Linker error by trying example program with ffmpeg

Question

I think this might be a stupid question and I'm just blind but that thing is driving me nuts for hours now. I downloaded ffmpeg and build it. Now I want to try the thing out in a program but I can't setup cmake to link ffmpeg properly and have no idea what is wrong.

The linker always tells me this:

christoph@christoph-ThinkPad-T490:~/develop/ffmpg_example/build$ make
[ 50%] Linking CXX executable test
CMakeFiles/test.dir/main.cxx.o: In function `main':
main.cxx:(.text+0x180): undefined reference to `av_register_all()'
main.cxx:(.text+0x1a7): undefined reference to `avformat_open_input(AVFormatContext**, char const*, AVInputFormat*, AVDictionary**)'
main.cxx:(.text+0x1ce): undefined reference to `avformat_find_stream_info(AVFormatContext*, AVDictionary**)'
main.cxx:(.text+0x206): undefined reference to `av_dump_format(AVFormatContext*, int, char const*, int)'
main.cxx:(.text+0x2bb): undefined reference to `avcodec_find_decoder(AVCodecID)'
main.cxx:(.text+0x2fc): undefined reference to `avcodec_alloc_context3(AVCodec const*)'
main.cxx:(.text+0x316): undefined reference to `avcodec_copy_context(AVCodecContext*, AVCodecContext const*)'
main.cxx:(.text+0x361): undefined reference to `avcodec_open2(AVCodecContext*, AVCodec const*, AVDictionary**)'
main.cxx:(.text+0x377): undefined reference to `av_frame_alloc()'
main.cxx:(.text+0x383): undefined reference to `av_frame_alloc()'
main.cxx:(.text+0x3ba): undefined reference to `avpicture_get_size(AVPixelFormat, int, int)'
main.cxx:(.text+0x3d0): undefined reference to `av_malloc(unsigned long)'
main.cxx:(.text+0x3ff): undefined reference to `avpicture_fill(AVPicture*, unsigned char const*, AVPixelFormat, int, int)'
main.cxx:(.text+0x43d): undefined reference to `sws_getContext(int, int, AVPixelFormat, int, int, AVPixelFormat, int, SwsFilter*, SwsFilter*, double const*)'
main.cxx:(.text+0x465): undefined reference to `av_read_frame(AVFormatContext*, AVPacket*)'
main.cxx:(.text+0x49f): undefined reference to `avcodec_decode_video2(AVCodecContext*, AVFrame*, int*, AVPacket const*)'
main.cxx:(.text+0x4fd): undefined reference to `sws_scale(SwsContext*, unsigned char const* const*, int const*, int, int, unsigned char* const*, int const*)'
main.cxx:(.text+0x545): undefined reference to `av_free_packet(AVPacket*)'
main.cxx:(.text+0x556): undefined reference to `av_free(void*)'
main.cxx:(.text+0x565): undefined reference to `av_frame_free(AVFrame**)'
main.cxx:(.text+0x574): undefined reference to `av_frame_free(AVFrame**)'
main.cxx:(.text+0x580): undefined reference to `avcodec_close(AVCodecContext*)'
main.cxx:(.text+0x58f): undefined reference to `avcodec_close(AVCodecContext*)'
main.cxx:(.text+0x59e): undefined reference to `avformat_close_input(AVFormatContext**)'
collect2: error: ld returned 1 exit status
CMakeFiles/test.dir/build.make:87: recipe for target 'test' failed
make[2]: *** [test] Error 1
CMakeFiles/Makefile2:75: recipe for target 'CMakeFiles/test.dir/all' failed
make[1]: *** [CMakeFiles/test.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

The cmake list looks like this:

 cmake_minimum_required(VERSION 3.16)                                                                                                                                     
 
 project(ffmpeg_test)
 
 add_library(avformat STATIC IMPORTED)
 set_target_properties(avformat
     PROPERTIES IMPORTED_LOCATION /home/christoph/develop/FFmpeg/build/lib/libavformat.a
 )
 add_library(avcodec STATIC IMPORTED)
 set_target_properties(avcodec
     PROPERTIES IMPORTED_LOCATION /home/christoph/develop/FFmpeg/build/lib/libavcodec.a
 )
 add_library(swscale STATIC IMPORTED)
 set_target_properties(swscale
     PROPERTIES IMPORTED_LOCATION /home/christoph/develop/FFmpeg/build/lib/libswscale.a
 )
 add_library(avutil STATIC IMPORTED)
 set_target_properties(avutil
     PROPERTIES IMPORTED_LOCATION /home/christoph/develop/FFmpeg/build/lib/libavutil.a
 )
 add_executable(test main.cxx)
 
 target_link_libraries(test PRIVATE
     /home/christoph/develop/FFmpeg/build/lib/libavformat.a
     avcodec
     swscale
     avutil
 )
 target_include_directories(test PRIVATE /home/christoph/develop/FFmpeg/build/include)

And here are the ffmpeg libs:

christoph@christoph-ThinkPad-T490:~/develop/FFmpeg/build/lib$ ll
total 277840
drwxr-xr-x  3 christoph christoph      4096 Dez  7 23:59 ./
drwxr-xr-x 17 christoph christoph      4096 Dez  7 23:59 ../
-rw-r--r--  1 christoph christoph 173479270 Dez  7 23:59 libavcodec.a
-rw-r--r--  1 christoph christoph   2174910 Dez  7 23:59 libavdevice.a
-rw-r--r--  1 christoph christoph  37992438 Dez  7 23:59 libavfilter.a
-rw-r--r--  1 christoph christoph  59222040 Dez  7 23:59 libavformat.a
-rw-r--r--  1 christoph christoph   4759514 Dez  7 23:59 libavutil.a
-rw-r--r--  1 christoph christoph    695698 Dez  7 23:59 libswresample.a
-rw-r--r--  1 christoph christoph   6164398 Dez  7 23:59 libswscale.a
drwxr-xr-x  2 christoph christoph      4096 Dez  7 23:59 pkgconfig/

And this is the example code:

#include<stdio.h>

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>

// compatibility with newer API
#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(55,28,1)
#define av_frame_alloc avcodec_alloc_frame
#define av_frame_free avcodec_free_frame
#endif

void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
  FILE *pFile;
  char szFilename[32];
  int  y;
  
  // Open file
  sprintf(szFilename, "frame%d.ppm", iFrame);
  pFile=fopen(szFilename, "wb");
  if(pFile==NULL)
    return;
  
  // Write header
  fprintf(pFile, "P6\n%d %d\n255\n", width, height);
  
  // Write pixel data
  for(y=0; y<height; y++)
    fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
  
  // Close file
  fclose(pFile);
}

int main(int argc, char *argv[]) {
  // Initalizing these to NULL prevents segfaults!
  AVFormatContext   *pFormatCtx = NULL;
  int               i, videoStream;
  AVCodecContext    *pCodecCtxOrig = NULL;
  AVCodecContext    *pCodecCtx = NULL;
  AVCodec           *pCodec = NULL;
  AVFrame           *pFrame = NULL;
  AVFrame           *pFrameRGB = NULL;
  AVPacket          packet;
  int               frameFinished;
  int               numBytes;
  uint8_t           *buffer = NULL;
  struct SwsContext *sws_ctx = NULL;

  if(argc < 2) {
    printf("Please provide a movie file\n");
    return -1;
  }
  // Register all formats and codecs
  av_register_all();
  
  // Open video file
  if(avformat_open_input(&pFormatCtx, argv[1], NULL, NULL)!=0)
    return -1; // Couldn't open file
  
  // Retrieve stream information
  if(avformat_find_stream_info(pFormatCtx, NULL)<0)
    return -1; // Couldn't find stream information
  
  // Dump information about file onto standard error
  av_dump_format(pFormatCtx, 0, argv[1], 0);
  
  // Find the first video stream
  videoStream=-1;
  for(i=0; i<pFormatCtx->nb_streams; i++)
    if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO) {
      videoStream=i;
      break;
    }
  if(videoStream==-1)
    return -1; // Didn't find a video stream
  
  // Get a pointer to the codec context for the video stream
  pCodecCtxOrig=pFormatCtx->streams[videoStream]->codec;
  // Find the decoder for the video stream
  pCodec=avcodec_find_decoder(pCodecCtxOrig->codec_id);
  if(pCodec==NULL) {
    fprintf(stderr, "Unsupported codec!\n");
    return -1; // Codec not found
  }
  // Copy context
  pCodecCtx = avcodec_alloc_context3(pCodec);
  if(avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {
    fprintf(stderr, "Couldn't copy codec context");
    return -1; // Error copying codec context
  }

  // Open codec
  if(avcodec_open2(pCodecCtx, pCodec, NULL)<0)
    return -1; // Could not open codec
  
  // Allocate video frame
  pFrame=av_frame_alloc();
  
  // Allocate an AVFrame structure
  pFrameRGB=av_frame_alloc();
  if(pFrameRGB==NULL)
    return -1;

  // Determine required buffer size and allocate buffer
  numBytes=avpicture_get_size(AV_PIX_FMT_RGB24, pCodecCtx->width,
                  pCodecCtx->height);
  buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
  
  // Assign appropriate parts of buffer to image planes in pFrameRGB
  // Note that pFrameRGB is an AVFrame, but AVFrame is a superset
  // of AVPicture
  avpicture_fill((AVPicture *)pFrameRGB, buffer, AV_PIX_FMT_RGB24,
         pCodecCtx->width, pCodecCtx->height);
  
  // initialize SWS context for software scaling
  sws_ctx = sws_getContext(pCodecCtx->width,
               pCodecCtx->height,
               pCodecCtx->pix_fmt,
               pCodecCtx->width,
               pCodecCtx->height,
               AV_PIX_FMT_RGB24,
               SWS_BILINEAR,
               NULL,
               NULL,
               NULL
               );

  // Read frames and save first five frames to disk
  i=0;
  while(av_read_frame(pFormatCtx, &packet)>=0) {
    // Is this a packet from the video stream?
    if(packet.stream_index==videoStream) {
      // Decode video frame
      avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
      
      // Did we get a video frame?
      if(frameFinished) {
    // Convert the image from its native format to RGB
    sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data,
          pFrame->linesize, 0, pCodecCtx->height,
          pFrameRGB->data, pFrameRGB->linesize);
    
    // Save the frame to disk
    if(++i<=5)
      SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, 
            i);
      }
    }
    
    // Free the packet that was allocated by av_read_frame
    av_free_packet(&packet);
  }
  
  // Free the RGB image
  av_free(buffer);
  av_frame_free(&pFrameRGB);
  
  // Free the YUV frame
  av_frame_free(&pFrame);
  
  // Close the codecs
  avcodec_close(pCodecCtx);
  avcodec_close(pCodecCtxOrig);

  // Close the video file
  avformat_close_input(&pFormatCtx);
  
  return 0;
}

Answer 1

Your file name "main.cxx" tells the compiler that this file should be compiled in c++ mode. Either you tell the compiler explicitly that this is a C file or you need to add

extern "C" { }

around your includes of the avlib headers.