socketcan J1939过滤器在python中的使用

如何解决socketcan J1939过滤器在python中的使用

在 Python 中，我尝试使用 linux 内核文档中提到的 J1939 过滤：https://www.kernel.org/doc/html/latest/networking/j1939.html

以下代码在 setsockopt() 行失败（设置过滤器）：

import socket
import struct

def pack_J1939_filters(can_filters):
    can_filter_fmt = "=" + "2Q2B2I" * len(can_filters)
    filter_data = []
    for can_filter in can_filters:
        name = can_filter['name']
        name_mask = can_filter['name_mask']
        addr = can_filter['addr']
        addr_mask = can_filter['addr_mask']
        pgn = can_filter['pgn']
        pgn_mask = can_filter['pgn_mask']

        filter_data.append(name)
        filter_data.append(name_mask)
        filter_data.append(addr)
        filter_data.append(addr_mask)
        filter_data.append(pgn)
        filter_data.append(pgn_mask)

    return struct.pack(can_filter_fmt,*filter_data)


s = socket.socket(socket.PF_CAN,socket.SOCK_DGRAM,socket.CAN_J1939) 

interface = "vcan0"

src_name = socket.J1939_NO_NAME
src_pgn = socket.J1939_NO_PGN 
src_addr = 0x81 
src_sck_addr = (interface,src_name,src_pgn,src_addr)
s.bind(src_sck_addr)

filters = [{"name": 0,"name_mask":0,"addr":0,"addr_mask":0,"pgn": 0,"pgn_mask": 0}]
packed_filters = pack_J1939_filters(filters)

# socket.SOL_CAN_J1939 does not seem to exist
SOL_CAN_BASE = 100
CAN_J1939 = 7
SOL_CAN_J1939 = SOL_CAN_BASE + CAN_J1939

s.setsockopt(SOL_CAN_J1939,socket.SO_J1939_FILTER,packed_filters) 

s.recvfrom(128)
s.close()

首先，内核文档提到使用 SOL_CAN_J1939 作为第一个参数。但是 socket.SOL_CAN_J1939 在 socket 包中不存在。所以查看这个位置的代码，我能够理解这个 int 值应该是 107：http://socket-can.996257.n3.nabble.com/RFC-v3-0-6-CAN-add-SAE-J1939-protocol-td7571.html

至于 setsockopt() 第三个参数，我打包过滤器以匹配 j1939_filter 结构（26 字节，如上一个链接的代码中所述）。这类似于在 can.interfaces.socketcan.utils 中为原始 CAN 所做的。

我做错了什么导致setsockopt()失败？

解决方法

第一个问题是 struct.pack 格式 (can_filter_fmt) 错误。我首先假设内核 j1939_filter 结构大小是成员的总和。这是错误的，因为编译器添加了填充。这可以作为 x 添加到 struct.pack 格式中，例如 2Q2I2B6x。请参阅Why isn't sizeof for a struct equal to the sum of sizeof of each member?

第二个问题是 can_filter_fmt 不是打包为 2Q2B2I 而是打包为 2Q2I2B6x（addr 成员在中间）。

至于 SOL_CAN_J1939 我是正确的，需要在文件中创建，因为它还没有在包中。

最终代码如下：

#!/usr/bin/env python3

import socket
import struct

def pack_J1939_filters(can_filters=None):
    if can_filters is None:
        # Pass all messages
        can_filters = [{}]

    can_filter_fmt = "=" + "2Q2I2B6x" * len(can_filters)
    filter_data = []
    for can_filter in can_filters:
        if 'name' in can_filter:
            name = can_filter['name']
        else:
            name = 0

        if 'name_mask' in can_filter:
            name_mask = can_filter['name_mask']
        else:
            name_mask = 0

        if 'pgn' in can_filter:
            pgn = can_filter['pgn']
        else:
            pgn = 0

        if 'pgn_mask' in can_filter:
            pgn_mask = can_filter['pgn_mask']
        else:
            pgn_mask = 0
        
        if 'addr' in can_filter:
            addr = can_filter['addr']
        else:
            addr = 0

        if 'addr_mask' in can_filter:
            addr_mask = can_filter['addr_mask']
        else:
            addr_mask = 0

        filter_data.append(name)
        filter_data.append(name_mask)
        filter_data.append(pgn)
        filter_data.append(pgn_mask)
        filter_data.append(addr)
        filter_data.append(addr_mask)

    return struct.pack(can_filter_fmt,*filter_data)

def print_msg(data,sck_addr):
    print(f"SA:{hex(sck_addr[3])} PGN:{hex(sck_addr[2])}")

    for j in range(len(data)):
        if j % 8 == 0 and j != 0:
            print()

        if j % 8 == 0:
            print(f"bytes {j} to {j+7}: ",end="")

        print(f"{hex(data[j])} ",end="")

    print()
    print()
    

def main():
    s = socket.socket(socket.PF_CAN,socket.SOCK_DGRAM,socket.CAN_J1939) 

    # allows to receive broadcast messages
    s.setsockopt(socket.SOL_SOCKET,socket.SO_BROADCAST,1)
    
    interface = "vcan0"

    src_name = socket.J1939_NO_NAME
    src_pgn = socket.J1939_NO_PGN # always no PGN for source,unless filtering is needed
    src_addr = 0x81 # recvfrom() will not return destination specific messages for other addresses
    src_sck_addr = (interface,src_name,src_pgn,src_addr)
    s.bind(src_sck_addr)

    packed_filters = pack_J1939_filters()

    SOL_CAN_BASE = 100
    CAN_J1939 = 7
    SOL_CAN_J1939 = SOL_CAN_BASE + CAN_J1939

    s.setsockopt(SOL_CAN_J1939,socket.SO_J1939_FILTER,packed_filters) 

    (recv_data,recv_sck_addr) = s.recvfrom(128)
    print_msg(recv_data,recv_sck_addr)

    s.close()


if __name__ == "__main__":
    main()

谢谢。

,

要使 J1939 与 SocketCAN 一起使用，您需要做两件事：

1. 内核 5.4+
2. can-j1939 内核模块已启用

---

测试 can-1939：

如果你安装了 can-utils 并且在 sudo modprobe can-j1939 之后你得到的是致命错误，或者如果你从 can-utils 启动 testj1939 并且你得到不支持协议的错误，那么这意味着 can-j1939 是未在您的内核中启用，您需要手动编译它。

---

以下是我在 Debian 10 内核中启用 can-j1939 的说明：

https://github.com/linux-can/can-utils/blob/master/can-j1939-install-kernel-module.md