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

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Date:	Mon, 20 Aug 2007 08:31:47 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	David Brownell <david-b@...bell.net>
cc:	Anton Altaparmakov <aia21@....ac.uk>,
	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 Sun, 19 Aug 2007, David Brownell wrote:

> 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).

Yup, sizeof(long) >= sizeof(void *) should always be true in Linux C.
I bet a lot of code out there depends on this.

BTW, just curious to know, but which (if any) are the platforms that have
sizeof(long) > sizeof(void *)? [i.e. greater-than but not equal?]

The reason I ask is that gcc will also complain (understandably so) with
"warning: cast from pointer to integer of different size" i.e. even if
it's a conversion from smaller size to greater size, and not really a
case of truncation. Therefore, I wonder if the stricter assertion:
sizeof(long) == sizeof(void *) holds true for Linux C, actually.

> 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.

Hmm, if you could fish out those testcases ...

> > 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.  

I suspect the root cause of the pain was that you used "int" or "long"
to talk between kernel and userspace in the first place. You shouldn't,
we have __u32 / __u64 / etc for that.

Satyam