Understanding IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG in CMake

Context

• Linking refers to the process of combining object files from different targets to create an executable or library.
• Targets can be created within your project (local targets) or imported from external projects (imported targets).
• In CMake, targets represent the building blocks of your project, such as executables, libraries, or custom commands.

IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG Property

• The <CONFIG> part allows you to set this information for different build configurations (e.g., Debug, Release).
• It specifies the programming languages used to create the object files within the imported target for a particular configuration.
• This property is associated with imported targets (targets from other projects).

Purpose

• This is crucial for correct linking and avoiding compatibility issues.
• It ensures that the linker uses the appropriate compiler flags and libraries specific to the languages involved.
• CMake uses this property to determine how to link against the imported target.

When to Use

• However, if you're integrating an imported target whose build system didn't set this property correctly, you might need to set it manually.
• This property is typically set by the build system that generates the imported target. You wouldn't normally modify it directly in your CMakeLists.txt.

Example

# Hypothetical CMakeLists.txt from another project (creating "my_library")
# (This part wouldn't be in your project)
add_library(my_library SHARED source.cpp)
set_target_properties(my_LIBRARY PROPERTIES
  IMPORTED_LINK_INTERFACE_LANGUAGES_DEBUG "C++"
  IMPORTED_LINK_INTERFACE_LANGUAGES_RELEASE "C++"
)

• In your project's CMakeLists.txt:

# Assuming "my_library" is imported correctly
target_link_libraries(my_program PUBLIC IMPORTED::my_library)

• CMake will use the appropriate C++ compiler and libraries when linking your my_program with the imported my_library.

• You usually wouldn't modify it directly unless necessary.
• This information is crucial for the linker to use the correct compiler flags and libraries.
• It specifies the programming languages used to create the object files within the imported target for a specific configuration.
• IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG is a target property for imported targets.

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Scenario

• You have an external library (my_mixed_lib) built with both C and C++ code, but the build system didn't set the IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG property correctly.

CMakeLists.txt in your project

# Assuming "my_mixed_lib" is imported but missing the property
find_package(MyMixedLib REQUIRED)  # Replace with your actual find_package call

# Set the property manually (might not be ideal in most cases)
set_target_properties(MyMixedLib::my_mixed_lib PROPERTIES
  IMPORTED_LINK_INTERFACE_LANGUAGES_DEBUG "C;C++"
  IMPORTED_LINK_INTERFACE_LANGUAGES_RELEASE "C;C++"
)

target_link_libraries(my_program PUBLIC IMPORTED::MyMixedLib::my_mixed_lib)

1. We use find_package to import the external library (MyMixedLib).
2. Since the build system didn't set the property, we manually set IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG using set_target_properties.
3. We specify C;C++ for both DEBUG and RELEASE configurations, indicating that the library contains object files compiled with both C and C++ languages.
4. Finally, we link our executable (my_program) with the imported library.

Important Notes

• This example is for illustrative purposes only. In practice, consult the documentation for the external library to determine the appropriate languages used.
• It's preferable to have the build system that generates the imported library set this property correctly.
• Manual intervention is usually not recommended as it can lead to issues if the actual languages used in the imported library differ.

• If possible, try contacting the maintainers of the external library and request them to set the IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG property during the build process. This ensures proper configuration for all users.

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1. • If the external project provides an interface library, use that instead of directly linking against the implementation library. Interface libraries typically encode language information without requiring the IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG property.

2. Compiler and Linker Flags
   

   • Limited Applicability
     This approach has limitations. You might be able to specify language-specific compiler and linker flags in your project's CMakeLists.txt. However, this is less robust than IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG as it doesn't explicitly tell CMake about the languages involved.
   • Example (C++ flags)
     

   target_link_libraries(my_program PUBLIC IMPORTED::my_library)
   set_target_properties(my_program PROPERTIES
     CXX_STANDARD 17  # Assuming the library uses C++17
     CXX_FLAGS "-std=c++17"
   )
   

   Caution
   Be cautious when using this approach, as incorrect flags can lead to compilation errors or unexpected behavior.

3. Build System Integration
   

   • If you have some control over the external project's build system, consider modifying it to set the IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG property during the build process. This is the ideal solution to ensure proper configuration for all users of the library.

Choosing the Right Approach

• Compiler and linker flags are a last resort, and their use requires caution and a thorough understanding of the languages involved.
• Use IMPORTED_LINK_INTERFACE_LANGUAGES_CONFIG only when necessary, such as for legacy libraries that don't provide interface libraries.
• Interface libraries should be the preferred option when available, as they encapsulate language information and dependencies cleanly.