Understanding BUILD_INTERFACE and INSTALL_INTERFACE in target_include_directories
When working with CMake, you might encounter the terms BUILD_INTERFACE and INSTALL_INTERFACE within the context of target_include_directories. These options play a crucial role in managing include paths for your project during both the build process and the installation process.
The Problem:
Imagine you have a library project with header files that need to be included by other projects during compilation. The header files are located within the library's source directory, but you want to ensure they are accessible to dependent projects even after installation. This poses a challenge because the library's source directory might not be the same as the installation directory.
Scenario and Code:
# Library source directory
set(LIBRARY_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/src")
# Library target
add_library(my_library ${LIBRARY_SRC_DIR}/my_library.cpp)
# Include directories for the build process
target_include_directories(my_library PUBLIC
${LIBRARY_SRC_DIR}/include
)
# Install header files
install(DIRECTORY ${LIBRARY_SRC_DIR}/include DESTINATION include)
In this example, the library's header files are located in src/include. We use target_include_directories to add this directory to the include path for the library's build. However, this setting only affects the build process. After installation, dependent projects won't be able to find the header files unless they are explicitly told where they are located.
The Solution:
This is where BUILD_INTERFACE and INSTALL_INTERFACE come into play. They allow you to define separate include paths for the build and install phases.
# Library source directory
set(LIBRARY_SRC_DIR "${CMAKE_CURRENT_SOURCE_DIR}/src")
# Library target
add_library(my_library ${LIBRARY_SRC_DIR}/my_library.cpp)
# Include directories for the build process
target_include_directories(my_library PUBLIC
${LIBRARY_SRC_DIR}/include
)
# Include directories for the install process
target_include_directories(my_library INTERFACE
${CMAKE_INSTALL_PREFIX}/include
)
# Install header files
install(DIRECTORY ${LIBRARY_SRC_DIR}/include DESTINATION include)
In this revised code:
-
target_include_directories(my_library PUBLIC ${LIBRARY_SRC_DIR}/include)defines the include path for the library's build process. This ensures the header files are accessible during compilation. -
target_include_directories(my_library INTERFACE ${CMAKE_INSTALL_PREFIX}/include)defines the include path for the installed library. This ensures dependent projects can find the header files after installation.
Analysis and Explanation:
-
BUILD_INTERFACE: Specifies include directories that are relevant during the build phase. These directories are used by the compiler and linker to find necessary header files and libraries.
-
INSTALL_INTERFACE: Specifies include directories that are relevant after the project is installed. These directories are used by dependent projects to find the installed headers and libraries.
-
PUBLIC: Indicates that the include directories are accessible to all dependent projects.
-
INTERFACE: Indicates that the include directories are only accessible to dependent projects that directly link against the target.
Benefits of Using BUILD_INTERFACE and INSTALL_INTERFACE:
-
Improved Build System Organization: Separating build and install include paths ensures a cleaner and more maintainable build system.
-
Centralized Installation: Install directories are defined centrally, eliminating the need for individual dependent projects to specify paths to installed libraries.
-
Cross-Platform Compatibility: The use of
INSTALL_INTERFACEensures consistency across different platforms and build environments.
Example:
Suppose you have a library named my_library and an application named my_app. You want my_app to use the headers from my_library after installation. By using INSTALL_INTERFACE in your my_library project, you provide my_app with the necessary include path to the installed library headers. This allows my_app to find the header files even though they are no longer located in the original source directory.
In Summary:
BUILD_INTERFACE and INSTALL_INTERFACE in target_include_directories are powerful tools for managing include paths in CMake projects. They allow you to clearly define where header files are located during both the build and install phases, ensuring proper accessibility and compatibility for dependent projects.
Further Reading:
