Video4Linux ioctl error (#25) when attempting to read device information from /dev/video0

I am currently attempting to retrieve device information for a built in web-cam using the following code:

#include <fcntl.h>
#include <unistd.h>
#include <linux/media.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

int main(int argc, char **argv) {
   int fd = open("/dev/video0", O_RDONLY, 0);
   if (fd > 0) {
       struct media_device_info *device_data = (struct media_device_info *) malloc (sizeof(struct media_device_info) * 1);

       if (ioctl(fd, MEDIA_IOC_DEVICE_INFO, device_data) ==  0)
            printf("Media Version: %u\nDriver: %s\nVersion: %d\nSerial: %s\n", (unsigned int) device_data->media_version, device_data->driver, (int) device_data->driver_version, device_data->serial);
        else {
               fprintf(stderr, "Couldn't get device info: %d: %s\n", errno, strerror(errno));
       }

       close(fd);
       free(device_data);
    }

    return 0;
}

When the code executes the else block is entered thus giving the following:

Couldn't get device info: 25: Inappropriate ioctl for device

From this it would seem that the device is being opened in the wrong manner such that ioctl cannot use the file descriptor. I must be missing something; could anyone here help me with regards to opening the /dev/video0 device?

Thanks!

p.s. If this has been answered before elsewhere please let me know. If this question is invalid in anyway then please accept my apologies.

Solution

It seems that the /dev/video* devices may be bound to separate /dev/media* devices, and you need to issue your MEDIA_IOC_DEVICE_INFO ioctl against the corresponding /dev/media* device for your /dev/video* device.

As to how to locate that corresponding device id, the best I have come up with is to search for media* files within the /sys/class/video4linux/video{N}/device directory.

For example, for a given device /dev/video0 on my system (kernel 4.15.0-34-generic), searching for media* files under /sys/class/video4linux/video0/device turned up media10, which I was then able to use to recover the serial number (open /dev/media10, issue the ioctl command).

I don't know whether this method of finding the corresponding media devices is consistent across distros/versions/kernels/etc.