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RCFLAGS vs. CMAKE_RC_FLAGS: Setting Resource Compilation Flags in CMake


What is RCFLAGS?

RCFLAGS is a CMake environment variable that allows you to specify default compilation flags for resource files (.rc) during the build process. These flags are used by the resource compiler (usually windres on Windows or a platform-specific tool) to control how the resource files are compiled.

How it Works

    • CMake retrieves the initial value of RCFLAGS from the environment where you run the CMake command.

  2. First Configuration

    • During the first configuration of your build tree for a project that uses resource files, CMake uses the value of RCFLAGS (if set) along with its own built-in default flags for the toolchain.
    • This combined set of flags is used to initialize the internal CMake cache variable CMAKE_RC_FLAGS.

  3. Subsequent Configurations

    • For any subsequent configuration runs (including the first one if CMAKE_RC_FLAGS is already defined), CMake ignores the RCFLAGS environment variable.
    • It relies solely on the cached value stored in CMAKE_RC_FLAGS to determine the resource compilation flags.

Key Points

  • Once set, the cached value takes precedence over any future changes to the RCFLAGS environment variable.
  • It's only used during the initial configuration to set the CMAKE_RC_FLAGS cache variable.
  • RCFLAGS provides a way to customize resource compilation behavior at build time.

Example Usage

If you want to add a specific flag (e.g., -I/custom/include/path) to the resource compilation process, you could set the RCFLAGS environment variable before running CMake:

export RCFLAGS="-I/custom/include/path"  # On Linux/macOS
set RCFLAGS="-I/custom/include/path"    # On Windows
cmake ..

Alternative Approach

Instead of using RCFLAGS, you can directly set the CMAKE_RC_FLAGS cache variable after the first configuration:

cmake -DRCFlags="-I/custom/include/path" ..

Example 1: Setting RCFLAGS in the Environment

# Assuming you want to include headers from a custom directory
export RCFLAGS="-I/custom/include/path"  # On Linux/macOS
set RCFLAGS="-I/custom/include/path"    # On Windows

# Configure your CMake project
cmake ..

In this example, the RCFLAGS environment variable is set before running cmake to include headers from the /custom/include/path directory during resource compilation.

Example 2: Setting CMAKE_RC_FLAGS in CMakeLists.txt

# Within your CMakeLists.txt file

# Check if RCFLAGS is set (optional, for informative purposes)
if(NOT DEFINED ENV{RCFLAGS})
  message(STATUS "RCFLAGS environment variable is not set")
endif()

# Set CMAKE_RC_FLAGS directly
set(CMAKE_RC_FLAGS "-I/custom/include/path" CACHE STRING "" FORCE)

# Add your resource files here (e.g., using target_include_directories)
target_include_directories(your_target PRIVATE /path/to/resource/files)

# Build your project
add_executable(your_target your_source_files.cpp ${your_resource_files.rc})

This example demonstrates setting CMAKE_RC_FLAGS directly within your CMakeLists.txt file. It also includes a check for the existence of RCFLAGS (optional for informational purposes). This approach keeps the resource compilation configuration within your project's build system, making it more self-contained.

  • For project-specific configurations or to avoid reliance on external environment variables, setting CMAKE_RC_FLAGS within your CMakeLists.txt file is generally recommended.
  • If you need to apply the same resource compilation flags across multiple projects, setting RCFLAGS in the environment might be convenient.

CMAKE_RC_FLAGS Cache Variable

This is the recommended approach within your CMake project. It allows you to directly set the flags within your CMakeLists.txt file:

set(CMAKE_RC_FLAGS "-I/custom/include/path" CACHE STRING "" FORCE)

  • Disadvantages

    • May require modifying existing CMakeLists.txt files to set the flags.
    • More control within your CMake project.
    • Makes the build configuration self-contained and independent of external environment variables.
    • Flags can be conditioned based on project-specific requirements.

Target Properties

For specific build targets, you can use target properties to define resource compilation flags:

target_compile_options(your_target PRIVATE "-I/custom/include/path")

  • Disadvantages

    • May not be ideal for global or project-wide resource compilation flags.
    • May require additional configuration for each target that needs specific flags.

  • Advantages

    • Allows applying flags to specific targets within your project.
    • Useful if different targets require different resource compilation settings.

Choosing the Best Alternative

The best alternative depends on your project's needs:

  • If different targets require specific resource compilation flags, use target properties.
  • If you have project-wide resource compilation flags, use CMAKE_RC_FLAGS in your CMakeLists.txt.

Additional Considerations

  • If you need to manage complex resource compilation configurations, consider using custom CMake modules or tools to centralize and automate flag management.
  • Some CMake generators (e.g., Visual Studio) might have built-in mechanisms for setting resource compilation flags, potentially overriding CMake's approach. Consult your generator's documentation for details.