>

struct can_frame {
    canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
    __u8    can_dlc; /* frame payload length */
    __u8    data[8] __attribute__((aligned(8)));
};

CAN in C - Frames (2)

#include <linux/can.h>

struct can_frame frame;
read(s, &frame, sizeof(struct can_frame));

/* do something with frame */

write(s, &frame, sizeof(struct can_frame));

CAN in Python - Frames

struct can_frame {
    canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
    __u8    can_dlc; /* frame payload length */
    __u8    data[8] __attribute__((aligned(8)));
};

import struct

frame_layout = "=IB3x8s"
frame_size = struct.calcsize(frame_layout)
frame = can_socket.recv(frame_size)
can_id, can_dlc, data = struct.unpack(frame_layout, frame)

# do something with frame

frame = struct.pack(frame_layout, can_id+1, len(data), data)
can_socket.send(frame)

What Else?

  • Interface Index 0 (Python: empty interface name ⟶ all interfaces)

  • Alternative system calls: recvfrom() and sendto() if one wants to know the interface on which a frame is received

  • The rest is Unix

    • File descriptors

    • Event loops

    • ⟶ well-know client/server techniques

  • Realtime … is a completely different story

    • Can be done if needed

    • Caution: if you don’t need it don’t use it!