Re: [PATCH v2] qemu: replace "" with <> in headers

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>,
	Dmitry Fleytman <dmitry.fleytman@gmail.com>,
	Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>,
	Li Zhijian <lizhijian@cn.fujitsu.com>,
	David Hildenbrand <david@redhat.com>,
	Jeff Cody <jcody@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	qemu-devel@nongnu.org, BALATON Zoltan <balaton@eik.bme.hu>,
	Keith Busch <keith.busch@intel.com>,
	Max Filippov <jcmvbkbc@gmail.com>,
	Hannes Reinecke <hare@suse.com>,
	Gerd Hoffmann <kraxel@redhat.com>, Fam Zheng <famz@redhat.com>,
	Max Reitz <mreitz@redhat.com>, Eric Blake <eblake@redhat.com>,
	Josh Durgin <jdurgin@redhat.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Alberto Garcia <berto@igalia.com>,
	zhanghailiang <zhang.zhanghailiang@huawei.com>,
	Ben Warren <ben@skyportsystems.com>,
	Stefan Berger <stefanb@linux.vnet.ibm.com>,
	Yongbok Kim <yongbok.kim@mips.com>,
	Michael Roth <mdroth@linux.vnet.ib
Subject: Re: [PATCH v2] qemu: replace "" with <> in headers
Date: Wed, 21 Mar 2018 15:19:22 +0000	[thread overview]
Message-ID: <20180321151922.GR8551@redhat.com> (raw)
In-Reply-To: <1521642402-197739-1-git-send-email-mst@redhat.com>

On Wed, Mar 21, 2018 at 04:46:32PM +0200, Michael S. Tsirkin wrote:
> Our current scheme is to use
>  #include ""
> for internal headers, and
>  #include <>
> for external ones.
> 
> Unfortunately this is not based on compiler support: from C point of
> view, the "" form merely looks up headers in the current directory
> and then falls back on <> directories.
> 
> Thus, for example, a system header trace.h - should it be present - will
> conflict with our local trace.h

If our local "trace.h" is in the current directory, then using ""
is right and you can still use <trace.h> to get the system version.

If our local trace.h is in include/ top level, then it is going to
block use of the system trace.h regardless of whether we use <> or ""

Fortunately our include/ tree uses sub-dirs, so we would typically
use  #include "$subdir/trace.h" and  #include <trace.h> would still
find the system header.

We just have to be careful we don't add stuff at the top level of
our include/ dir with names that are liable to clash. This might
suggest renaming  include/elf.h to include/qemu/elf.h, or just
moving elf.h to the qemu/ subdirectory. Likewise include/glib-compat.h
might be better moved to qemu/ subdirectory.

> As another example of problems, a header by the same name in the source
> directory will always be picked up first - before any headers in
> the include directory.

There's only a couple of headers in the top level of our include/
directory - everything else is pulled in with a named path
eg #include "block/block_int.h", so that would not conflict with
reference to a bare #include "block_int.h" from the current directory.

> Let's change the scheme: make sure all headers that are not
> in the source directory are included through a path
> starting with qemu/ , thus:
> 
>  #include <>
> 
> headers in the same directory as source are included with
> 
>  #include ""
> 
> as per standard.

As stated before, I consider this a step backwards - it is a
good clear standard to use "" for project local includes and
<> for 3rd party / system includes IMHO. The change doesn't
do anything beneficial for the two scenarios described above
AFAICT.

Regards,
Daniel
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