How Can I Merge Multiple Static Libraries into One Using CMake?

Combining Static Libraries into One with CMake

When working with projects dependent on multiple static libraries, merging these libraries into a single cohesive unit becomes essential. CMake offers a robust solution to this challenge, eliminating the need for downstream dependencies maintenance.

The Problem

Attempts to combine static libraries using target_link_libraries directly often prove unsuccessful. However, when the combined library is used as a dependency for an executable, CMake seems to automatically merge the libraries.

The Solution

A custom target can be employed to achieve the desired result:

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
        )

This target extracts object files from the individual static libraries and combines them into a new static library.

Variant Using libtool

If desired, the libtool utility can be leveraged for the same purpose:

add_custom_target(combined
        COMMAND libtool -static -o ${C_LIB} $<TARGET_FILE:a> $<TARGET_FILE:b>
        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
        DEPENDS a b
        )

Conclusion

While it may seem there should be a more straightforward approach, this custom target effectively combines static libraries into one. Needless to say, this technique simplifies dependency management and ensures the seamless delivery of combined libraries to downstream consumers.

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