之前介绍过GYP，它是Google早期用来维护chromium项目的meta-build system，GN则是用来替代GYP的工具，目前chromium及相关的开源项目都迁移到基于GN来管理。并且GN是基于C++编写，效率上比基于python的GYP快了近20倍。

环境准备

配置depot_tools (ninja)

参考：http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html

git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git

配置gn

虽然depot_tools已经带了gn，但它其实只是一个shell脚本，直接使用会报错，找不到真正的gn二进制文件。

如果编译过webrtc或chromium，gn相关的工具链已经下载，可以在src/buildtools/mac/gn找到。

自己编译可以基于gn的git仓库编译，可以参考：

#!/bin/bash 
set -e
set -v

# Get the sources
mkdir gn-standalone
cd gn-standalone
mkdir tools
cd tools
git clone https://chromium.googlesource.com/chromium/src/tools/gn
cd ..
mkdir -p third_party/libevent
cd third_party/libevent
wget --no-check-certificate https://chromium.googlesource.com/chromium/chromium/+archive/master/third_party/libevent.tar.gz
tar -xvzf libevent.tar.gz
cd ../..
git clone https://chromium.googlesource.com/chromium/src/base
git clone https://chromium.googlesource.com/chromium/src/build
git clone https://chromium.googlesource.com/chromium/src/build/config
mkdir testing
cd testing
git clone https://chromium.googlesource.com/chromium/testing/gtest
cd ..

# Build
cd tools/gn
./bootstrap/bootstrap.py -s

# At this point, the resulting binary is at:
# gn-standalone/out/Release/gn

使用gn的例子

gn的例子可以参考gn的git仓库中带的example来开始。比如，以下测试的代码根目录是src

src根目录下增加.gn文件

.gn文件内容

buildconfig = "//build/BUILDCONFIG.gn"

.gn文件所在的目录是GN工具认定的工程的source root，.gn的内容基本就是指定buildconfig，//是指source root所在的路径。

建立build/BUILDCONFIG.gn

gn的build目录的配置可以参考gn仓库中example的build配置，但是需要适当修改在才能正常使用

BUILDCONFIG.gn 内容如下：

set_default_toolchain("//build/toolchains:gcc")

cflags_cc = [ "-std=c++11" ]

第一行指定要使用的 toolchain，//build/toolchains:gcc 指的是 build/toolchains/BUILD.gn 里面定义的 gcc toolchain

建立 build/toolchains/BUILD.gn

因为gn没有内建的toolchain规则，都需要自己定义

# Copyright 2014 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

toolchain("gcc") {
  tool("cc") {
    depfile = ".d"
    command = "gcc -MMD -MF $depfile     -c  -o "
    depsformat = "gcc"
    description = "CC "
    outputs = [
      "/..o",
    ]
  }

  tool("cxx") {
    depfile = ".d"
    command = "g++ -MMD -MF $depfile     -c  -o "
    depsformat = "gcc"
    description = "CXX "
    outputs = [
      "/..o",
    ]
  }

  tool("alink") {
    rspfile = ".rsp"
    command = "rm -f  && ar rcs  @$rspfile"
    description = "AR "
    rspfile_content = ""
    outputs = [
      "/",
    ]
    default_output_extension = ".a"
    output_prefix = "lib"
  }

  tool("solink") {
    soname = ""  # e.g. "libfoo.so".
    sofile = "/$soname"
    rspfile = soname + ".rsp"

    command = "g++ -shared  -o $sofile -Wl,-soname=$soname @$rspfile"
    rspfile_content = "-Wl,--whole-archive   -Wl,--no-whole-archive "

    description = "SOLINK $soname"

    # Use this for  expansions unless a target manually
    # overrides it (in which case  will be what the target
    # specifies).
    default_output_extension = ".so"

    # Use this for  expansions unless a target manually overrides
    # it (in which case  will be what the target specifies).
    default_output_dir = ""

    outputs = [
      sofile,
    ]
    link_output = sofile
    depend_output = sofile
    output_prefix = "lib"
  }

  tool("link") {
    outfile = ""
    rspfile = "$outfile.rsp"
    command = "g++  -o $outfile @$rspfile  "
    description = "LINK $outfile"
    default_output_dir = ""
    rspfile_content = ""
    outputs = [
      outfile,
    ]
  }

  tool("stamp") {
    command = "touch "
    description = "STAMP "
  }

  tool("copy") {
    command = "cp -af  "
    description = "COPY  "
  }
}

还可以参考下 chromium配置toolchain的做法

src根目录下建立 BUILD.gn

group("default") {
  deps = [
    "//test:hello_world",
  ]
}

src/test目录下增加build.gn配置与源文件

• 增加 BUILD.gn

executable("hello_world") {
  sources = [
    "test.cpp",
  ]
}

• 增加 test.cpp

#include <iostream>

int main() {
  std::cout << "hello world" << std::endl;
  return 0;
}

编译

$ gn gen out/def --ide=xcode
Generating Xcode projects took 0ms
Done. Made 2 targets from 5 files in 6ms
$
$ ninja -C out/def
ninja: Entering directory `out/def'
[3/3] STAMP obj/default.stamp
$
$ ./out/def/hello_world 
hello world

参考

• https://blog.simplypatrick.com/posts/2016/01-23-gn/
• GN Quick Start guide
• GN Language and Operation