SocketCAN read from socket only returns 11cobid

Question

I am trying to read from a socketCAN and the msg is always filtered for the 11bit identifier. This should be a problem fixable with setting the rpoper flags for the 29bit identifier but I can`t find where if anyone can help...

    struct can_frame message;
    struct sockaddr_can addr;
    struct ifreq ifr;
    int   fd = -1;                  // file descriptor (itÂ´s a socket)

    if((fd = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0)
    {
        LE_INFO("cannot open socket");
        return;
    }
    strcpy(ifr.ifr_name, "can0");
    ioctl(fd, SIOCGIFINDEX, &ifr);
    addr.can_family = AF_CAN;
    addr.can_ifindex = ifr.ifr_ifindex;
    if(bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
    {
        printf("cannot bind socket\n");
        return;
    }

    uint8_t nbytes;

    message.can_id |= CAN_EFF_FLAG;

    while(1)
    {
        nbytes = read(fd, &message, sizeof(struct can_frame));

        if (nbytes < 0) {
                perror("can raw socket read");
                return;
        }

        /* paranoid check ... */
        if (nbytes < sizeof(struct can_frame)) {
                fprintf(stderr, "read: incomplete CAN frame\n");
                return;
        }

        printf("READ COB_ID:%x\n",message.can_id | CAN_EFF_FLAG);
        
    }
    return;

I am sending a CAN frame with idx x901 and this is what is printed:

READ COB_ID:80000101

I have troubleshooted this in many different ways and it seems that the C code is working as it should, but I suspect the problem to be with the kernel module for either mcp251x which is not correctly receiving the extended flag? Or it may be with some initialization I need to do before running the kernel module???

Thank you in advance to anyone who can help.

`if (nbytes < sizeof(struct can_frame)) {` <<-- this is not an error; it is perfectly legal for read() to return less than its third argument. — wildplasser, Jun 11 '21 at 08:36
@wildplasser: While nbytes can even be negative, in SocketCAN having nbytes different then size of can_frame means that we have incomplete can frame - so it is an error. — avra, Sep 03 '21 at 06:38

score 0 · Accepted Answer · answered Jun 11 '21 at 08:31

Your understanding of CAN flags and filtering is not correct. Take a look at extract from linux can.h:

/* special address description flags for the CAN_ID */
#define CAN_EFF_FLAG 0x80000000U /* EFF/SFF is set in the MSB */
#define CAN_RTR_FLAG 0x40000000U /* remote transmission request */
#define CAN_ERR_FLAG 0x20000000U /* error message frame */

/* valid bits in CAN ID for frame formats */
#define CAN_SFF_MASK 0x000007FFU /* standard frame format (SFF) */
#define CAN_EFF_MASK 0x1FFFFFFFU /* extended frame format (EFF) */
#define CAN_ERR_MASK 0x1FFFFFFFU /* omit EFF, RTR, ERR flags */

Here is an example that works for both SFF and EFF messages:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

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

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

int main(int argc, char **argv)
{
    struct can_frame message;
    struct sockaddr_can addr;
    struct ifreq ifr;
    int   fd = -1;                  // file descriptor (itÂ´s a socket)

    if((fd = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0)
    {
        printf("cannot open socket");
        return -9;
    }
    strcpy(ifr.ifr_name, "vcan0");
    ioctl(fd, SIOCGIFINDEX, &ifr);
    addr.can_family = AF_CAN;
    addr.can_ifindex = ifr.ifr_ifindex;
    if(bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0)
    {
        printf("cannot bind socket\n");
        return -1;
    }

    u_int8_t nbytes;

    message.can_id |= CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_EFF_MASK;

    while(1)
    {
        nbytes = read(fd, &message, sizeof(struct can_frame));

        if (nbytes < 0) {
                perror("can raw socket read");
                return -2;
        }

        /* paranoid check ... */
        if (nbytes < sizeof(struct can_frame)) {
                fprintf(stderr, "read: incomplete CAN frame\n");
                return -3;
        }

        printf("READ COB_ID: %x\n", message.can_id & CAN_EFF_MASK);

    }

    return 0;
}

Now sending these messages:

cansend vcan0 00000123#FFFFFFFFFFFFFFFF
cansend vcan0 12345678#FFFFFFFFFFFFFFFF

gives correct output:

READ COB_ID: 123
READ COB_ID: 12345678

this is correct, I did found out about the problem and solved the issue. thank you for your answer to it. — homer69, Sep 01 '21 at 09:16
@Claudio: Then could you please mark my answer as a correct one? — avra, Sep 03 '21 at 06:34

SocketCAN read from socket only returns 11cobid

1 Answers1