使用CMake构建libpqxx C++程序时链接器报错

问题概述 #

使用CMake构建基于libpqxx的C程序时，Linux和macOS均出现链接器错误，无法找到pqxx相关的C符号。但find_package(PostgreSQL)能正常识别libpq（C语言库），且通过clang++ main.cpp -o main -lpqxx -lpq命令可成功编译运行，证明libpqxx和libpq已正确安装。

macOS下Ninja编译错误输出 #

[2/2] Linking CXX executable main
FAILED: main 
: && /Library/Developer/CommandLineTools/usr/bin/c++ -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX13.1.sdk -mmacosx-version-min=12.6 -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/main.dir/src/main.cpp.o -o main  /usr/local/lib/libpq.dylib && :
Undefined symbols for architecture x86_64:
  "pqxx::connection::init(char const*)", referenced from:
      pqxx::connection::connection(char const*) in main.cpp.o
  "pqxx::connection::close()", referenced from:
      pqxx::connection::~connection() in main.cpp.o
  "pqxx::internal::demangle_type_name(char const*)", referenced from:
      ___cxx_global_var_init in main.cpp.o
      ___cxx_global_var_init.2 in main.cpp.o
      ___cxx_global_var_init.3 in main.cpp.o
      ___cxx_global_var_init.4 in main.cpp.o
  "pqxx::internal::check_pqxx_version_7_8()", referenced from:
      pqxx::check_version() in main.cpp.o
  "pqxx::connection::dbname() const", referenced from:
      _main in main.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

现有配置文件 #

CMakeLists.txt #

cmake_minimum_required(VERSION 3.25)
project(main VERSION 0.1 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(PROJECT_SOURCES 
    src/main.cpp)

add_executable(main ${PROJECT_SOURCES})

find_package(PostgreSQL REQUIRED)

target_link_libraries(main PRIVATE PostgreSQL::PostgreSQL)

main.cpp #

#include <iostream>
#include <pqxx/pqxx>

int main(int argc, const char *argv[])
{
    pqxx::connection c;

    std::cout << "Connected to " << c.dbname() << ".\n";

    return 0;
}

可正常工作的C语言版本（main.c） #

#include <stdio.h>
#include <libpq-fe.h>

int main(int argc, const char *argv[])
{
    int lib_ver = PQlibVersion();

    printf("Version of libpq: %d\n", lib_ver);

    return 0;
}

环境信息 #

• Intel架构Mac，macOS 12.6.8，通过Homebrew安装PostgreSQL、libpq、libpqxx
• Ubuntu 22.04，通过apt安装PostgreSQL、libpq-dev，libpqxx源码编译后安装至/usr/local

解决方案 #

问题核心：原CMake配置仅链接了libpq（PostgreSQL的C语言库），但未链接libpqxx这个独立的C++库。手动编译时通过-lpqxx指定了该库，因此CMake需要显式添加对libpqxx的链接。

方案1：使用libpqxx的CMake配置文件（推荐） #

如果libpqxx安装时提供了CMake配置模块（如Homebrew安装的版本），修改CMakeLists.txt如下：

cmake_minimum_required(VERSION 3.25)
project(main VERSION 0.1 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(PROJECT_SOURCES 
    src/main.cpp)

add_executable(main ${PROJECT_SOURCES})

find_package(PostgreSQL REQUIRED)
# 查找libpqxx的CMake配置
find_package(PQXX REQUIRED)

# 同时链接libpq和libpqxx
target_link_libraries(main PRIVATE PostgreSQL::PostgreSQL PQXX::PQXX)

方案2：手动查找libpqxx库（适用于无CMake配置的源码安装版本） #

如果libpqxx没有提供CMake配置文件，通过find_library和find_path手动定位库和头文件：

cmake_minimum_required(VERSION 3.25)
project(main VERSION 0.1 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(PROJECT_SOURCES 
    src/main.cpp)

add_executable(main ${PROJECT_SOURCES})

find_package(PostgreSQL REQUIRED)
# 手动查找libpqxx库和头文件
find_library(PQXX_LIBRARY pqxx REQUIRED)
find_path(PQXX_INCLUDE_DIR pqxx/pqxx.h REQUIRED)

# 添加头文件路径
target_include_directories(main PRIVATE ${PQXX_INCLUDE_DIR})
# 链接libpq和libpqxx
target_link_libraries(main PRIVATE PostgreSQL::PostgreSQL ${PQXX_LIBRARY})

内容的提问来源于stack exchange，提问作者Mehrshad Khansarian