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Optimizing Your CMake Build Process: Alternatives to add_custom_target()


Purpose

  • Provides a way to integrate tasks that aren't directly related to compiling source code.
  • Executes a set of shell commands during the build.
  • Creates a custom target in your CMake project's build process.

Basic Syntax

add_custom_target(TARGET_NAME COMMAND COMMANDS...)
  • COMMANDS
    A sequence of shell commands (or command-line arguments) that you want to execute during the build process. These can be any valid commands that your build system understands (e.g., shell scripts, Python scripts, etc.).
  • TARGET_NAME
    A unique name for your custom target. This name will be used to reference it within your CMakeLists.txt file.

Optional Arguments

  • COMMENT
    (Optional) Adds a human-readable comment to describe the purpose of the custom target.
  • DEPENDS
    (Optional) Lists other targets (executable, library, or custom) that this custom target depends on. The custom target will only be built if any of its dependencies have changed.
  • WORKING_DIRECTORY
    (Optional) Specifies the working directory where the commands should be executed. Defaults to the current build directory.

Example

add_custom_target(generate_documentation
  COMMAND ${CMAKE_COMMAND} -E make_directory documentation
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/src
  COMMENT "Generate project documentation using Makefile"
  DEPENDS main.cpp)

In this example:

  • A comment is added to explain the purpose of the custom target.
  • The custom target depends on the main.cpp file. This means that if main.cpp changes, the generate_documentation target will be rebuilt.
  • The command is run in the src directory using the WORKING_DIRECTORY option.
  • It executes the make_directory command from CMake to create a directory named documentation in the src subdirectory of the source tree.
  • A custom target named generate_documentation is created.

Key Points

  • Use dependencies wisely to avoid unnecessary rebuilds.
  • Custom targets are always considered out of date, meaning they'll be rebuilt whenever CMake reruns the build process.
  • Use add_custom_command() if you need to create files as outputs from the custom commands.
  • Any task that needs to be part of the build process but isn't directly related to compilation.
  • Performing code generation or analysis tasks.
  • Creating documentation.
  • Downloading or generating external files.
  • Running build scripts or other build system tools.

Running a Shell Script

add_custom_target(run_tests
  COMMAND /path/to/myscript.sh
  COMMENT "Run unit tests using a custom script")

This example executes a shell script named myscript.sh located at /path/to/myscript.sh during the build process.

Running a Python Script

add_custom_target(generate_data
  COMMAND python ${CMAKE_CURRENT_SOURCE_DIR}/data_generator.py output.txt
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/data
  COMMENT "Generate data file using a Python script")

This example executes the Python script data_generator.py located in the data subdirectory of the source tree. The script creates a file named output.txt in the same directory.

Downloading a File

add_custom_target(download_dependency
  COMMAND curl -L https://example.com/dependency.zip -o dependency.zip
  COMMENT "Download a dependency from a URL")

This example downloads a file named dependency.zip from the specified URL using the curl command.

add_custom_command(
  OUTPUT processed_data.txt
  COMMAND python ${CMAKE_CURRENT_SOURCE_DIR}/process_data.py input.txt processed_data.txt
  DEPENDS input.txt
  COMMENT "Process data using a Python script")

add_custom_target(my_program
  DEPENDS processed_data.txt
  # ... rest of your target definition using processed_data.txt)
  • Another custom target named my_program depends on processed_data.txt, ensuring it's built before my_program.
  • The command depends on input.txt so it only runs if the input file changes.
  • The OUTPUT keyword specifies the generated file.
  • It defines a command that processes input.txt using a Python script and creates an output file named processed_data.txt.
  • This example demonstrates add_custom_command().

    • If your custom command generates an output file that another target depends on, you can use add_custom_command() directly. This allows you to specify the output file and dependencies for the command.

    • Example

      add_custom_command(
  OUTPUT processed_data.txt
  COMMAND python ${CMAKE_CURRENT_SOURCE_DIR}/process_data.py input.txt processed_data.txt
  DEPENDS input.txt
  COMMENT "Process data using a Python script")

add_executable(my_program main.cpp processed_data.txt)

    • This approach is more concise when you have a clear dependency relationship between the generated file and another target.

Choosing the Right Approach

The best alternative to add_custom_target() depends on the complexity and purpose of your custom task:

  • Explore existing CMake features for common tasks before resorting to custom targets.
  • For complex build systems, consider external projects or modules.
  • For commands with outputs used by other targets, add_custom_command() directly is a good option.
  • For simple commands without output files, add_custom_target() is sufficient.