On Tue, Jan 09, 2007 at 19:14:10 +0300, Tomasz Kvarsin wrote:
> Let's assume that I write function that will return
> struct buffer_head *, and want also signal about error
> can I use ERR_PTR mechanizm for this?
>
> And more general question, when ERR_PTR bad decision,
> and when not?
If I recall correctly (someone please correct me if not -- I only used it in
2.4), the virtual addresses at the very end will never be allocated by
kmem_cache_alloc (or kmalloc, which is the same) and can therefore be used to
represent small integeral error codes, namely 1 to 999. Errno constants are
known to statisfy that restriction. So if you want to return buffer_head or
errno value, you can use ERR_PTR. For other kinds of error values it depends
whether they are small enough.
> I see some places in kernel, with such functions:
>
> struct foo *f(int *err)
> {
> *err = ESOMERR;
> return NULL;
> }
>
> why so?
Either because the pointer is not necessarily pointer to kernel internal
structure, or, more likely, because whoever wrote simply didn't realize he
could use the ERR_PTR trick. In fact I rarely saw such trick outside of linux
kernel, though small positive numbers would work well in userland (in fact
I think "small positive integers" are required to be invalid pointers so that
((struct whatever *)NULL).member is caught).
--
Jan 'Bulb' Hudec <bulb@xxxxxx>
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