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Exploring LINK_INTERFACE_LIBRARIES: A Powerful Tool for CMake Dependency Management


Purpose

In CMake, the LINK_INTERFACE_LIBRARIES property is used to specify the public interface libraries required by a target (usually a library itself). These libraries are essential for other targets (its dependents) that link against the current target to function correctly.

Key Points

  • Explicit Control
    LINK_INTERFACE_LIBRARIES provides a way to explicitly control the interface libraries for a target. This can be useful in several scenarios:
    • Overriding Defaults
      You can override the default transitive dependencies to specify a custom set of required libraries.
    • Breaking Circular Dependencies
      In complex project structures, circular dependencies between libraries can occur. By carefully managing interface libraries, you can break these cycles, ensuring a clean build process.
    • Hiding Implementation Details
      By excluding certain internal libraries from the interface, you can prevent them from being directly linked by dependents, promoting better encapsulation.
  • Transitive Dependencies
    When a target is linked using target_link_libraries(), the linker automatically includes not only the specified libraries but also their transitive dependencies (libraries they depend on). This ensures that all necessary libraries are included in the final executable.

Advanced Usage

For even finer control, CMake offers related properties:

  • INTERFACE_LINK_LIBRARIES_DIRECT_EXCLUDE: Excludes specific libraries from the direct link dependencies passed to dependents.
  • INTERFACE_LINK_LIBRARIES_DIRECT: Specifies the direct link dependencies for a target's dependents.

Example

# Define a library target named "mylib"
add_library(mylib SHARED source.cpp)

# Link mylib against the system library "pthread"
target_link_libraries(mylib PRIVATE pthread)

# Set the interface libraries for mylib, excluding pthread (implementation detail)
set_target_properties(mylib PROPERTIES INTERFACE_LINK_LIBRARIES "")  # Start from an empty list
target_link_libraries(mylib INTERFACE some_other_library)  # Add the explicitly required library

In this example, mylib depends on pthread internally but doesn't expose it as an interface dependency. The dependent target will only need to link against some_other_library.

  • For simpler cases, the default transitive dependency behavior might be sufficient.
  • Use LINK_INTERFACE_LIBRARIES judiciously, as overly complex configurations can make project maintenance difficult.

Overriding Default Transitive Dependencies

This example shows how to override the default transitive dependencies for a target.

# Define a library "libfoo" that depends on "libbar"
add_library(libfoo SHARED foo.cpp)
target_link_libraries(libfoo PRIVATE libbar)  # Default transitive dependency

# Define another library "libgui"
add_library(libgui SHARED gui.cpp)
target_link_libraries(libgui PUBLIC libfoo)  # Links against libfoo and its transitive dependencies

# Override the interface libraries for libfoo, excluding "libbar" (not required by dependents)
set_target_properties(libfoo PROPERTIES INTERFACE_LINK_LIBRARIES "")  # Start from an empty list
target_link_libraries(libfoo INTERFACE some_other_library)  # Add the explicitly required library

# Now, when linking against "libgui", only "some_other_library" will be needed
target_link_libraries(myprogram gui)

Breaking Circular Dependencies

This example showcases how to break a circular dependency between libraries using LINK_INTERFACE_LIBRARIES.

# Define libraries "libfoo" and "libbar" with a circular dependency
add_library(libfoo SHARED foo.cpp)
add_library(libbar SHARED bar.cpp)
target_link_libraries(libfoo PUBLIC libbar)
target_link_libraries(libbar PUBLIC libfoo)  # This creates a circular dependency

# Break the dependency by excluding one library from the interface of the other
set_target_properties(libfoo PROPERTIES INTERFACE_LINK_LIBRARIES "")
target_link_libraries(libfoo INTERFACE some_other_library)

set_target_properties(libbar PROPERTIES INTERFACE_LINK_LIBRARIES "")
target_link_libraries(libbar INTERFACE libfoo)  # One-way dependency is allowed

# Now, linking against either library will not cause build errors due to the circular dependency
target_link_libraries(myprogram1 foo)
target_link_libraries(myprogram2 bar)

Hiding Implementation Details

This example demonstrates how to hide internal implementation details from dependents.

# Define a library "mylib"
add_library(mylib SHARED source.cpp)

# Link mylib against an internal helper library "helper"
add_library(helper STATIC helper.cpp)
target_link_libraries(mylib PRIVATE helper)  # Helper is for internal use

# Set the interface libraries for mylib, excluding "helper"
set_target_properties(mylib PROPERTIES INTERFACE_LINK_LIBRARIES "")
target_link_libraries(mylib INTERFACE some_other_library)  # Expose only the required library

# Dependents will only need to link against "some_other_library"
target_link_libraries(myprogram mylib)

target_link_libraries() with PUBLIC/PRIVATE

  • Limitations
    This approach becomes cumbersome for complex projects with many dependencies. It can be difficult to track and manage transitive dependencies accurately.
  • Behavior
    You can use the PUBLIC and PRIVATE keywords with target_link_libraries() to achieve basic dependency management. Public libraries are propagated to dependents, while private libraries are linked only for the current target.

Include Directories

  • Limitations
    This approach doesn't handle linking dependencies. If the library requires a compiled library file (e.g., .so, .a), this method alone wouldn't suffice.
  • Behavior
    Setting include directories for header files can sometimes be enough for some dependencies, especially for header-only libraries. This allows dependents to find the necessary headers for compilation.

Package Managers

  • Limitations
    Package managers introduce an additional layer of complexity and might require specific configuration for your project. They might not always have all the required libraries available.
  • Behavior
    Consider using package managers like vcpkg or Conan. These tools manage dependencies for you, including downloading, building, and linking.
  • External Packages
    Consider package managers if you rely on external libraries that have well-maintained packages available.
  • Complex Projects
    For larger projects with intricate dependencies, using LINK_INTERFACE_LIBRARIES is highly recommended for its clarity and control over the interface.
  • Simple Projects
    For smaller projects with well-defined dependencies, target_link_libraries() with PUBLIC/PRIVATE might be sufficient.