How Can I Combine Multiple Static Libraries into a Single Library Using CMake?

Combining Multiple Static Libraries into a Single Library Using CMake

When building projects that rely on numerous static libraries, it can be desirable to merge these libraries into a single unit for release. CMake does have specific methods for combining target libraries directly, such as target_link_libraries(), however, it may not perform the desired action in all cases.

Custom Target Approach

One approach involves creating a custom target that directly manipulates the static libraries. By executing commands such as ar or libtool, it is possible to extract object files from individual libraries and then recombine them into a single archive. This can be achieved through the following CMake code:

# Add the individual static libraries
add_library(a a.cpp a.h)
add_library(b b.cpp b.h)

# Create the combined library target
set(C_LIB ${CMAKE_BINARY_DIR}/libcombi.a)
add_custom_target(combined
    COMMAND ar -x $<TARGET_FILE:a>
    COMMAND ar -x $<TARGET_FILE:b>
    COMMAND ar -qcs ${C_LIB} *.o
    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
    DEPENDS a b
)

# Import the combined library
add_library(c STATIC IMPORTED GLOBAL)
add_dependencies(c combined)
set_target_properties(c
    PROPERTIES
    IMPORTED_LOCATION ${C_LIB}
)

# Link the combined library to the executable
target_link_libraries(main c)

This method effectively aggregates the object files from the individual libraries into a single archive, providing the desired combined library.

Alternative Considerations

While the custom target approach achieves the objective, there may be alternative approaches to consider. CMake provides the link_libraries() function, which has the potential to link multiple libraries into a single output. Additionally, investigation into using the add_library() function with the OBJECT or INTERFACE flags may offer a solution.

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