Access 64-bit driver's ioctl from 32-bit Application

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If you try to access ioctls of 64-bit driver from your 32-bit application, it will fail with Error -ENOTTY.  This is because the value of IOCTL Command will be different when the application is compiled for 32-bit and 64-bit.

In order for 32-bit application, you need to define .compat_ioctl in your file_operations structure.

When you call the ioctl(fd, IOCTL_COMMAND, &arg) from a 32-bit application on a 64-bit kernel, the kernel will divert the ioctl to the .compat_ioctl function in the file_operations struct.

.unlocked_ioctl is used when 64-bit application accesses 64-bit kernel.

static struct file_operations driver_fops =
{
    .owner = THIS_MODULE,
    .open = my_open,
    .release = my_close,
    .compat_ioctl = my_ioctl
    .unlocked_ioctl = my_ioctl
};

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