在Chromium中调用自定义共享库出现未定义符号错误求助

问题背景 #

在Chromium中通过C++调用外部自定义共享库的函数，编译时触发链接错误：ld.lld: error: undefined symbol: my_class::print_a_dot()

编译错误详情 #

ubuntu:~/chromium/src> autoninja -C out/Default chrome
ninja: Entering directory `out/Default'
[16888/17733] SOLINK ./libcontent.so
FAILED: libcontent.so libcontent.so.TOC
python3 "../../build/toolchain/gcc_solink_wrapper.py" --readelf="../../third_party/llvm-build/Release+Asserts/bin/llvm-readelf" --nm="../../third_party/llvm-build/Release+Asserts/bin/llvm-nm"  --sofile="./libcontent.so" --tocfile="./libcontent.so.TOC" --output="./libcontent.so" -- ../../third_party/llvm-build/Release+Asserts/bin/clang++ -shared -Wl,-soname="libcontent.so" -Werror -fuse-ld=lld -Wl,--fatal-warnings -Wl,--build-id -fPIC -Wl,-z,noexecstack -Wl,-z,relro -Wl,--color-diagnostics -Wl,--undefined-version -Wl,--no-call-graph-profile-sort -m64 -no-canonical-prefixes -Wl,--gdb-index -rdynamic -Wl,-z,defs -Wl,--as-needed -nostdlib++ --sysroot=../../build/linux/debian_bullseye_amd64-sysroot -Wl,-rpath=\$ORIGIN -o "./libcontent.so" @"./libcontent.so.rsp"
ld.lld: error: undefined symbol: my_class::print_a_dot()
>>> referenced by delegated_frame_host.cc:457 (../../content/browser/renderer_host/delegated_frame_host.cc:457)
>>>               obj/content/browser/browser/delegated_frame_host.o:(content::DelegatedFrameHost::DidCopyStaleContent(std::Cr::unique_ptr<viz::CopyOutputResult, std::Cr::default_delete<viz::CopyOutputResult>>))
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
[16913/17733] CXX irt_x64/obj/ipc/mojom/ipc.mojom.o
ninja: build stopped: subcommand failed.

自定义共享库详情（~/learn目录） #

1. my_class.cpp #

#include <stdio.h>
#include <iostream>
#include <memory>
#include <stdint.h>
#include "my_class.h"


extern void my_class::print_a_dot()
{
    std::cout << ".";
}

2. my_class.h #

#ifndef MY_CLASS_H    // To make sure you don't declare the function more than once by including the header multiple times.
#define MY_CLASS_H

class my_class {

  public:
      static int some_number;
      static bool initialised;
      void print_a_dot();
      int init_encoder();
  };


#endif

3. CMakeLists.txt #

cmake_minimum_required(VERSION 3.15)

project(MyProject)

add_library(myexample SHARED my_class.cpp)

Chromium中的配置（third_party/dude4目录） #

目录结构 #

third_party % find dude4
dude4
dude4/BUILD.gn
dude4/include
dude4/include/my_class.h

BUILD.gn内容 #

config("my_class_import") {
  include_dirs = ["include"]
  libs = ["/home/ubuntu/learn/build/libmyexample.so"]
}

group("my_class") {
  public_configs = [":my_class_import"]
}

Chromium代码中的调用（delegated_frame_host.cc） #

头部引用 #

#include "third_party/dude4/include/my_class.h"

bool my_class::initialised = false;
int my_class::some_number = 999;

函数内调用逻辑 #

void DelegatedFrameHost::DidCopyStaleContent(
    std::unique_ptr<viz::CopyOutputResult> result) {
  // host may have become visible by the time the request to capture surface is
  // completed.

    my_class mc;
    mc.initialised = false;
    std::cout << "my_class::some_number: ";
    std::cout << my_class::some_number << std::endl;
    my_class::some_number = 888;
    std::cout << "mc.some_number: ";
    std::cout << mc.some_number << std::endl;
    my_class nc;
    std::cout << "nc.some_number: ";
    std::cout << nc.some_number << std::endl;

    if (mc.initialised == false) {
        std::cout << "initialising encoder" << std::endl;
        mc.initialised = true;
    }
    mc.print_a_dot();
    // 后续代码...
}

错误原因分析及解决步骤 #

核心原因 #

1. 成员函数定义语法错误：my_class.cpp中extern void my_class::print_a_dot()的写法不符合C++类成员函数的定义规范，extern关键字在这里是多余的，会导致编译器无法正确识别这是类的成员函数实现，最终生成的共享库中缺少my_class::print_a_dot()的有效符号。
2. 链接依赖未正确传递：Chromium的GN构建系统中，group目标本身不会主动将依赖的库链接到最终目标，需要确保libcontent.so对应的构建目标（如content/browser:browser）明确添加对third_party/dude4:my_class的依赖，否则链接器不会去查找指定的共享库。

修复步骤 #

步骤1：修正共享库的函数定义

修改my_class.cpp，去掉多余的extern关键字：

#include <stdio.h>
#include <iostream>
#include <memory>
#include <stdint.h>
#include "my_class.h"

void my_class::print_a_dot()
{
    std::cout << ".";
}

重新编译共享库：

cd ~/learn
mkdir -p build && cd build
cmake .. && make

步骤2：确保Chromium目标正确依赖自定义库

找到content/browser/BUILD.gn中构建libcontent.so的目标（通常是source_set("browser")或shared_library("content")），在其deps列表中添加"//third_party/dude4:my_class"：

source_set("browser") {
  # 其他已有配置...
  deps = [
    # 已有依赖项...
    "//third_party/dude4:my_class",
  ]
}

步骤3：重新编译Chromium

autoninja -C out/Default chrome

内容的提问来源于stack exchange，提问作者Duke Dougal