Re: Re: [PATCH resent] uapi libc compat: allow non-glibc to opt out of uapi definitions

On Wed, Mar 08, 2017 at 07:51:29PM -0500, Carlos O'Donell wrote:

On 03/08/2017 07:14 PM, Szabolcs Nagy wrote:

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* Carlos O'Donell [off-list ref] [2017-03-08 10:53:00 -0500]:

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On 11/11/2016 07:08 AM, Felix Janda wrote:

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fixes the following compiler errors when <linux/in6.h> is included
after musl <netinet/in.h>:

./linux/in6.h:32:8: error: redefinition of 'struct in6_addr'
./linux/in6.h:49:8: error: redefinition of 'struct sockaddr_in6'
./linux/in6.h:59:8: error: redefinition of 'struct ipv6_mreq'

Do you have plans for fixing the error when the inclusion order is the other way?

the other way (linux header included first) is
problematic because linux headers don't follow
all the standards the libc follows, they violate
namespace rules in their struct definitions, so
the libc definitions are necessarily incompatible
with them and thus different translation units can
end up refering to the same object through
incompatible types which is undefined.
(even if the abi matches and thus works across
the syscall interface, a sufficiently smart
toolchain can break such code at link time,
and since the libc itself uses its own definitons
that's what user code should use too).

there should be a way to include standard conform
libc headers and linux headers into the same tu,
at least the case when all conflicting definitions
come from the libc should work and i think that
should be the scope of these libc-compat.h changes.
(of course if glibc tries to support arbitrary
interleavings then the changes should not break that)

You can get non-standard defines even when including the
linux headers _after_ libc headers because linux headers
should rightly continue to define things that are required
for linux-specific applications.

IMO the fact that the UAPI headers may cause problems with
standards conformance is orthogonal to the discussion of 
_how_ we fix inclusion order issues.

Some of the network headers can be used in relative safety
and need to be used for some applications. It is those cases
where I'd like to see an inclusion guard design that works
for both inclusion orders.

The issue has been discussed on our side (musl) and our position so
far is that we don't want to try to support the case of including the
kernel headers before the libc headers, at least not at this time.
It's a big rabbit hole of stuff that could go wrong. This doesn't
preclude the kernel folks trying to make things so that it _can_ be
supported more smoothly.

Rich