CODEWITHSUNDEEP
c++cmakecross-platformdlib
mascoj
mascoj

Reputation: 1320

CMake: Override libraries added by target_link_libraries

So I am running into a pretty large headache building my software with CMake.

I am building a third party library statically (dlib) which requires zlib and libpng (both also static, I prebuilt these) libraries to support PNG functionality. The CMakeFile provided by the COTS dlib library is doing a basic:

target_link_libraries( dlib ${dlib_required_libs})

This is making all of its libraries configured as "general" libraries, which end up being used for both Release and Debug builds.

This isn't an issue in Linux, but Windows has a lovely "feature" of specifying the runtime library (/MT or /MD or /MTd or /MDd). Any mismatches between these flags cause multiple symbol definition errors at link time. i.e. If libpng was built with /MT and my software is using /MTd, they will be incompatible.

To alleviate this, I have two built versions of zlib and libpng. One set using the /MT flag for Release builds and the other /MTd for Debug builds. These happily link into my own software using optimized/debug flags on target_link_libraries where they are used. HOWEVER, dlib (3rd party) is only linking the Release set of the zlib and libpng libs by the way it's CMakeFile has been written.

My main question is, is there a way I can 'override' what dlib is linking without modifying it's provided CMakeFile? I attempted overwriting dlib_LIB_DEPENDS and forcing it into the cache out of desperation, to no avail.

Upvotes: 1

Views: 2158

Answers (2)

Tsyvarev
Tsyvarev

Reputation: 65860

FindPNG.cmake script, like many other Find*.cmake scripts used by find_package(), does not care about multiconfig builds. So it just searched single library, while multiconfig builds naturally want library-per-config.

For make searched PNG library to be per-config, one can to write own FindPNG.cmake script (and set CMAKE_MODULE_PATH to point to the directory with this script).

But for concrete usage it is simpler to just rewrite output of the original script, that is set PNG_LIBRARY cache variable pointed to library-per-config:

optimized png-lib-release debug png-lib-debug

or, using generator expressions,

$<$<CONFIG:Release>:png-lib-release>$<$<CONFIG:Debug>:png-lib-debug>`

(instead of png-lib-release and png-lib-debug should be paths to the release and debug version of the library conrrespondingly).

Both these values are expected to be used with target_link_libraries command, producing per-config linkage.

Upvotes: 3

Davis King
Davis King

Reputation: 4791

Dlib comes with copies of libpng and zlib in dlib/external. By default, dlib's CMakeLists.txt will build them the correct way and statically link them into your program. So the solution is to let it do that. Don't try to create a separate static library because, as you noted, that causes a lot of trouble on windows.

Upvotes: 0

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