Find bus number and device number with device file symlink

Question

I have a device file(SYMLINK) /dev/CDMAModem generated by a udev rule. I want to find the bus number and device number of the actual device. Actually I want to perform USBDEVFS_RESET ioctl on device /dev/bus/usb/BUS_NO/DEVICE_NO in my C++ program.

----udev rule----

SUBSYSTEMS=="usb", ACTION=="add", DRIVERS=="zte_ev", ATTRS{bNumEndpoints}=="03", SYMLINK+="CDMAModem"
SUBSYSTEMS=="usb", ACTION=="remove", DRIVERS=="zte_ev", ATTRS{bNumEndpoints}=="03", SYMLINK-="CDMAModem"

Answer 1

I think libudev will give you that:

#include <libudev.h>
#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
#include <unistd.h>

int main(int argc, char **argv)
{

    struct udev *udev;
    struct udev_enumerate *enumerate;
    struct udev_list_entry *devices, *dev_list_entry;
    struct udev_device *dev;

    udev = udev_new();

    enumerate = udev_enumerate_new(udev);
    udev_enumerate_add_match_subsystem(enumerate, "CDMAModem");
    udev_enumerate_scan_devices(enumerate);
    devices = udev_enumerate_get_list_entry(enumerate);

    udev_list_entry_foreach(dev_list_entry, devices) {
        const char *path;
    
        path = udev_list_entry_get_name(dev_list_entry);
        dev = udev_device_new_from_syspath(udev, path);

        fprintf(stderr, "devnum: %s\n",
            udev_device_get_sysattr_value(dev, "devnum"));
        fprintf(stderr, "busnum: %s\n",
            udev_device_get_sysattr_value(dev, 'busnum:));
        udev_device_unref(dev);
    }

    udev_enumerate_unref(enumerate);
    udev_unref(udev);

    return 0;
}

You can then use this information with ioctl() as in:

[charles@localhost 2-1]$ cd /sys/class/mem/random
[charles@localhost 2-1]$echo $PWD
/sys/devices/pci0000:00/0000:00:11.0/0000:02:00.0/usb2/2-1
             
            

Answer 2

You can perform an ioctl on the file represented by the symlink /dev/CDMAModem as you would on the file under the /dev/bus/ structure.

#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/usbdevice_fs.h>

int f = open("/dev/CDMAModem", O_RDWR);
ioctl(f, USBDEVFS_RESET);

If you actually want to find where this link is pointing, the file command will tell you.

> file /dev/CDMAModem
/dev/CDMAModem: symbolic link to `bus/usb/BUS/DEV'