[CanBus] Example C code for SocketCAN

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下面是 SocketCAN Librar 使用的介紹，完整程式碼請參考: kaka-lin/Notes/Network/canbus/can_examples

搭配 Qt 製作自己的 CanBus Tool 如下圖:

1. Create a CAN socket

The first step before doing anything is to create a socket.

int can_fd;

// PF_CAN or AF_CAN
if ((can_fd = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
  perror("Error while Opening Socket");
  return 1;
}

其中 socket() 的定義如下:

#include <sys/socket.h>

int socket(int domain, int type, int protocol)

  • domain: 選擇要用於通信的協議系列，常用的如下

    • AF_INET: IPv4 Internet protocols
    • AF_INET6: IPv6 Internet protocols
    • AF_CAN: Controller Area Network automotive bus protocol

  • type: Socket 的類型，常用的如下

    • SOCK_STREAM: Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported, like TCP or FTP.

    • SOCK_DGRAM: Supports datagrams (connectionless, unreliable messages of a fixed maximum length), like UDP.

    • SOCK_RAW: Provides raw network protocol access, like CAN.

  • protocol: 與 Socket 一起使用的特定協議，如下

    • IPPROTO_TCP
    • IPPTOTO_UDP
    • CAN_RAW

2. Bind the socket to the CAN Interface:

Next, we must retrieve the interface index for the interface name (can0, can1, vcan0 etc) we wish to use. To do this we send an I/O control call and pass an ifreq structure containing the interface name:

#include <sys/ioctl.h>
#include <net/if.h>

struct ifreq ifr;

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

Then bind the socket to the CAN interface:

struct sockaddr_can addr;

memset(&addr, 0, sizeof(addr));
addr.can_family = AF_CAN;
addr.can_ifindex = ifr.ifr_ifindex;

if (bind(can_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
  perror("Error in Socket bind");
  return 1;
}

3. Sending/Reading a frame

Sending a frame

To send a CAN frame, one must initialize a can_frame structure and populate it with data.

struct can_frame {
  canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
  __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
  __u8    __pad;   /* padding */
  __u8    __res0;  /* reserved / padding */
  __u8    __res1;  /* reserved / padding */
  __u8    data[8] __attribute__((aligned(8)));
};

To send a frame, initialize a can_frame with an ID of 0x123, a payload containing “0xDEADBE11” and send it using the write() system call:

#include <linux/can.h>
#include <linux/can/raw.h>

struct can_frame frame;
frame.can_id  = 0x123;
frame.can_dlc = 4;
frame.data[0] = 0xDE;
frame.data[1] = 0xAD;
frame.data[2] = 0xBE;
frame.data[3] = 0x11;

if (write(can_fd, &frame, sizeof(struct can_frame)) != sizeof(struct can_frame)) {
  perror("Write");
  return 1;
}

Reading a frame

To read a frame, initialize a can_frame and call the read() system call. This will block until a frame is available.

int nbytes;
struct can_frame frame;

nbytes = read(can_fd, &frame, sizeof(struct can_frame));
if (nbytes < 0) {
  perror("Read");
  return 1;
}

printf("0x%03X [%d] ",frame.can_id, frame.can_dlc);

for (int i = 0; i < frame.can_dlc; i++)
  printf("%02X ",frame.data[i]);
printf("\r\n");

4. Closing the socket

if (close(s) < 0) {
   perror("Close");
   return 1;
}

Reference

Kaka LinSoftware Engineer

AIoT