[vox-tech] inline keyword solves "undefined reference" issue

[Hai Yi]

thanks Jeff, I wrapped the #include with

#ifdef __cplusplus
extern "C"{
#endif

...

#ifdef __cplusplus
}
#endif

and the inline problem is gone, it's no long needed, the app works like a
charm!

Thought: I did wrap with "extern C{}" before, but I didn't put the
conditional macro (#ifdef __cplusplus) then as I did it now, guess I used it
the wrong way then.

Thanks again, Hai


On Wed, Jun 9, 2010 at 2:31 AM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us>wrote:

> Hai Yi wrote:
> > Thanks Jeff. However, the header file of those C routines are included
> > in ffmpegmgr.h, which is in turn included by ffmpegmgr.cpp.
> >
> > The "undefined reference " normally indicates that the functions's
> > definitions are not found, and it takes place when doing the linker.
>
> I am aware of that.  However, if you don't have your calls properly
> declared
> then the linking may fail.
>
> > The
> > libraries, libavcodec, etc, have been specified from the g++ command
> > line, and seems to work - only that I need to make the encapulated
> > member function in ffmpegmgr.cpp inline to eliminate the linker error:
> >
> > inline void FFMpegMgr::new_video_stream(AVFormatContext *oc)
> > {
> >    ...
> >     st = av_new_stream(oc, oc->nb_streams);
> >    ...
> > }
>
> I cannot explain that.  Adding the inline keyword to new_video_stream
> eliminates new_video_stream from the linking process (being placed inline,
> there is no need to link it), but should have no effect on the link-ability
> of av_new_stream.
>
> Are you wrapping your C declarations as seen from C++ in extern "C" {}
> blocks? This is often done by wrapping the #include statement for the C
> header file in your cpp or C++ header files.
>
> > Also, the lib that I specified is *.a, will it be different if the lib
> > is *.so ?
>
> If you declare your objects correctly, there should be no need for source
> code changes between static and shared library linking.
>
> > Thanks,
> > Hai
> >
> >
> > On Mon, Jun 7, 2010 at 1:26 AM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us
> > <mailto:jdnewmil at dcn.davis.ca.us>> wrote:
> >
> >     Seems more likely that you haven't included the declarations for the
> >     C routines in your cpp file. Would need to see more context to
> diagnose.
> >
> >     "Hai Yi" <yihai2004 at gmail.com <mailto:yihai2004 at gmail.com>> wrote:
> >
> >      >Hello all:
> >      >
> >      >I created a class FFMpegMgr and a method init(); inside init() I
> >     just put
> >      >three ffmpeg api there:
> >      >avcodec_register_all();
> >      >avdevice_register_all();
> >      > av_register_all();
> >      >
> >      >The source file ffmpegmgr.cpp look like this:
> >      >
> >      > void FFMpegMgr::init() {
> >      >    avcodec_register_all();
> >      >    avdevice_register_all();
> >      >    av_register_all();
> >      >
> >      >}
> >      >
> >      >Now I got a "undefined reference to avcodec_register_all" error.
> >      >
> >      >if I put that body in the class declaration or add a "inline"
> >     keyword, that
> >      >problem was gone and the compilation succeeded.
> >      >
> >      >question is, do those three functions, which are C functions
> >     defined in
> >      >avcodec.h,  have to be used only in the inline functions in C++?
> >      >
> >      >Sorry if it appear naive - I've been out of touch with C/C++ for a
> >     while.
> >      >
> >      >Thanks!
> >      >Hai
>
>
> --
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