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Re: [Libguestfs] [PATCH 5/7] New APIs: add-domain and add-libvirt-dom.



On Wed, Nov 10, 2010 at 12:47:55PM +0000, Richard W.M. Jones wrote:
> On Wed, Nov 10, 2010 at 12:26:03PM +0000, Daniel P. Berrange wrote:
> > On Wed, Nov 10, 2010 at 12:11:22PM +0000, Richard W.M. Jones wrote:
> > > On Wed, Nov 10, 2010 at 11:58:40AM +0000, Daniel P. Berrange wrote:
> > > > On Wed, Nov 10, 2010 at 11:46:31AM +0000, Richard W.M. Jones wrote:
> > > > > diff --git a/src/guestfs.h b/src/guestfs.h
> > > > > index 0f4f9fd..be7398a 100644
> > > > > --- a/src/guestfs.h
> > > > > +++ b/src/guestfs.h
> > > > > @@ -79,6 +79,11 @@ extern void guestfs_set_private (guestfs_h *g, const char *key, void *data);
> > > > >  extern void *guestfs_get_private (guestfs_h *g, const char *key);
> > > > >  
> > > > >  /*--- Structures and actions ---*/
> > > > > +
> > > > > +/* Hack to avoid needing to include <libvirt.h> */
> > > > > +typedef struct _virDomain virDomain;
> > > > > +typedef virDomain *virDomainPtr;
> > > > > +
> > > > >  #include <rpc/types.h>
> > > > >  #include <rpc/xdr.h>
> > > > >  #include <guestfs-structs.h>
> > > > 
> > > > Surely this will break any application whose code does
> > > > 
> > > >   #include <libvirt/libvirt.h>
> > > >   #include <guestfs.h>
> > > > 
> > > > because they'll get a duplicated definition of virDomain from
> > > > both headers. 
> > > 
> > > I checked and it works fine both ways round.  It would break if
> > > libvirt changed the definition, but you've boxed yourself into a
> > > corner there by documenting the current definition:
> > 
> > Surely the compiler should complain about duplicated typedefs, even
> > if they are identical, because C doesn't allow duplicate typedefs
> > within the same scope.
> > 
> >   http://david.tribble.com/text/cdiffs.htm#C99-typedefs
> > 
> > eg
> > 
> >   $ cat st.c 
> >   typedef struct foo foo;
> >   typedef struct foo foo;
> >   $ gcc -Wall -o st st.c
> >   st.c:3:20: error: redefinition of typedef ‘foo’
> >   st.c:2:20: note: previous declaration of ‘foo’ was here
> 
> Apparently "scope" there means something a bit different.  Try this
> program to see it definitely works (and also swap the order of the
> include and the typedef):
> 
> ----------------------------------------------------------------------
> #include <stdio.h>
> #include <stdlib.h>
> 
> #include <libvirt/libvirt.h>
> typedef struct _virDomain virDomain;
> typedef virDomain *virDomainPtr;
> 
> int main () { printf ("hello\n"); exit (0); }
> ----------------------------------------------------------------------
> 
> gcc -Wall -o test test.c
> 
> It doesn't need the #ifndef as suggested by that link.

This rather surprises me. I wonder if this is just an accident
of the current GCC warning checker impl, or actually the correct
standards compliant behaviour. I've always been under the impression
that all header files had to have all their typedefs/functions/etc
protected by #ifndef FOO/#define FOO/...content.../#endif to
prevent multiple declaration.

Daniel
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