libav - 对“av_frame_alloc”等的未定义引用

Question

I am trying to compile this tutorial from the libav doc: link

I changed nothing on the code!

But when I compile it with:

gcc test.c -lavformat -lswscale -lavdevice -lavformat -lavcodec -lavutil -lpthread -lm -o example

I get these errors:

undefined reference to `check_sample_fmt'
undefined reference to `select_sample_rate'
undefined reference to `select_channel_layout'
undefined reference to `av_frame_alloc'
undefined reference to `av_frame_free'

Searching Dr.google I read that it maybe has something to do with the linking order of the libs. But I did not found the correct one yet?!

EDIT: this 'possible dublicate' seems not to be related to my problem

Answer 1

You probably are running an old code with a newer version of FFmpeg. Ffmpeg changed some functions names, some create some depreciated warnings while others create errors.

According to the official Ffmpeg github :

av_frame_alloc(), av_frame_unref() and av_frame_free() now can and should be 
used instead of avcodec_alloc_frame(), avcodec_get_frame_defaults() and avcodec_free_frame() respectively

Answer 2

ffmpeg comes with an Makefile for the examples. I took this one and simply used it for my libav examples. Link

Answer 3

This doesn't apply in your case, but you get a similar issue if you try to use ffmpeg from C++.

#include <libavcodec/avcodec.h>

int main() {
  av_frame_alloc();
}

Unfortunately the ffmpeg headers do not have the proper extern "C" { wrappers so you have to do it yourself.

extern "C" {
#include <libavcodec/avcodec.h>
}

int main() {
  av_frame_alloc();
}

Answer 4

You are using GNU toolchain. On Windows, I have seen that sometimes MSVC linker also produces.lib files when generating shared/dynamic.dll libraries. These.lib only holds symbol information needed during linking step but have have no actual machine code. As a result, you still have to add them to linker input. In Visual studio they can be added to "Project -> properties -> Linker -> Input -> Additional Dependencies" For example: Append the following to the above path "avformat.lib; avcodec.lib; avutil.lib". This would solve the undefined symbol issue in the question. I am not sure how different, things are with GNU. But I hope this can help any Windows user facing a similar problem. On Windows do these three things:

1. Add all include files directories to "Project -> properties -> VC++ Directories -> General -> Include Directories"
2. Add all.lib files to "Project -> properties -> Linker -> Input -> Additional Dependencies"
3. Copy all the.dll files where your executable is produced.