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

Combining Multiple Static Libraries into a Single Library with CMake

Maintaining the dependencies of a project can be challenging when it relies on multiple static libraries. To streamline the process, merging these libraries into a single unit becomes desirable. While CMake provides facilities for linking libraries, it lacks functionality for combining static libraries directly.

To address this issue, a custom target can be employed to achieve the desired result. By utilizing commands such as ar or libtool, the individual object files from the static libraries can be extracted and merged into a new archive. For instance, consider the following code snippet:

cmake_minimum_required(VERSION 2.8.7)

add_library(b b.cpp b.h)
add_library(a a.cpp a.h)
add_executable(main main.cpp)

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
        )

add_library(c STATIC IMPORTED GLOBAL)
add_dependencies(c combined)

set_target_properties(c
        PROPERTIES
        IMPORTED_LOCATION ${C_LIB}
        )

target_link_libraries(main c)

In this example, the combined custom target executes the necessary commands to extract object files from the a and b libraries, and then merges them into a new archive called libcombi.a. This combined library can be imported into the main executable using the c target.

Alternatively, the libtool command from Apple's development tools can be used for this 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
        )

While this approach provides a robust solution, it would be beneficial to have a more native implementation within CMake for combining static libraries directly. This would streamline the process further and eliminate the need for custom targets.

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