Optimizing Library Linking in CMake for Xcode: XCODE_LINK_BUILD_PHASE_MODE and Alternatives

Purpose

• Introduced in CMake version 3.19.
• Controls how Xcode links libraries to your target (executable, shared library, framework, etc.) during the build process.

Values

• KNOWN_LOCATION
  
  • Employs "Link Binary With Libraries" phase for linking under the same conditions as BUILT_ONLY, plus:
    • Imported library targets (except those of unknown type).
    • Any non-target library specified directly with a path.
  • If the conditions aren't met, libraries are linked via linker flags.
• BUILT_ONLY
  
  • Uses "Link Binary With Libraries" phase for linking to other CMake targets under these conditions:
    • The target being linked to is a non-imported, non-interface library (regular library).
    • The output directory of the target being built hasn't been changed from its default (RUNTIME_OUTPUT_DIRECTORY or LIBRARY_OUTPUT_DIRECTORY).
• NONE
  Libraries are linked directly using linker flags, bypassing Xcode's build phase.

Considerations

• If multiple directories have libraries with identical filenames, some might be linked incorrectly due to Xcode's search path lookup limitations.
• Libraries linked through "Link Binary With Libraries" come after those linked with flags. This order matters if static libraries contain symbols with the same name; the latter takes precedence.

Setting XCODE_LINK_BUILD_PHASE_MODE

• Use the set_target_properties command within your CMakeLists.txt file:

  set_target_properties(your_target_name PROPERTIES XCODE_LINK_BUILD_PHASE_MODE "BUILT_ONLY")
  

Common Use Cases

• KNOWN_LOCATION
  For situations where you need more control over linking behavior, including imported libraries or libraries specified by paths.
• BUILT_ONLY
  For linking to other CMake targets within the same project, especially when using default output directories.
• Default (NONE)
  Suitable for simple projects or when you have specific control over linker flags.

• Consider KNOWN_LOCATION if you need more flexibility or encounter linking issues with imported libraries.
• BUILT_ONLY is often a good starting point for most CMake projects within Xcode.
• The optimal value depends on your project structure, library types, and preferences.

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Default Behavior (NONE)

# No explicit setting, libraries will be linked using linker flags
target_link_libraries(your_target_name some_library another_library)

Linking to a Built CMake Target (BUILT_ONLY)

# Assuming "my_library" is a CMake target within the same project
add_library(my_library some_source.cpp)  # Create the library target
target_link_libraries(your_target_name my_library)  # Link your target to "my_library"

# Optional: Set BUILT_ONLY explicitly (useful for clarity)
set_target_properties(your_target_name PROPERTIES XCODE_LINK_BUILD_PHASE_MODE "BUILT_ONLY")

# Assuming "external_library" is an imported library
find_package(SomeLibrary REQUIRED)  # Find and import the library
target_link_libraries(your_target_NAME SomeLibrary::SomeLib)

# Optional: Set KNOWN_LOCATION explicitly (useful for libraries with non-standard paths)
set_target_properties(your_target_name PROPERTIES XCODE_LINK_BUILD_PHASE_MODE "KNOWN_LOCATION")

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Linker Flags

• You explicitly specify linker flags for each library you want to link with:

  target_link_libraries(your_target_name PRIVATE some_library another_library PUBLIC -lSomeExternalLib)
  

  • PRIVATE: Links the library but hides it from the Xcode project's build phase (useful for internal dependencies).
  • PUBLIC: Links the library and makes it visible in the Xcode project's build phase.
  • -lSomeExternalLib: Flag for linking to an external library (replace with the actual flag).

• This is the default behavior if you don't set XCODE_LINK_BUILD_PHASE_MODE.

Target-Specific Linker Flags

• Use target_link_options to specify linker flags for a particular target:

  target_link_options(your_target_name PRIVATE "-Wl,-as-needed")  # Example flag
  

  • This approach provides more granular control over linker settings per target.

Choosing the Right Approach

• Target-specific linker flags can be useful if you need to apply different linker settings to different targets.
• XCODE_LINK_BUILD_PHASE_MODE can be helpful if you have numerous libraries and want Xcode to manage some of the linking process for built or known-location libraries.
• If you only have a few libraries and want a simple setup, linker flags might suffice.

• Using a combination of these approaches might be suitable depending on your project's structure.
• XCODE_LINK_BUILD_PHASE_MODE simplifies linking but might not provide the same level of control as linker flags.
• Linker flags offer more flexibility but can become cumbersome for complex projects.