Create a static library that refers to other static libraries

Question

I would like to create a static library that depends on another library, in this case ZLIB, for which I already have a static build (libz.a).

I have the following:

...
set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static")
set (BUILD_SHARED_LIBS OFF CACHE BOOL "" FORCE)
set (CMAKE_FIND_LIBRARY_SUFFIXES .a ${CMAKE_FIND_LIBRARY_SUFFIXES}) # to find the .a   instead of the .so
...
find_package(ZLIB REQUIRED)
if (ZLIB_FOUND)
  message(STATUS "ZLIB library: ${ZLIB_LIBRARIES}") # ZLIB library:     /usr/lib64/libz.a
  include_directories(${ZLIB_INCLUDE_DIRS})
  set (EXT_LIBS ${EXT_LIBS} ${ZLIB_LIBRARIES})
endif()
...
add_library (libTest ${MCCORE_SOURCES_CC})
target_link_libraries(libTest ${EXT_LIBS}) #EXT_LIBS = /usr/lib64/libz.a

However, the final step of the build is creating the static library but without any reference to /usr/lib64/libz.a

ex:

/usr/bin/ar cr libTest.a  object1.o object2.o ... objectN.o 

I would expect:

/usr/bin/ar cr libTest.a  object1.o object2.o ... objectN.o /usr/lib64/libz.a

It seems the final archive creation doesn't care for libraries set with target_link_libraries.

Any ideas?

What can I do to force this?

Answer 1

Under Windows (with Visual Studio) the following would do the trick:

add_library(fooStatic1 STATIC fooStatic.cpp)

set(LIBS_TO_COMBINE "${CMAKE_BINARY_DIR}/libfooStatic1.lib ${ZLIB_LIBRARIES}")

add_library(combined STATIC ${LIBS_TO_COMBINE} dummy.cpp) #dummy.cpp being empty
add_dependencies(combined fooStatic1)
set_source_files_properties(${LIBS_TO_COMBINE} PROPERTIES EXTERNAL_OBJECT TRUE GENERATED TRUE)
set_target_properties(combined PROPERTIES LINKER_LANGUAGE CXX)
set_target_properties(combined PROPERTIES STATIC_LIBRARY_FLAGS "${LIBS_TO_COMBINE}")

Unfortunately, it won't work under linux, as ar will just combine the archive files without unpacking them - creating something that is not really usable. In order to achieve your goal you need to extract the .o files and recombine them:

ar -x /usr/lib64/libz.a
ar -x libfooStatic1.a
ar -rc libcombined.a  *.o

I am not aware of a CMake macro that would help in the process. You could probably run ar -x in execute_process(...), glob the output, and then run ar -rc.