linux-kernel - Re: [PATCH] ptrdiff_t is not uintptr

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Date:	Sun, 19 Aug 2007 17:57:56 -0700
From:	David Brownell <david-b@...bell.net>
To:	Anton Altaparmakov <aia21@....ac.uk>
Cc:	Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ptrdiff_t is not uintptr_t, damnit

On Sunday 19 August 2007, Anton Altaparmakov wrote:
> >
> > ISTR we don't *have* a uintptr_t on all architectures, or that would
> > be the appropriate thing to use in these 32/64 bit ABI scenarios.
> >
> >
> >> Use unsigned long or uintptr_t instead.
> >
> > I suspect you mean "unsigned long long"...
> 
> No he doesn't.  "unsigned long" is guaranteed to be large enough to  
> hold a pointer (at least on Linux anyway).

And yet when I used that, I got compiler warnings on some systems.

ISTR that was the first solution I tried, but GCC really wanted to
issue warnings.  Either about casting 64-bit to pointer, or about
casting it to "unsigned long", either way lost precision.

> On a 32-bit arch "unsigned long" is 32-bit and pointers are 32-bit.
> 
> On a 64-bit archi "unsigned long" is 64-bit and pointers are 64-bit.

So with 32 bit userspace "unsigned long long" is the type to use
when talking to a 64-bit kernel; and with pure 64-bit code, it's
enough to write "unsigned long".

I'm fairly sure that's the root cause of the pain I recall here;
but I'd have to run experiments again to verify that.  

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