Building SDL2_image as a CMake external project

Question

I've been trying to create a CMake-based build-system for a project that is supposed to use SDL2_image library. I do not want to force user to install any libraries to the system to be able to build the project, so I took advantage of the CMake's ability to download and build dependencies (freetype, SDL2 and SDL2_image) from source code as External Projects.

Everything is fine with freetype and SDL2 (which both include CMakeLists.txt files out of the box), but I've ran out of ideas how to make it work for SDL2_image. CMake's external projects support custom configuration and building settings which I used in different variants with no success.

The CMake file itself can be found here, but the problematic part is this:

# SDL_image library
ExternalProject_Add(sdl2_image_project
    URL https://www.libsdl.org/projects/SDL_image/release/SDL2_image-2.0.0.tar.gz
    DEPENDS sdl2_project
    PREFIX ${LIBS_DIR}/SDL2_image
    CONFIGURE_COMMAND LDFLAGS=-L${SDL2_BIN} CFLAGS=-I${SDL2_SRC}/include SDL2_CONFIG=${SDL2_BIN}/sdl2-config <SOURCE_DIR>/configure --prefix=<INSTALL_DIR> --enable-shared=no
    BUILD_COMMAND make
    INSTALL_COMMAND ""
)

An error occurs while building sdl2_image_project. Some trivial research discovered that the error is generated by the undefined references to parts of libdl. Here is a tiny part of the hole error:

libtool: link: gcc -I/home/snikitin/_src/img_glypher/libs/SDL2/src/sdl2_project/include -I/usr/local/include/SDL2 -D_REENTRANT -o showimage showimage.o -Wl,-rpath -Wl,/usr/local/lib -pthread  -L/home/snikitin/_src/img_glypher/libs/SDL2/src/sdl2_project-build ./.libs/libSDL2_image.a -L/usr/local/lib -lSDL2 -pthread
/home/snikitin/_src/img_glypher/libs/SDL2/src/sdl2_project-build/libSDL2.a(SDL_dynapi.c.o): In function `get_sdlapi_entry':
/home/snikitin/_src/img_glypher/libs/SDL2/src/sdl2_project/src/dynapi/SDL_dynapi.c:227: undefined reference to `dlopen'

I think the problem takes place due to the fact that linker tries to create a shared version of SDL2_image library while linking it to a static libSDL2.a. The thing is - if this is right - SDL2 building step creates both static and shared versions of itself so one would assume that linker would use libSDL2-2.0.so instead (I do not actually need a shared library - just the static one, but I do not know how to prevent the build system from trying to create it apart from passing --enable-shared=no to SDL2_image configure script, which does not help in this case).

After a lot of googling I've discovered that the possible source of the problem is that sdl2-config (which is called to get some flags for compiler during SDL_image building) may be called with wrong arguments and produces wrong cflags which confuse everything else. But I'm not sure that is the case and also I do not know how to influence sdl2_config call from CMake (configure --help does not seem to unveil any useful options for this situation).

I am running Ubuntu 14.04 x64 if it matters in any way. Would appreciate any advice!

Answer 1

Looks like you need to link some libraries like m and dl. It can be fixed by providing custom sdl2-config file. Copy sdl2-config from extracted archive and substitute --libs result:

--libs)
  echo -L${exec_prefix}/lib -Wl,-rpath,${libdir} -pthread -lSDL2 -lm -ldl
  ;;

Note that order is important (that's why just modifying LIBS not works for me). Now this file can be used in your ExternalProject_Add command instead of SDL2_CONFIG=${SDL2_BIN}/sdl2-config:

...
... CFLAGS=-I${SDL2_SRC}/include SDL2_CONFIG=${CMAKE_CURRENT_LIST_DIR}/sdl2-config <SOURCE_DIR>/configure
...