Understanding CMAKE_MODULE_LINKER_FLAGS_CONFIG for Module Linking

Purpose

• Similar to CMAKE_C_FLAGS_<CONFIG>, but it targets the linker stage during module creation.
• Controls linker flags specifically for creating modules (shared libraries, DLLs, etc.) in your CMake project.

Functionality

• Accessible within CMake scripts to customize the linking process for your modules.
• Flags can be set for different build configurations (e.g., Debug, Release) using the <CONFIG> suffix.
• Stores linker flags as a string value.

Usage

1. Setting Values
   

   • Use the set command to define the variable:

   set(CMAKE_MODULE_LINKER_FLAGS_DEBUG "-g")
   set(CMAKE_MODULE_LINKER_FLAGS_RELEASE "-O3")
   

   Here, -g enables debugging symbols for the Debug configuration, and -O3 optimizes for performance in the Release configuration.

2. Accessing Values
   

   • Use the variable within CMake code to incorporate the flags into your build process:

   find_library(MY_LIBRARY ...)  # Find a library
   
   target_link_libraries(MY_MODULE PUBLIC MY_LIBRARY)  # Link against the library
   target_link_options(MY_MODULE PRIVATE "${CMAKE_MODULE_LINKER_FLAGS_<CONFIG>}")  # Apply linker flags
   

   In this example, target_link_options applies the appropriate linker flags based on the current build configuration (<CONFIG>) when linking your MY_MODULE.

Key Points

• Offers flexibility for customizing module creation.
• Integrates seamlessly with CMake's build system.
• Provides configuration-specific control over module linking.

• Consider using inheritance with the CMAKE_MODULE_LINKER_FLAGS variable to set common flags for all configurations and then override specific flags for individual configurations using CMAKE_MODULE_LINKER_FLAGS_CONFIG.
• You can combine this variable with other linker flags using string manipulation commands like append or prepend.

---

# Set common flags for all configurations (inherited)
set(CMAKE_MODULE_LINKER_FLAGS "-Wl,-rpath,$ORIGIN")

# Set configuration-specific flags
set(CMAKE_MODULE_LINKER_FLAGS_DEBUG "-g")
set(CMAKE_MODULE_LINKER_FLAGS_RELEASE "-O3")

# Find a library
find_library(MY_LIBRARY ...)

# Create a module with configuration-specific linking
add_library(MY_MODULE MODULE source1.cpp source2.cpp)
target_link_libraries(MY_MODULE PUBLIC MY_LIBRARY)

# Apply linker flags based on configuration
target_link_options(MY_MODULE PRIVATE "${CMAKE_MODULE_LINKER_FLAGS_<CONFIG>}")

# Add additional flags (specific to Release configuration)
if(CMAKE_BUILD_TYPE STREQUAL "Release")
  target_link_options(MY_MODULE PRIVATE "-Wl,--no-as-needed")
endif()

1. • set(CMAKE_MODULE_LINKER_FLAGS "-Wl,-rpath,$ORIGIN") defines a common flag (-Wl,-rpath,$ORIGIN) applied to all configurations using the base CMAKE_MODULE_LINKER_FLAGS variable. This flag instructs the linker to search for dependent libraries in the directory containing the module itself ($ORIGIN).

2. Configuration-Specific Flags
   

   • set(CMAKE_MODULE_LINKER_FLAGS_DEBUG "-g") and set(CMAKE_MODULE_LINKER_FLAGS_RELEASE "-O3") define flags specific to the Debug and Release configurations, respectively. These override the common flags for those configurations.

3. Finding Library and Creating Module
   

   • find_library(MY_LIBRARY ...) locates a library named MY_LIBRARY in the project.
   • add_library(MY_MODULE MODULE source1.cpp source2.cpp) creates a module named MY_MODULE from source files source1.cpp and source2.cpp.

4. Linking and Applying Flags
   

   • target_link_libraries(MY_MODULE PUBLIC MY_LIBRARY) links the MY_LIBRARY to the MY_MODULE.
   • target_link_options(MY_MODULE PRIVATE "${CMAKE_MODULE_LINKER_FLAGS_<CONFIG>}") applies the appropriate configuration-specific linker flags defined earlier. CMake automatically expands <CONFIG> to the current build configuration (e.g., Debug or Release).

5. Additional Flags for Release
   

   • The if statement checks if the current build type is Release. If so, an additional flag -Wl,--no-as-needed is appended using target_link_options specifically for the Release configuration. This flag tells the linker to avoid unnecessary symbol exporting from the module.

---

target_link_options with Conditions

• You can directly apply linker flags within the target_link_options command using conditional statements. This approach avoids a separate variable for configuration-specific flags.

find_library(MY_LIBRARY ...)

add_library(MY_MODULE MODULE source1.cpp source2.cpp)
target_link_libraries(MY_MODULE PUBLIC MY_LIBRARY)

target_link_options(MY_MODULE PRIVATE
    "$<CONFIG:DEBUG>" "-g"  # Debug flag for Debug configuration
    "$<CONFIG:RELEASE>" "-O3" # Release flag for Release configuration
)

Here, CMake evaluates the condition within $<CONFIG:...> based on the current build configuration and applies the corresponding flag.

CMAKE_SHARED_LINKER_FLAGS (Limited Flexibility)

• If your configuration-specific flags are more generic and apply to all shared libraries (not just modules), you can use CMAKE_SHARED_LINKER_FLAGS_<CONFIG>. This offers less granularity as it applies to all shared libraries, not just modules.

Custom CMake Modules

• For complex linking requirements with intricate logic, you might create a custom CMake module. This module can encapsulate the configuration logic and flag manipulation for linking modules. However, this approach has a steeper learning curve.

• For advanced scenarios or reusability, a custom CMake module might be worth exploring.
• If you need to share linker flags across multiple shared libraries, CMAKE_SHARED_LINKER_FLAGS_<CONFIG> might be suitable, but consider its limitations.
• For simple configuration-specific linking, target_link_options with conditions is concise and readable.