Re: [git pull] core kernel updates for v2.6.28

From: Linus Torvalds
Date: Thu Oct 16 2008 - 18:57:58 EST

On Fri, 17 Oct 2008, Frédéric Weisbecker wrote:
> 2008/10/17 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>:
> > The fact is, that second argument was a "ptrdiff_t", which is neither
> > "int" nor "long". It should be "%td" I think. But the thing is, when you
> > fix a warning, you should damn well know what the hell you're doing, not
> > just shut it up.
> Sorry, I made some falses assumptions about the printed type I guess...

Well, the thing is, on 32-bit x86, ptrdiff_t is "int". And on 64-bit, it's
"long". And on some (most?) other architectures, it's "long" regardless of
whether it's 32-bit or 64-bit.

So you fixed a warnign on x86-32, but you introduced it just about
everywhere else.

And it so happens that the old use of "%ld" was better than "%d", because
regardless of the exact type of ptrdiff_t, with gcc it is essentially
always going to be at least the same _size_ as "long". IOW, even when it's
"int", it will always print out correctly with "%ld", despite the format
warning. IOW, the type may be "wrong" from a C standards standpoint, but
it will work in practice.

In contrast, using "%d" can actually print it out wrong, because it will
be literally the wrong physical size, not just a type issue on a C level.
So depending on calling conventions, you might end up with the upper bits
cleared, or even the wrong bits printed out.

Using "%td" is always right, assuming the underlying printing library is
recent enough to know about it. And the kernel has known about %td for the
last three years.

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