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• g++ Linking Error on Mac while compiling FFMPEG

  7 mai 2013, par Saptarshi Biswas

  g++ on Snow Leopard is throwing linking errors on the following piece of code

  test.cpp

  #include <iostream>
  
  using namespace std;
  
  #include <libavcodec></libavcodec>avcodec.h>    // required headers
  
  #include <libavformat></libavformat>avformat.h>
  
  int main(int argc, char**argv) {
  
      av_register_all();             // offending library call
  
      return 0;
  
  }
  
  </iostream>

  When I try to compile this using the following command

  g++ test.cpp -I/usr/local/include -L/usr/local/lib \
  
  -lavcodec -lavformat -lavutil -lz -lm -o test

  I get the error
  Undefined symbols :
  "av_register_all()", referenced from :
  _main in ccUD1ueX.o
  ld : symbol(s) not found
  collect2 : ld returned 1 exit status

  Interestingly, if I have an equivalent c code,
  test.c

  #include 
  
  #include <libavcodec></libavcodec>avcodec.h>
  
  #include <libavformat></libavformat>avformat.h>
  
  int main(int argc, char**argv) {
  
      av_register_all();
  
      return 0;
  
  }

  gcc compiles it just fine

  gcc test.c -I/usr/local/include -L/usr/local/lib \
  
  -lavcodec -lavformat -lavutil -lz -lm -o test

  I am using Mac OS X 10.6.5

  $ g++ --version
  
  i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
  
  $ gcc --version
  
  i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)

  FFMPEG's libavcodec, libavformat etc. are C libraries and I have built them on my machine like thus :

  ./configure --enable-gpl --enable-pthreads --enable-shared \
  
  --disable-doc --enable-libx264
  
  make && sudo make install

  As one would expect, libavformat indeed contains the symbol av_register_all

  $ nm /usr/local/lib/libavformat.a | grep av_register_all
  
  0000000000000000 T _av_register_all
  
  00000000000089b0 S _av_register_all.eh

  I am inclined to believe g++ and gcc have different views of the libraries on my machine. g++ is not able to pick up the right libraries. Any clue ?

• Python SWIG bindings with SomeType ** as function argument

  2 mai 2013, par vericule

  I couldn't find any working Python bindings for ffmpeg, so I decided to generate one with SWIG. Generation was quick and easy (no customization, just default SWIG interface), but these a problem using some functions like int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options); from libavformat/avformat.h. Using C this can be run simply by :

  AVFormatContext *pFormatCtx = NULL;
  
  int status;
  
  status = avformat_open_input(&pFormatCtx, '/path/to/my/file.ext', NULL, NULL);

  In Python I try following :

  >>> from ppmpeg import *
  
  >>> av_register_all()
  
  >>> FormatCtx = AVFormatContext()
  
  >>> FormatCtx
  
   >
  
  >>> avformat_open_input(FormatCtx, '/path/to/my/file.ext', None, None)
  
  Traceback (most recent call last):
  
     File "<stdin>", line 1, in <module>
  
  TypeError: in method &#39;avformat_open_input&#39;, argument 1 of type &#39;AVFormatContext **&#39;
  
  </module></stdin>

  Problem is that Python do not have & equivalent. I tried to use cpointer.i and its pointer_class (%pointer_class(AVFormatContext, new_ctx)), but new_ctx() returns pointer and this is not I want definitely. %pointer_class(AVFormatContext *, new_ctx) is illegal and gives syntax error. I would be grateful for any help. Thanks.

  EDIT :
  I forgot to mention I tried to use typemaps, but don't know how to write custom typemap for struct and documentation has only examples for basic types like int or float...

• how do I write to avlibs AVOptions from C

  19 mai 2014, par vinay samuel

  I want to do the equivalent of the following command line from a C program.

  ffmpeg ..... -movflags frag_keyframe

  I have narrowed it down to setting some fields in priv_class in AVOutputFormat somehow. But I don’t understand how AVOptions work, I could not find a good example that explains how AVOptions are set and used. I did come across av_opt_set but if I set the name field to "movflags" what is the val and do I set a separate option for frag_keyframe or is it embedded in the flags set for "movflags" ?
  Thanks in advance for your time.