Cmake link static lib against shared one

Question

I'm new to CMake and I don't really know what I'm doing ... I am currently trying to link a static library (libavcodec.a) against a shared one (GreenVideo). I keep having errors saying that my functions provided by my static lib are "undefined reference". My CMakeLists.txt looks like this:

PROJECT(GreenVideo)

INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR})
INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR}/include/)

link_directories(/home/nde/svn/Visio/trunk/applications/dorsalistaApp/components/GreenVideo/lib)


ADD_LIBRARY( GreenVideo SHARED
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/xvmc.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/vdpau.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/dxva2.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/avcodec.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/avfft.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/dv_profile.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/vaapi.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/vda.h
            ${CMAKE_CURRENT_SOURCE_DIR}/include/libavcodec/version.h

            metadatagen.h metadatagen.cpp

            greenvideofactory.h
            greenvideofactory.cpp
        )



target_link_libraries(GreenVideo /home/nde/svn/Visio/trunk/applications/dorsalistaApp/components/GreenVideo/lib/libavcodec.a)

Is there anything suspicious in the way I'm doing it ?

Regards, Nicolas

Answer 1

Your static library obviously has some undefined references . Static library does not store the information about how those reference can be resolved, unlike shared library (.so).

In fact, static library is just an arcive, a collection of objecf files (.o).

To fix your issues you must add more link libraries afrer .../ibavcodec.a . All the libraries that libavcodec.a may need, such as for example: c , stdc++ , rt etc..

Edit:

Okay, it seems i figured out your problem. It's not with the way you link your library, it's because of the way you include libavcodec 's header files.

Where you include header file like:

#include <libavcodec/avcodec.h>

you must do

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

You can also wrap many includes in one extern "C" { ... } like:

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

---

What's the problem? C++ symbols have name mangling . So your linker looks for C++ symbols in libavcodec.a but those are not there.