开搞

创建文件//tools/gn/tutorial/BUILD.gn

  1. declare_args() {
  2.   enable_teleporter = true
  3.   enable_doom_melon = false
  4. }
  6. shared_library("hello") {
  7.   sources = [
  8.     "hello.cc",
  9.   ]
  10.   all_dependent_configs = [
  11.     ":hello_config"
  12.   ]
  13. }
  15. executable("say_hello") {
  16.   sources = [
  17.     "say_hello.cc",
  18.   ]
  19.   deps = [
  20.     ":hello",
  21.   ]
  22. }
  24. config("hello_config") {
  25.   defines = [
  26.     "TWO_PEOPLE",
  27.   ]
  28. }

修改//BUILD.gn

  1. diff --git a/BUILD.gn b/BUILD.gn
  2. index a1249eda399b..c93ffbd7f34c 100644
  3. --- a/BUILD.gn
  4. +++ b/BUILD.gn
  5. @@ -80,6 +80,7 @@ group("gn_all") {
  6.      "//tools/binary_size:binary_size_trybot_py",
  7.      "//tools/ipc_fuzzer:ipc_fuzzer_all",
  8.      "//tools/metrics:metrics_metadata",
  9. +    "//tools/gn/tutorial:say_hello",
  10.      "//ui/base:ui_base_unittests",
  11.      "//ui/gfx:gfx_unittests",
  12.      "//url:url_unittests",

运行命令

  1. gn gen out/Default
  2. ninja -C out/Default say_hello

报错了

  1. ninja: Entering directory `out/Default'
  2. [1/1] Regenerating ninja files
  3. [2/2] LINK ./say_hello
  4. FAILED: say_hello 
  5. TOOL_VERSION=1567226261 ../../build/toolchain/mac/linker_driver.py -Wcrl,strippath,/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip ../../third_party/llvm-build/Release+Asserts/bin/clang++ -B /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/  -Wl,-fatal_warnings -stdlib=libc++ -arch x86_64 -segprot PROTECTED_MEMORY rw r -Werror -nostdlib++ -isysroot ../../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.10.0 -Wl,-ObjC -Wl,-rpath,@loader_path/. -Wl,-rpath,@loader_path/../../.. -o "./say_hello" -Wl,-filelist,"./say_hello.rsp" ./libhello.dylib ./libc++.dylib 
  6. Undefined symbols for architecture x86_64:
  7.   "Hello(char const*, char const*)", referenced from:
  8.       _main in say_hello.o
  9. ld: symbol(s) not found for architecture x86_64
  10. clang: error: linker command failed with exit code 1 (use -v to see invocation)

思考发现，缺少编译动态链接库的选项

  1. -Wl,-undefined -Wl,dynamic_lookup

or

  1. clang -shared -undefined dynamic_lookup -o libfoo.so foo.c

尝试修改BUILD.gn, 添加ldflags

  1. executable("say_hello") {
  2.   sources = [
  3.     "say_hello.cc",
  4.   ]
  5.   deps = [
  6.     ":hello",
  7.   ]
  8.   ldflags = [
  9.     "-Wl,-undefined",
  10.     "-Wl,dynamic_lookup",
  11.   ]
  12. }

编译通过，但是运行报错
Screen Shot 2019-10-18 at 10.22.14 PM.png

WTF!! 为啥，为啥？？

编译一个依赖base动态库的可执行程序，貌似没有这种问题。对比检查编译参数。完全一样的，有木有！！
Screen Shot 2019-10-22 at 8.21.20 AM.png

经过冷静思考，突然想起//base/baseexport.h这个文件。符号被隐藏了！！
赶紧在函数名前面追加`_attribute((visibility(_"default"_)))`，链接通过了！！

查看base库的编译选项，发现了问题根源-fvisibility=hidden

  1. gn desc out/Default "base:base" cflags --all

加—blame参数，查找定义位置

  1. gn desc out/Default "base:base" cflags --all --blame

Screen Shot 2019-10-22 at 8.32.28 AM.png

OK, 目前为止，原因已经弄清楚了，模仿base_export.h，写一个头文件就好了。

解决方案一

运行命令

  1. gn desc out/Default "tools/gn/tutorial/linking_demo:simple" cflags --all --tree --blame

发现

  1. From //build/config/gcc:symbol_visibility_hidden
  2.      (Added by //build/config/BUILDCONFIG.gn:456)
  3.   -fvisibility=hidden

更改设置即可

  1.   configs -= [
  2.     "//build/config/gcc:symbol_visibility_hidden",
  3.   ]

解决方案二

模仿base库，添加宏定义SIMPLE_IMPLEMENTATION

  1. config("simple_implementation") {
  2.   defines = [ "SIMPLE_IMPLEMENTATION" ]
  3.   configs = [ "//build/config/compiler:wexit_time_destructors" ]
  4. }

在目标中依赖添加该设置

  1. shared_library("simple") {
  2.   sources = [
  3.     "simple.cc",
  4.     "simple.h",
  5.   ]
  6.   configs += [
  7.     ":simple_implementation",
  8.   ]
  9. }

OK， 添加export.h

  1. #ifndef TOOLS_GN_TUTORIAL_LINKING_DEMO_EXPORT_H_
  2. #define TOOLS_GN_TUTORIAL_LINKING_DEMO_EXPORT_H_
  4. #if defined(COMPONENT_BUILD)
  5. #if defined(WIN32)
  7. #if defined(SIMPLE_IMPLEMENTATION)
  8. #define SIMPLE_EXPORT __declspec(dllexport)
  9. #else
  10. #define SIMPLE_EXPORT __declspec(dllimport)
  11. #endif  // defined(SIMPLE_IMPLEMENTATION)
  13. #else  // defined(WIN32)
  14. #if defined(SIMPLE_IMPLEMENTATION)
  15. #define SIMPLE_EXPORT __attribute__((visibility("default")))
  16. #else
  17. #define SIMPLE_EXPORT
  18. #endif  // defined(SIMPLE_IMPLEMENTATION)
  19. #endif
  21. #else  // defined(COMPONENT_BUILD)
  22. #define SIMPLE_EXPORT
  23. #endif
  25. #endif  // TOOLS_GN_TUTORIAL_LINKING_DEMO_EXPORT_H_

修改头文件

  3. #ifndef TOOLS_GN_TUTORIAL_LINKING_DEMO_SIMPLE_H
  4. #define TOOLS_GN_TUTORIAL_LINKING_DEMO_SIMPLE_H
  5. #include "tools/gn/tutorial/linking_demo/export.h"
  7. void SIMPLE_EXPORT simpleCall(void);
  9. #endif // TOOLS_GN_TUTORIAL_LINKING_DEMO_SIMPLE_H

参考

https://stackoverflow.com/questions/21907504/how-to-compile-shared-lib-with-clang-on-osx