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Understanding CMake's CMAKE_LANG_ARCHIVE_CREATE Variable


Purpose

  • It acts as a replacement for the legacy CMAKE_<LANG>_CREATE_STATIC_LIBRARY variable on certain platforms, particularly those that can handle a large number of object files within a single archive.
  • It's a rule variable in CMake that dictates how to create a static archive (.a file) for a specific programming language.

When to Use

  • You generally wouldn't need to set this variable manually unless you're encountering issues with the default behavior or have very specific requirements for archive creation.
  • CMake automatically employs CMAKE_LANG_ARCHIVE_CREATE when it detects the need to generate static libraries, especially on platforms that benefit from its capabilities.

Relationship with Other Variables

  • It works in conjunction with two other related variables:
    • CMAKE_<LANG>_ARCHIVE_APPEND: Used to add object files incrementally to an existing static archive.
    • CMAKE_<LANG>_ARCHIVE_FINISH: Signals the completion of archive creation.

Benefits

  • Enables CMake to leverage platform-specific tools or techniques for creating static libraries that can accommodate a significant number of object files efficiently.

In Summary

  • It's usually not necessary to modify this variable directly in your CMakeLists.txt files.
  • CMAKE_LANG_ARCHIVE_CREATE is an internal mechanism managed by CMake to construct static archives effectively on various platforms.
  • If you have advanced requirements for static library creation beyond the defaults, it's recommended to consult the CMake documentation for platform-specific considerations and potentially explore custom commands or targets within your CMake project.
  • While the documentation mentions CMAKE_<LANG> (where <LANG> represents a language like C or C++), it's important to note that defining custom versions of these variables (e.g., CMAKE_ASM_ARCHIVE_CREATE) for assembly language is currently ignored by CMake.

Example 1: Creating a Static Library (C++)

# Add source files to your target
add_library(my_library OBJECT source1.cpp source2.cpp)

# CMake automatically handles archive creation using CMAKE_LANG_ARCHIVE_CREATE
target_link_libraries(my_program my_library)

In this example, CMake will use the appropriate rule based on your platform (determined by CMAKE_LANG_ARCHIVE_CREATE) to generate the static library (my_library.a) when you build the target my_program.

Example 2: Adding Files to an Existing Archive (Custom Command)

# Create an empty archive
archive_create TEST my_library.a

# Add object files incrementally (might use CMAKE_<LANG>_ARCHIVE_APPEND internally)
target_sources(my_library PRIVATE source3.cpp)

# CMake will likely use CMAKE_LANG_ARCHIVE_FINISH to finalize the archive
target_link_libraries(my_program my_library)

Here, you're manually creating an empty archive (my_library.a) using the archive_create command. Then, when you add more source files to the target my_library, CMake might employ CMAKE_<LANG>_ARCHIVE_APPEND internally to efficiently update the existing archive. Finally, linking my_library to my_program would likely involve CMAKE_LANG_ARCHIVE_FINISH to ensure the archive is properly finalized.


Leveraging CMake's Static Library Support

  • If you encounter issues, consider:
    • Upgrading CMake to the latest version to benefit from potential improvements or bug fixes related to archive creation.
    • Checking your compiler and platform documentation for specific flags or techniques you might need to incorporate in your CMake project.
  • In most cases, the best approach is to let CMake handle static library creation for you. It employs CMAKE_LANG_ARCHIVE_CREATE effectively on most platforms.

Custom Commands for Advanced Control (For Experts)

Important Considerations

  • This approach is generally recommended only for advanced users who have exhausted other options and have specific requirements not met by CMake's defaults.
  • Custom commands require a deeper understanding of archive creation on your target platform and can introduce complexity to your CMake project.
  • Consider custom commands only as a last resort for very specific needs, requiring advanced CMake expertise.
  • If you encounter issues, investigate CMake version compatibility and platform-specific solutions.
  • For most scenarios, it's advisable to let CMake handle static library creation through CMAKE_LANG_ARCHIVE_CREATE.