gn编译webrtc介绍

说明:因为需要通过gn编译webrtc在此介绍源代码和自研代码的添加gn命令和格式。

概述:我们通过gclient实现webrtc下载源代码后使用gn此时，生成相应平台的文件(.ninja)。

gn参考：https://www.chromium.org/developers/gn-build-configuration

gn介绍：(generate ninja )顾名思义就是生成ninja文件是编译工具GYP(Generate Your Project)替代工具，由google开源c 实现编写，主要实现交叉编译，可指定输出平台目标和多个目标平台。

gn gen out/Default --args='target_os="linux" target_cpu="x64"

既然是解释器，就会有相应的语法。windows以平台为例.gn语法介绍。

BUILD.gn截取一段文件分析如下：

1：首先import.gni该文件通常定义为公共功能。

2.编译器将找到相应的目标进行编译。目标的意思是我们的模块需要编译什么，如静态库、动态库或操作程序。只有指定的目标编译器才能生成。主要类型如下：

executable：生成exe程序。group：包含多个目标虚拟节点。shared_library：生成静态库lib或.a。loadable_module：动态库或者.so。source_set：轻量级静态库(常用)。app：用于mac/ios。android_apk：生成andorid平台apk.

3:依赖每个目标deps包括。此时将涉及路径问题，当地相同路径的开头为冒号:。如果是根目录，则为//开头。

4:通过一些变量gn的args传递。

# Copyright (c) 2013 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. # This is the root build file for GN. GN will start processing by loading this # file, and recursively load all dependencies until all dependencies are either # resolved or known not to exist (which will cause the build to fail). So if # you add a new build file, there must be some path of dependencies from this # file to your new one or GN won't know about it. import("//build/config/compiler/compiler.gni") ... if (is_android) {   import("//build/config/android/config.gni") }  # This file defines the following two main targets: # # "gn_all" is used to create explicit dependencies from the root BUILD.gn to # each top-level component that we wish to include when building everything via # "all". This is required since the set of targets built by "all" is determined # automatically based on reachability from the root BUILD.gn (for details, see # crbug.com/503241). Builders should typically use "all", or list targets # explicitly, rather than relying on "gn_all". # # "gn_visibility": targets that are normally not visible to top-level targets, # but are built anyway by "all". Since we don't want any such targets, we have # this placeholder to make sure hidden targets that aren't otherwise depended # on yet are accounted for. group("gn_all") {   testonly = true   deps = [     ":gn_visibility",     "//base:base_perftests",     "//base:base_unittests",     "//base/util:base_util_unittests",     "//chrome/installer",     "//chrome/updater",     "//components:components_unittests",     "//components/gwp_asan:gwp_asan_unittests",     "//net:net_unittests",     "//services:services_unittests",     "//services/service_manager/public/cpp",     "//skia:skia_unittests",     "//sql:sql_unittests",     "//third_party/flatbuffers:flatbuffers_unittests",     "//tools/binary_size:binary_size_trybot_py",     "//tools/ipc_fuzzer:ipc_fuzzer_all",     "//tools/metrics:metrics_metadata",     "//ui/base:ui_base_unittests",     "//ui/color:color_unittests",     "//ui/gfx:gfx_unittests",     "//url:url_unittests",   ]   if (!is_component_build) {     deps  = [ "//third_party/abseil-cpp:absl_tests" ]   }   if (!is_component_build) {     deps  = [ "//third_party/abseil-cpp:absl_tests" ]   }   if (!is_android && !is_chromecast) {     deps  = [       "//crypto:crypto_unittests",       "//google_apis/gcm:gcm_unit_tests",     ]   }   if (enable_js_type_check) {     deps  = [ ":webui_closure_compile" ]   }   if (!is_ios && !is_ios && !is_android && !is_chromecast && !is_fuchsia) {     deps  = [       "//chrome",       "//chrome/browser/ui/color:dump_colors",       "//chrome/test:browser_tests",       "//chrome/test:interactive_ui_tests",       "//chrome/test:sync_integration_tests",       "//chrome/test/chromedriver:chromedriver_unittests",       "//components/subresource_filter/tools:subresource_filter_tools",       "//components/zucchini:zucchini",       "//components/zucchini:zucchini_unittests",       "//gpu/gles2_conform_support:gles2_conform_test",       "//gpu/khronos_glcts_support:khronos_glcts_test",       "//jingle:jingle_unittests",       "//net:hpack_example_generator",       "//ppapi:ppapi_unittests",       "//ppapi/examples/2d",       "//ppapi/examples/audio",       "//ppapi/examples/audio_input",       "//ppapi/examples/crxfs",       "//ppapi/examples/enumerate_devices",       "//ppapi/examples/file_chooser",       "//ppapi/examples/flash_topmost",       "//ppapi/examples/font",       "//ppapi/examples/gamepad",       "//ppapi/examples/gles2",       "//ppapi/examples/gles2_spinning_cube",       "//ppapi/examples/ime",       "//ppapi/examples/input",       "//ppapi/examples/media_stream_audio",       "//ppapi/examples/media_stream_video",       "//ppapi/examples/mouse_cursor",       "//ppapi/examples/mouse_lock",       "//ppapi/examples/printing",       "//ppapi/examples/scalin",
      "//ppapi/examples/scripting",
      "//ppapi/examples/stub",
      "//ppapi/examples/threading",
      "//ppapi/examples/url_loader",
      "//ppapi/examples/video_capture",
      "//ppapi/examples/video_decode",
      "//ppapi/examples/video_encode",
      "//printing:printing_unittests",
      "//third_party/SPIRV-Tools/src:SPIRV-Tools",
      "//third_party/SPIRV-Tools/src/test/fuzzers",
      "//third_party/pdfium/samples:pdfium_test",
      "//tools/perf/clear_system_cache",
      "//tools/polymer:polymer_tools_python_unittests",
      "//ui/accessibility:accessibility_perftests",
      "//ui/accessibility:accessibility_unittests",
      "//ui/accessibility/extensions:extension_tests",
      "//ui/accessibility/extensions:extensions",
    ]
  }
  if (!is_ios) {
    deps += [
      "//cc:cc_unittests",
      "//components/policy:policy_templates",
      "//components/ui_devtools/viz",
      "//components/url_formatter/tools:format_url",
      "//components/viz:viz_perftests",
      "//components/viz:viz_unittests",
      "//content/shell:content_shell",
      "//content/test:content_browsertests",
      "//content/test:content_perftests",
      "//content/test:content_unittests",
      "//gpu:gpu_benchmark",
      "//gpu:gpu_unittests",
      "//ipc:ipc_tests",
      "//media:media_unittests",
      "//media/midi:midi_unittests",
      "//media/mojo:media_mojo_unittests",
      "//mojo",
      "//mojo:mojo_unittests",
      "//net:net_perftests",
      "//storage:storage_unittests",
      "//third_party/angle/src/tests:angle_end2end_tests",
      "//third_party/angle/src/tests:angle_unittests",
      "//third_party/blink/common:blink_common_unittests",
      "//third_party/blink/renderer/controller:blink_unittests",
      "//third_party/blink/renderer/platform:blink_platform_unittests",
      "//third_party/blink/renderer/platform/heap:blink_heap_unittests",
      "//third_party/blink/renderer/platform/wtf:wtf_unittests",
      "//tools/imagediff",
      "//ui/display:display_unittests",
      "//ui/events:events_unittests",
      "//ui/gl:gl_unittests",
      "//ui/latency:latency_unittests",
      "//ui/native_theme:native_theme_unittests",
      "//ui/touch_selection:ui_touch_selection_unittests",
      "//url/ipc:url_ipc_unittests",
      "//v8:gn_all",
    ]
  }
  if (is_win || (is_linux && !is_chromeos) || is_android) {
    deps += [
      "//weblayer/shell:weblayer_shell",
      "//weblayer/test:weblayer_browsertests",
      "//weblayer/test:weblayer_unittests",
    ]
    if (is_android) {
      deps += [ "//weblayer/browser/android/javatests:weblayer_instrumentation_test_apk" ]
    }
  }
  if (!is_ios && !is_android) {
    deps += [
      "//components/cronet:cronet_tests",
      "//components/cronet:cronet_unittests",
      "//components/viz/demo:viz_demo",
    ]
  }
  if (enable_openscreen) {
    deps += [ "//chrome/browser/media/router:openscreen_unittests" ]
  }
  if (!is_ios && !is_fuchsia) {
    deps += [
      "//chrome/test:telemetry_perf_unittests",
      "//chrome/test:unit_tests",
      "//components:components_browsertests",
      "//device:device_unittests",
      "//google_apis/gcm:mcs_probe",
      "//media/capture:capture_unittests",
      "//media/cast:cast_unittests",
      "//third_party/angle/src/tests:angle_white_box_tests",
      "//third_party/catapult/telemetry:bitmaptools($host_toolchain)",
    ]
  } else if (is_ios) {
    deps += [
      "//ios:all",
      "//third_party/crashpad/crashpad:crashpad_tests",
    ]
  } else if (is_fuchsia) {
    deps += [
      ":d8_fuchsia",
      "//fuchsia:gn_all",
      "//headless:headless_non_renderer",
    ]
  }
  deps += root_extra_deps
  if (enable_extensions) {
    deps += [
      "//extensions:extensions_browsertests",
      "//extensions:extensions_unittests",
      "//extensions/shell:app_shell_unittests",
    ]
    # TODO(crbug.com/981112): This target is failing to compile consistently in
    # coverage build, remove this condition when the bug is fixed.
    if (!use_clang_coverage) {
      deps += [ "//extensions/browser/api/declarative_net_request/filter_list_converter" ]
    }
  }
  if (enable_remoting) {
    deps += [ "//remoting:remoting_all" ]
  }
  if (toolkit_views) {
    deps += [
      "//ui/views:views_unittests",
      "//ui/views/examples:views_examples",
      "//ui/views/examples:views_examples_unittests",
      "//ui/views/examples:views_examples_with_content",
    ]
  }
  if (use_aura) {
    deps += [
      "//ui/aura:aura_demo",
      "//ui/aura:aura_unittests",
      "//ui/wm:wm_unittests",
    ]
  }
  if (use_ozone) {
    deps += [
      "//ui/ozone",
      "//ui/ozone:ozone_unittests",
      "//ui/ozone/demo",
      "//ui/ozone/gl:ozone_gl_unittests",
    ]
    if (ozone_platform_x11) {
      deps += [ "//ui/ozone:ozone_x11_unittests" ]
    }
  }
...

比如meida模块中audio的模块BUILD.gn文件中具体某个静态库的编译,可以看出一个模块指定生成静态库后，会将其包含的源文件以及依赖库都进行加载中，其中依赖库包含本地模块的静态库，也包含一些第三方库。不同的类型库不同的加载方式。

source_set("audio") {
sources = [
    "agc_audio_stream.h",
    "alive_checker.cc",
    "alive_checker.h",
    "audio_debug_file_writer.cc",
    "audio_debug_file_writer.h",
    "audio_debug_recording_helper.cc",
    "audio_debug_recording_helper.h",
    "audio_debug_recording_manager.cc",
    "audio_debug_recording_manager.h",
    "audio_debug_recording_session.h",
    "audio_debug_recording_session_impl.cc",
    "audio_debug_recording_session_impl.h",
    "audio_device_description.cc",
    "audio_device_description.h",
    ]
 if (is_win) {
    sources += [
      "win/audio_device_listener_win.cc",
      "win/audio_device_listener_win.h",
      "win/audio_low_latency_input_win.cc",
      "win/audio_low_latency_input_win.h",
      "win/audio_low_latency_output_win.cc",
      "win/audio_low_latency_output_win.h",
      "win/audio_manager_win.cc",
      "win/audio_manager_win.h",
]
deps += [
      ":aaudio_stubs",
      "//media/base/android:media_jni_headers",
    ]
 libs += [
      "dxguid.lib",
      "setupapi.lib",
      "winmm.lib",
    ]

所以如果我们需要减少编译生成模块大小，和添加自己实现的模块，都可以通过修改gn文件进行。