linux-kernel - Re: Redefinition of struct in6_addr in <netinet/in.h> and <linux/in6.h>

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Message-ID: <1358356211.2923.25.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Wed, 16 Jan 2013 17:10:11 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Mike Frysinger <vapier@...too.org>
CC:	<libc-alpha@...rceware.org>,
	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>,
	Cong Wang <amwang@...hat.com>,
	Thomas Backlund <tmb@...eia.org>,
	Eric Blake <eblake@...hat.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <libvirt-list@...hat.com>,
	<tgraf@...g.ch>, David Miller <davem@...emloft.net>,
	<schwab@...e.de>, <carlos@...temhalted.org>
Subject: Re: Redefinition of struct in6_addr in <netinet/in.h> and
 <linux/in6.h>

On Wed, 2013-01-16 at 12:04 -0500, Mike Frysinger wrote:
> On Wednesday 16 January 2013 10:47:12 Ben Hutchings wrote:
> > On Wed, 2013-01-16 at 23:21 +0900, YOSHIFUJI Hideaki wrote:
> > > Cong Wang wrote:
> > > > (Cc'ing some glibc developers...)
> > > > 
> > > > Hello,
> > > > 
> > > > In glibc source file inet/netinet/in.h and kernel source file
> > > > include/uapi/linux/in6.h, both define struct in6_addr, and both are
> > > > visible to user applications. Thomas reported a conflict below.
> > > > 
> > > > So, how can we handle this? /me is wondering why we didn't see this
> > > > before.
> > 
> > [...]
> > 
> > > This is not a new issue.  In addition to this,
> > > netinet/in.h also conflits with linux/in.h.
> > > 
> > > We might have
> > > 
> > >  #if !defined(__GLIBC__) || !defined(_NETINET_IN_H)
> > >  
> > >  #endif
> > > 
> > > around those conflicting definitions in uapi/linux/in{,6}.h.
> > 
> > This only solves half the problem, as <netinet/in.h> might be included
> > after <linux/in.h>.  Also, not all Linux userland uses glibc.
> 
> certainly true, but the current expectation is that you don't mix your ABIs.

Whose expectation?  Which ABIs are being mixed?

> if you're programming with the C library API, then use the C library headers.  
> if you're banging directly on the kernel, then use the kernel headers.  not 
> saying it's a perfect solution, but it works for the vast majority of use 
> cases.

In practice most C programs for Linux will use a mixture of thinly
wrapped system calls and higher-level APIs from the C library, and never
really call the kernel directly (as that requires inline assembler).
Userland programmers will work around this historical mess by tweaking
the #include order or splitting source files.  But they shouldn't have
to.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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