Error compiling kernel module for simple device driver

Question

I've got such a function:

void cleanup_module(void)
{
    /* 
     * Unregister the device 
     */
    if(unregister_chrdev(Major, DEVICE_NAME)<0) 
        printk(KERN_ALERT "Error in unregister_chrdev:\n");
}

and error:

/home/student/kernel/hello.c: In function ‘cleanup_module’:
/home/student/kernel/hello.c:39:2: error: void value not ignored as it ought to be

This line is the one with if statement. Do you know what I'm doing wrong?

Answer 1

This means that unregister_chrdev doesn't have a return value (it's void ), but you have put it in an if. That is, you are using a void value which should have been ignored. Hence the error message.

Check out this question which asks why the return value was changed to void.

Answer 2

Based on this unregister_chrdev() used to return an int but its return type was changed to void as the returned value was meaningless. Remove the if entirely from the posted code:

unregister_chrdev(Major, DEVICE_NAME);

Answer 3

The error indicates that the unregister_chrdev() function is a void type function, ie it returns nothing. However, you are checking its return value with the < operator.