Re: error: template with C linkage

["Kevin P. Fleming" <kpfleming at digium dot com>]

On 09/19/2011 02:19 PM, Ian Lance Taylor wrote:
> Marc Glisse<marc.glisse@inria.fr>  writes:
>
> > On Sun, 18 Sep 2011, Jeffrey Walton wrote:
> >
> > > On Sat, Sep 17, 2011 at 4:21 PM, Jonathan Wakely<jwakely.gcc@gmail.com>  wrote:
> > > > On 17 September 2011 14:23, Jeffrey Walton wrote:
> > > > > /usr//include/cryptopp/cryptlib.h:99: error: template with C linkage
> > > > ...
> > > > > I've tried adding '-x c++' to force c++ in the files.
> > > >
> > > > That's obviously not going to help.
> > > >
> > > > If the compiler complains about a template, it's not compiling C,
> > > > because there's no such thing in C.  Ditto for C linkage; everything
> > > > in C has C linkage, so there's no concept of any other kind of
> > > > language linkage, so no reason a C compiler would ever mention "C
> > > > linkage".  So the compiler was already treating the files as
> > > > containing C++ (just apparently assuming that everything in a system
> > > > directory gets an implicit extern "C" around it.)
> > > OK, thanks. I'm not sure what the implications of "system compiler"
> > > are, but OpenBSD's configuration did not like those files in
> > > /usr/include.
> > >
> > > $ g++ -v
> > > Reading specs from /usr/bin/../lib/gcc-lib/i386-unknown-openbsd4.9/4.2.1/specs
> > > Target: i386-unknown-openbsd4.9
> > > Configured with: OpenBSD/i386 system compiler
> > > Thread model: posix
> > > gcc version 4.2.1 20070719
> > > $
> > >
> > > Do you have any ideas why a location would make a difference?
> >
> > Antique platforms have system headers that don't know about C++, so
> > g++ implicitly adds extern "C" around them. And being a system header
> > is a property that depends on the directory the file is in.
>
> This approach is controlled on a target-by-target basis, by the way.  If
> OpenBSD's header files are C++ ready, gcc's OpenBSD target configuration
> should be changed to define NO_IMPLICIT_EXTERN_C.

Does '-isystem' factor into this in any way? I would hope that using 
'-isystem' to specify an additional directory of 'system' header files 
would not also turn on implicit 'extern "C"' mode.

If you are curious, we use '-isystem' to have GCC relax our 
warning/error modes on header files that are nominally part of our 
project but are installed into 'system' locations (like 
/opt/<package>/include).

--
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming@digium.com | SIP: kpfleming@digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org