linux-kernel - Re: Userspace compiler support of "long long"

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Message-Id: <A934E115-A126-45A5-AE5F-E19215DEFF5E@mac.com>
Date:	Thu, 28 Jun 2007 08:18:50 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Matthew Wilcox <matthew@....cx>, Adrian Bunk <bunk@...sta.de>,
	LKML Kernel <linux-kernel@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>, david@...g.hm,
	Andi Kleen <ak@...e.de>, linux-arch@...r.kernel.org
Subject: Re: Userspace compiler support of "long long"

On Jun 28, 2007, at 08:08:03, Jakub Jelinek wrote:
> On Thu, Jun 28, 2007 at 07:53:51AM -0400, Kyle Moffett wrote:
>> Oh, ok, that makes it even easier to say this with certainty:  
>> Changing the other 64-bit archs to use "long long" for their 64- 
>> bit numbers will not cause additional warnings.  I'm also almost  
>> certain there are no architectures which use "long long" for 128- 
>> bit integers. (Moreover, I can't find hardly anything which does  
>> 128-bit integers at all).
>
> unsigned long and unsigned long long have the same size, precision  
> and alignment on all LP64 arches, that's true.  But they have  
> different ranks and more importantly they mangle differently in C+ 
> +.  So, whether some user exposed type uses unsigned long or  
> unsigned long long is part of the ABI, whether that's size_t,  
> uintptr_t, uint64_t, u_int64_t or any other type, you can't change  
> it without breaking the ABI.

That sounds *extraordinarily* broken.  Hopefully this would *not*  
affect the type of a function which is passed a C "struct" containing  
the "long long", right?

Hmm, I guess the question is:  Do we support people directly passing  
__u64 to C++ functions in userspace?  I could understand, perhaps,  
passing around structures defined in the kernel headers, but  
certainly not the kernel-internal types.  The only reason we even  
export those is so we can have a private set of bit-size-defined  
types with which to define kernel ABI structures.

Cheers,
Kyle Moffett

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