在 C++ 项目中链接 libpq 时未定义的引用

编程问答 2022-05-16

问题描述

我正在尝试在 C++ 项目中使用 libpqxx（并隐式使用 libpq）。
我使用 vcpkg 作为 submodule 通过设置 CMAKE_TOOLCHAIN_FILE 来获取我的库。

当我尝试构建时，出现以下错误：

/usr/bin/ld: /home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a(fe-auth.o): in function `pg_fe_sendauth':
fe-auth.c:(.text+0x4fe): undefined reference to `pg_md5_encrypt'
/usr/bin/ld: fe-auth.c:(.text+0x51b): undefined reference to `pg_md5_encrypt'
/usr/bin/ld: /home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a(fe-auth.o): in function `pg_fe_getauthname':
fe-auth.c:(.text+0x838): undefined reference to `pqGetpwuid'
/usr/bin/ld: fe-auth.c:(.text+0x8a4): undefined reference to `pg_strerror_r'
/usr/bin/ld: /home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a(fe-auth.o): in function `PQencryptPassword':
fe-auth.c:(.text+0x936): undefined reference to `pg_md5_encrypt'
/usr/bin/ld: /home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a(fe-auth.o): in function `PQencryptPasswordConn':
fe-auth.c:(.text+0x9ed): undefined reference to `pg_md5_encrypt'
/usr/bin/ld: /home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a(fe-auth-scram.o): in function `build_client_final_message':
fe-auth-scram.c:(.text+0xdb): undefined reference to `scram_SaltedPassword'
/usr/bin/ld: fe-auth-scram.c:(.text+0xee): undefined reference to `scram_ClientKey'
/usr/bin/ld: fe-auth-scram.c:(.text+0xfe): undefined reference to `scram_H'
/usr/bin/ld: fe-auth-scram.c:(.text+0x113): undefined reference to `scram_HMAC_init'
/usr/bin/ld: fe-auth-scram.c:(.text+0x12d): undefined reference to `scram_HMAC_update'
/usr/bin/ld: fe-auth-scram.c:(.text+0x141): undefined reference to `scram_HMAC_update'
/usr/bin/ld: fe-auth-scram.c:(.text+0x15b): undefined reference to `scram_HMAC_update'
/usr/bin/ld: fe-auth-scram.c:(.text+0x16f): undefined reference to `scram_HMAC_update'
/usr/bin/ld: fe-auth-scram.c:(.text+0x185): undefined reference to `scram_HMAC_update'
/usr/bin/ld: fe-auth-scram.c:(.text+0x195): undefined reference to `scram_HMAC_final'
/usr/bin/ld: fe-auth-scram.c:(.text+0x1e4): undefined reference to `pg_b64_enc_len'
/usr/bin/ld: fe-auth-scram.c:(.text+0x213): undefined reference to `pg_b64_encode'
/usr/bin/ld: fe-auth-scram.c:(.text+0x344): undefined reference to `pg_b64_enc_len'
/usr/bin/ld: fe-auth-scram.c:(.text+0x368): undefined reference to `pg_b64_encode'
...

我在这里分解了链接命令：

/usr/bin/cmake -E cmake_link_script CMakeFiles/hello-pq.dir/link.txt --verbose=1
/usr/bin/c++     CMakeFiles/hello-pq.dir/CMakeFiles/3.16.3/CompilerIdCXX/CMakeCXXCompilerId.cpp.o CMakeFiles/hello-pq.dir/main.cpp.o  
-o ../bin/hello-pq  
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libssl.a 
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libcrypto.a 
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libpqxx-7.3.a 
-lpthread 
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a -ldl

我的 CMakeLists.txt 看起来像这样：

cmake_minimum_required(VERSION 3.16)

set(CMAKE_TOOLCHAIN_FILE ${CMAKE_CURRENT_SOURCE_DIR}/../../third-party/vcpkg/scripts/buildsystems/vcpkg.cmake
        CACHE STRING "Vcpkg toolchain file")

project(hello-pq)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_required ON)

set(EXECUTABLE_OUTPUT_PATH ${CMAKE_CURRENT_SOURCE_DIR}/bin)

find_package(OpenSSL required) 
find_package(libpqxx CONfig required)

file(GLOB_RECURSE PROJECT_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/src/*.cpp)
add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})

target_link_libraries(${PROJECT_NAME}
                        PRIVATE OpenSSL::SSL 
                        PRIVATE OpenSSL::Crypto
                        PRIVATE libpqxx::pqxx)

我查看了 libpq.a installed 目录中的 vcpkg 和 nm，我可以看到，例如，pg_md5_encrypt 显示为 未定义：

U pg_md5_encrypt

没看懂，pg_md5_encrypt之类的这些缺失的函数在哪里？
是否还有其他 libpq... 需要链接或可能是其他版本？

解决方法

经过更多挖掘和测试后，我发现还有 2 个 postgres 静态库需要链接：

  • libpqcommon.a
  • libpgport.a

此外，链接顺序也很重要，虽然我原以为必须在 libs that are needed 之前通过 libs that need them，但这里的情况并非如此，而且感觉倒退了。
这是一个有效的链接器命令：

usr/bin/c++ CMakeFiles/hello-pq.dir/src/main.cpp.o \
-o ../bin/hello-pq \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libpqxx-7.3.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpgcommon.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpgport.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libssl.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libcrypto.a \
-lpthread \
-ldl

为了链接额外的 2 个静态库并实现上述顺序，我像这样更新了 CMakeLists.txt：

cmake_minimum_required(VERSION 3.16)

set(CMAKE_TOOLCHAIN_FILE ${CMAKE_CURRENT_SOURCE_DIR}/../../third-party/vcpkg/scripts/buildsystems/vcpkg.cmake
        CACHE STRING "Vcpkg toolchain file")

project(hello-pq)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(EXECUTABLE_OUTPUT_PATH ${CMAKE_CURRENT_SOURCE_DIR}/bin)

find_package(OpenSSL REQUIRED) 
#find_package(PostgreSQL REQUIRED) # Not needed (it will duplicate the libs)
find_package(libpqxx CONFIG REQUIRED)

file(GLOB_RECURSE PROJECT_SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/src/*.cpp)
add_executable(${PROJECT_NAME} ${PROJECT_SOURCES})

target_link_libraries(${PROJECT_NAME}
                        PRIVATE libpqxx::pqxx
                        PRIVATE PostgreSQL::PostgreSQL
                        PRIVATE OpenSSL::SSL 
                        PRIVATE OpenSSL::Crypto)

这转化为以下链接器命令：

/usr/bin/c++ \
CMakeFiles/hello-pq.dir/src/main.cpp.o \
-o ../bin/hello-pq \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libpqxx-7.3.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libpq.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libssl.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/debug/lib/libcrypto.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpq.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpgport.a \
/home/user/work/cxx-playground/third-party/vcpkg/installed/x64-linux/lib/libpgcommon.a \
-ldl \
-lpthread

我们可以看到 libpq.a 在链接参数中出现了两次：

  • 一次是由于 libpqxx::pqxx 目标（我假设）
  • 一次由于 PostgreSQL::PostgreSQL 目标 其中还包含需要链接的 2 个缺失的静态库（libpgport.a 和 libpgcommon.a）。

我的结论是这是一个 libpqxx 错误，它也应该将这 2 个静态库与 target_link_libraries(PUBLIC...) 链接到它公开的唯一目标 (libpqxx::pqxx)

c++libpqlibpqxxpostgresqlvcpkg