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Re: [qemu-s390x] [PATCH v2] qemu: replace "" with <> in headers

From: Daniel P . Berrangé
Subject: Re: [qemu-s390x] [PATCH v2] qemu: replace "" with <> in headers
Date: Wed, 21 Mar 2018 15:19:22 +0000
User-agent: Mutt/1.9.2 (2017-12-15)

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

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