使用Bazel构建依赖ITK库的C++二进制文件出现链接错误（未定义引用）

问题背景 #

我用Bazel管理一个项目，结构简化后如下：

./
┗━ utils
  ┗━ BUILD
┗━ tool1
  ┗━ BUILD
┗━ itk.BUILD
┗━ WORKSPACE

核心需求是构建依赖utils库的tool1二进制文件，utils本身依赖预编译在third_party/itk的ITK库。目前bazel build //utils:utils能成功构建，但bazel build //tool1时出现大量链接错误，仅剩libitkvnl_algo-4.13.a相关的报错，示例：

external/itk/lib/libitkvnl_algo-4.13.a(vnl_cholesky.cxx.o): In function `vnl_cholesky::vnl_cholesky(vnl_matrix<double> const&, vnl_cholesky::Operation)':
vnl_cholesky.cxx:(.text+0x99): undefined reference to `v3p_netlib_dpofa_'

这些缺失的符号均来自ITK自身库，且对应的.a文件已经出现在Bazel生成的链接命令中，但单独调整单个库的顺序无效，需要调整多个库的链接顺序。

项目各配置文件内容如下：

utils/BUILD #

cc_library(
    name = "utils",
    srcs = glob(
        [
            "utils/*.cpp",
            "include/**",
        ],
    ),
    hdrs = glob([
        "include/*.h",
    ]),
    copts = [
        "-DITK_USE_SYSTEM_ZLIB",
    ],
    includes = ["./include"],
    linkopts = ["-ldl", "-lz", "-lm", "-lgomp", "-lpthread", "-lcrypto", "-lrt"],
    visibility = ["//visibility:public"],
    deps = [
        "@itk//:itk_libs",
    ],
)

tool1/BUILD #

cc_binary(
    name = "tool1",
    srcs = ["tool1.cpp"],
    deps = [
        "//utils:utils",
    ],
)

WORKSPACE #

new_local_repository(
    name = "itk",
    path = "third_party/itk",
    build_file = "itk.BUILD",
)

itk.BUILD #

cc_library(
    name = "itk_libs",
    srcs = glob(["lib/**/*.a"]),
    hdrs = glob([
        "include/ITK-4.13/**/*.h",
        "include/ITK-4.13/*.h",
        "include/ITK-4.13/**/*.hxx",
        "include/ITK-4.13/*.hxx",
        "include/ITK-4.13/**/*.txx",
        "include/ITK-4.13/*.txx",
    ]),
    strip_include_prefix = "include/ITK-4.13",
    visibility = ["//visibility:public"],
)

问题原因 #

当前itk.BUILD把所有ITK静态库打包成了一个cc_library，但Bazel无法识别这些静态库内部的依赖关系。utils构建成功只是因为它的代码没有触发对跨库引用符号的调用，而tool1的代码路径用到了这些符号，就暴露了链接顺序错误的问题——需要被引用的库（比如libitkv3p_netlib-4.13.a）必须放在引用它的库（libitkvnl_algo-4.13.a）之后链接，但Bazel当前的处理顺序不满足这个要求。

解决办法 #

1. 拆分ITK库，显式声明依赖（推荐方案） #

不要把所有ITK库打包成一个集合，而是为每个.a文件单独创建cc_library，并明确写出它们之间的依赖关系。例如libitkvnl_algo-4.13.a依赖libitkvnl-4.13.a和libitkv3p_netlib-4.13.a，修改itk.BUILD如下：

# 定义最底层的依赖库
cc_library(
    name = "itkv3p_netlib",
    srcs = ["lib/libitkv3p_netlib-4.13.a"],
    hdrs = glob([
        "include/ITK-4.13/**/*.h",
        "include/ITK-4.13/**/*.hxx",
        "include/ITK-4.13/**/*.txx",
    ]),
    strip_include_prefix = "include/ITK-4.13",
    visibility = ["//visibility:public"],
)

cc_library(
    name = "itkvnl",
    srcs = ["lib/libitkvnl-4.13.a"],
    hdrs = glob([
        "include/ITK-4.13/**/*.h",
        "include/ITK-4.13/**/*.hxx",
        "include/ITK-4.13/**/*.txx",
    ]),
    strip_include_prefix = "include/ITK-4.13",
    visibility = ["//visibility:public"],
    deps = [":itkv3p_netlib"],  # 声明依赖关系
)

# 定义依赖上层的库
cc_library(
    name = "itkvnl_algo",
    srcs = ["lib/libitkvnl_algo-4.13.a"],
    hdrs = glob([
        "include/ITK-4.13/**/*.h",
        "include/ITK-4.13/**/*.hxx",
        "include/ITK-4.13/**/*.txx",
    ]),
    strip_include_prefix = "include/ITK-4.13",
    visibility = ["//visibility:public"],
    deps = [":itkvnl", ":itkv3p_netlib"],  # 声明依赖关系
)

# 创建集合库，方便其他模块引用
cc_library(
    name = "itk_libs",
    visibility = ["//visibility:public"],
    deps = [
        ":itkvnl_algo",
        ":itkvnl",
        ":itkv3p_netlib",
        # 按依赖顺序添加其他需要的ITK库
    ],
)

这样Bazel会根据依赖关系自动调整链接顺序，确保被依赖的库后链接，解决符号未定义问题。

2. 强制指定链接顺序（临时方案） #

如果拆分库的工作量太大，可以直接在tool1的cc_binary中通过linkopts手动指定ITK库的链接顺序，把引用其他库的静态库放在前面，被依赖的库放在后面：

cc_binary(
    name = "tool1",
    srcs = ["tool1.cpp"],
    deps = [
        "//utils:utils",
    ],
    linkopts = [
        "-Lexternal/itk/lib",
        "-litkvnl_algo-4.13",
        "-litkvnl-4.13",
        "-litkv3p_netlib-4.13",
        # 按正确顺序添加其他ITK库
    ],
)

这个方法需要明确所有库的依赖关系，维护成本较高，仅作为临时替代方案。

3. 检查预编译库的兼容性 #

确认预编译的ITK库与当前Bazel使用的编译器、编译选项（如C++标准、优化级别、宏定义）一致，避免因ABI不兼容导致符号无法识别。

验证 #

修改配置后执行bazel build //tool1，如果链接错误消失，则说明问题已解决；若仍有错误，检查是否遗漏了某些库的依赖声明。

内容的提问来源于stack exchange，提问作者xoreaxeax