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

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Wed, Mar 21, 2018 at 03:19:22PM +0000, Daniel P. Berrangé wrote:
> 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.
> 

This is exactly what this patch proposes, with a uniform scheme:
start everything with qemu/.

> 
> > 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.

We can not know that there are no system headers that start with block/ on
any current or future systems.

> > 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.

I think you are mistaken on the last point:
1. Everything will be under qemu/ so we never clash with a system file
2. A local stale file anywhere in source directory is completely ignored
   since source is not on -I path.

I hope this clarifies things.

> 
> Regards,
> Daniel
> -- 
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