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

[Jeff Newmiller]

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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