Re: container_of

[Greg KH <greg@kroah.com>]

On Tue, Dec 23, 2008 at 07:01:59AM +0100, Julia Lawall wrote:
> On Mon, 22 Dec 2008, Greg KH wrote:
> 
> > On Fri, Dec 19, 2008 at 06:56:46PM +0100, Julia Lawall wrote:
> > > Considering code such as the following (drivers/acpi/osl.c):
> > > 
> > >  	struct acpi_os_dpc *dpc = container_of(work, struct acpi_os_dpc, work);
> > > 	if (!dpc) {
> > >  		printk(KERN_ERR PREFIX "Invalid (NULL) context\n");
> > >  		return;
> > >  	}
> > > 
> > > Is it very likely that container_of can return NULL?  Container_of 
> > > computes an offset from a pointer, so I have the impression that if given 
> > > a NULL value it would normally return a negative number (or a very large 
> > > positive one, depending on how it is interpreted).
> > 
> > You are correct, that check will never happen, and should be removed.
> 
> Actually, someone else pointed out that one could call the function with 
> &x->fld, and if x is NULL, then container_of(&x->fld,...) would go back 
> down to NULL.  I haven't looked at the call sites though to see if that 
> can happen.

No, if you were to evaluate x->fld, you would oops :)

So again, the check is pointless.  And I say this as someone who has
written such checks many times in the past...

thanks,

greg k-h