Re: Warning for incompatible functions declared 'extern "C"'?

[JÄdrzej Dudkiewicz <jedrzej dot dudkiewicz at gmail dot com>]

On Fri, Jul 25, 2014 at 6:47 AM, Nick <nospam@codesniffer.com> wrote:
> On Thu, 2014-07-24 at 09:19 +0100, Jonathan Wakely wrote:
>> That is valid C++ code. C linkage doesn't mean the function has to
>> actually be usable from C.
>>
>> You can also declare functions with C linkage that take reference
>> parameters or throw exceptions.
>
> I didn't know that.  Out of curiosity...I assumed that one of the
> ramifications of declaring some code w/ C linkage was that it emitted
> symbols that have C mangling.  Is that true?  If so, how are C++ antics
> handled in this scenario?

Look at this:

[fedora@facs-bugtracker ~]$ cat y.cc
#include <string>

extern "C" {
        std::string GetVersion() { return "1"; }
}


[fedora@facs-bugtracker ~]$ objdump -t y.o

y.o:     file format elf64-x86-64

SYMBOL TABLE:
[....]
0000000000000000 g     F .text  0000000000000061 GetVersion
[....]

Name GetVersion is not mangled, so it's a bare name. Note, that if you
want to use it elsewhere, you'd have to declare it somehow. So if
GetVersion returns std::string, your header would wound up with
"#include <string>" and it won't be understood by C compiler, or you
could declare it as something else, but then you'd be lying to the
compiler, and then you're on your own. But if it's included in C++
sources, it will be ok, since function would be declared with proper
return value and it will be annotated as having C linkage, so C++
expects something that is called 'GetVersion' and returns std::string
- so everything is ok.

Of course it won't work if GetVersion is overloaded:

              ^
[fedora@facs-bugtracker ~]$ cat y.cc
#include <string>

extern "C" {
        std::string GetVersion() { return "1"; }
        int GetVersion(bool as_int) { return 1; }
}


[fedora@facs-bugtracker ~]$ g++ y.cc -c
y.cc: In function âint GetVersion(bool)â:
y.cc:5:28: error: declaration of C function âint GetVersion(bool)â
conflicts with
  int GetVersion(bool as_int) { return 1; }
                            ^
y.cc:4:14: error: previous declaration âstd::string GetVersion()â here
  std::string GetVersion() { return "1"; }

Please forgive me the most idiotic overload ever.
-- 
JÄdrzej Dudkiewicz

I really hate this damn machine, I wish that they would sell it.
It never does just what I want, but only what I tell it.