Understanding CMake's CMP0014: When Subdirectories Must Have CMakeLists.txt

CMP0014: Input Directories Must Have CMakeLists.txt

This policy controls CMake's behavior when encountering subdirectories referenced by add_subdirectory or subdirs commands that lack a CMakeLists.txt file.

Prior to CMake 2.8.0

• The subdirectory was treated as if it had an empty CMakeLists.txt, essentially bypassing the build process for that directory.
• CMake silently ignored the missing CMakeLists.txt file.

From CMake 2.8.0 onwards (controlled by CMP0014)

• CMake's behavior depends on the setting of the CMP0014 policy:
  • OLD Behavior (default if not explicitly set)
    
    • Issues a warning but continues with the build process, emulating the pre-2.8.0 behavior (potentially leading to unexpected results).
  • NEW Behavior (recommended)
    
    • Considers the missing CMakeLists.txt an error and halts the build process. This ensures that subdirectories are properly configured for building.

Setting the Policy

You can explicitly set the CMP0014 policy using the cmake_policy command:

cmake_policy(CMP0014 NEW)  # Enforce NEW behavior (error on missing CMakeLists.txt)
cmake_policy(CMP0014 OLD)   # Allow OLD behavior (warning, continue)

Recommendation

Setting CMP0014 to NEW is generally recommended for clarity and to prevent potential build issues arising from missing configuration files. This ensures that all subdirectories intended for inclusion in the project have a CMakeLists.txt file to define their build steps.

• The cmake_minimum_required command can also be used to set CMP0014 along with other required CMake version and policy settings.
• If you're working with legacy projects that might not have CMakeLists.txt files in all subdirectories, you can temporarily set CMP0014 to OLD to accommodate the build process. However, consider updating such projects to include proper CMakeLists.txt files for consistency and maintainability.

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Scenario 1: OLD Behavior (default if not set explicitly)

# This subdirectory (missing_subdir) might not have a CMakeLists.txt file

add_subdirectory(missing_subdir)  # Warning might be issued here, but build continues

# Rest of your CMakeLists.txt

In this case, if missing_subdir doesn't have a CMakeLists.txt file:

• With the default (OLD) behavior, CMake will issue a warning but continue building your project, potentially leading to the subdirectory being ignored.

Scenario 2: NEW Behavior (recommended)

# Set CMP0014 to enforce NEW behavior (error on missing CMakeLists.txt)
cmake_policy(CMP0014 NEW)

add_subdirectory(missing_subdir)  # This will result in an error if missing_subdir lacks CMakeLists.txt

Here, if missing_subdir lacks a CMakeLists.txt file:

• CMake will throw an error and halt the build process, preventing unexpected behavior.

# Project might have subdirectories without CMakeLists.txt (legacy)

# Temporarily allow OLD behavior for this project
cmake_policy(CMP0014 OLD)

add_subdirectory(legacy_subdir)  # Warning might be issued, but build continues

# Reset CMP0014 to recommended NEW behavior for future projects
cmake_policy(CMP0014 NEW)

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1. • You can write custom CMake code to iterate through subdirectories referenced by add_subdirectory or subdirs.
   • Within the loop, check if each subdirectory has a CMakeLists.txt file using the EXISTS command.
   • If a CMakeLists.txt is missing, you can issue a warning or error using message depending on your desired behavior.
   • This approach provides more granular control but requires writing additional code.

2. Advanced CMake Modules
   

   • Create a custom CMake module that encapsulates the desired behavior.
   • This module could take a list of subdirectories and perform the checks for missing CMakeLists.txt files.
   • You can then include this module in your project's main CMakeLists.txt and use it for all subdirectory checks.
   • This method offers reusability but might add complexity to your project structure.

3. External Tools
   

   • In some workflows, you can leverage external tools like pre-build scripts or linters to identify missing CMakeLists.txt files before invoking CMake.
   • These tools can scan your project directory structure and raise warnings or errors about missing configuration files.
   • This approach might be more suitable for integrating with CI/CD pipelines but may require additional tooling setup.

Choosing the Right Approach

• For integrating with build pipelines
  Explore external tools to automate early detection of missing configuration files.
• For reusability across projects
  Create a custom CMake module for centralized and reusable checks.
• For complex projects with legacy code
  Consider the manual checks approach to handle missing CMakeLists.txt gracefully while building.
• For simple projects
  Opt for setting CMP0014 to the desired behavior (NEW for clarity and error prevention).