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

Combining Multiple Static Libraries into a Single One with CMake

In software development scenarios, it is common to have multiple static libraries that serve as dependencies for a larger project. To streamline distribution and reduce dependency management complexity, it may be desirable to combine these libraries into a single static library. CMake offers several approaches to achieve this goal.

One approach is to leverage the target_link_libraries command. However, it is important to note that this command simply adds the specified libraries as link dependencies, it does not combine them into a single library.

For a more seamless solution, consider using a custom target together with the ar command. Here's a sample CMake script that demonstrates this approach:

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)

This script builds two static libraries (a and b) and an executable (main) that depends on a combined library (c). The combined custom target uses the ar command to extract the object files from the individual libraries and combine them into c.a.

Alternatively, consider using Apple's libtool to perform the combination:

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 these custom target approaches provide a means to combine multiple static libraries, it is worth noting that the CMake community is still actively seeking a more elegant solution for this common task.

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