add a new toolchain

To add a new tool chain to the GN build system, you need to:

1. Create a tool chain definition file. This file will define the tool chain’s name, the compilers and linkers it uses, and any other relevant configuration. For example, the following tool chain definition file defines a tool chain called my_toolchain that uses the Clang compiler and linker:

tool("my_toolchain") {
  toolchain_type = "clang"
  cxx = "clang++"
  ar = "llvm-ar"
  ld = "clang++"
}

2. Add the tool chain definition file to your build directory. This will make it available to GN when it generates build files. For example, if you are building in the out/my_build directory, you would copy the tool chain definition file to out/my_build/BUILDCONFIG.gn.

3. Specify the tool chain to use in your build files. You can do this using the toolchain argument to the target() function. For example, the following target will use the my_toolchain tool chain to compile the my_binary executable:

executable("my_binary") {
  toolchain = "my_toolchain"
  sources = ["my_source.cc"]
}

4. Generate build files for your tool chain. You can do this using the gn gen command. For example, to generate build files for the my_build directory, you would run the following command:

gn gen out/my_build

5. Build your project. Once you have generated build files for your tool chain, you can build your project using the ninja command. For example, to build the my_binary executable, you would run the following command:

ninja -C out/my_build my_binary

If you are adding a new tool chain to a large project, it is a good idea to start by adding it to a small test build. This will help you to identify any potential problems before you deploy the new tool chain to the entire project.

Here are some additional tips for adding a new tool chain to GN:

• You can use the toolchain_args argument to the tool() function to specify additional arguments to be passed to the compiler and linker.
• You can use the target_toolchain argument to the target() function to override the tool chain specified in the BUILDCONFIG.gn file. This can be useful for building specific targets with a different tool chain.
• You can use the gn check command to verify that your tool chain definition is correct and that GN is able to generate build files for it.

declare_args

The declare_args() function in GN is used to declare build arguments. Build arguments are variables that can be used to control the build process, such as the compiler to use, the optimization level, or whether to generate debug symbols.

To use declare_args(), you simply create a block of code and assign values to the arguments you want to declare. For example, the following code declares two build arguments: enable_debug and compiler:

declare_args() {
  enable_debug = false
  compiler = "clang++"
}

Once you have declared build arguments, you can use them in your build files. For example, the following code uses the enable_debug argument to control whether to generate debug symbols:

executable("my_binary") {
  sources = ["my_source.cc"]
  if (enable_debug) {
    debug = true
  }
}

You can also use build arguments to pass values to tools. For example, the following code uses the compiler argument to specify the compiler to use:

tool("my_tool") {
  toolchain_type = "clang"
  cxx = compiler
}

Build arguments can be overridden on the command line when you generate build files. For example, to override the enable_debug argument, you would run the following command:

gn gen out/my_build --args="enable_debug=true"

This would generate build files with the enable_debug argument set to true.

Here are some tips for using declare_args() in GN:

• Use build arguments to control the build process, not to specify the location of source files or other resources.
• Use descriptive names for your build arguments.
• Avoid declaring build arguments that are only used in a single file or target. Instead, consider using a variable or constant.
• Document your build arguments in the comments of your build files.

code

GN is an open-source meta-build system that is written in C++. It is used to generate build files for Ninja, a build system that is known for its speed and efficiency.

GN source code is divided into a number of different modules, including:

• gn/: This module contains the core GN functionality, such as the parser, evaluator, and generator.
• gn/toolchain/: This module contains the code for generating build files for different toolchains.
• gn/target/: This module contains the code for generating build files for different types of targets.
• gn/action/: This module contains the code for generating build files for custom actions.
• gn/test/: This module contains the code for generating build files for running tests.

GN source code is well-written and well-documented. It is easy to read and understand, even for developers who are new to GN.

If you are interested in learning more about how GN works, I encourage you to read the GN documentation and to look at the GN source code.

Here are some tips for reading and understanding GN source code:

• Start by reading the GN documentation. This will give you a good overview of how GN works and the different components of the GN build system.
• Look at the examples in the gn/examples/ directory. These examples show how to use GN to build different types of projects.
• Read the comments in the GN source code. The GN source code is well-commented, and the comments provide valuable information about how the code works.
• Don’t be afraid to experiment. The best way to learn how GN works is to try it out. Create a simple GN build file and try building your project.

GN is a powerful tool that can help you to build your project more efficiently. By learning how to read and understand GN source code, you can customize the build process to meet your specific needs.