如何在Mac上的c ++中使用LevelDB，为什么我得到“ ld：架构x86_64找不到符号”

Question

我是C ++的新手，现在我想学习LevelDB，但遇到一些问题。

brew install leveldb --cc=gcc-4.8

• 我的代码：

  #include <iostream>
  #include "leveldb/db.h"
  int main() {
      leveldb::Options options;
      leveldb::DB *db;
      options.create_if_missing = true;
      leveldb::Status status = leveldb::DB::Open(options, "./.db", &db);
      assert(status.ok())
      std::cout << "Hello, World!" << std::endl;
      return 0;
  }
  

• CMakeLists.txt

  cmake_minimum_required(VERSION 3.6)
  project(ldb)
  set(CMAKE_CXX_STANDARD 14)
  
  find_library(LevelDB_LIBRARY leveldb)
  
  set(SOURCE_FILES main.cpp)
  add_executable(ldb ${SOURCE_FILES})
  
  target_link_libraries(ldb /usr/local/lib/libleveldb.a)
  
  set(THREADS_PREFER_PTHREAD_FLAG ON)
  find_package(Threads REQUIRED)
  TARGET_LINK_LIBRARIES(ldb pthread)
  
  set(CMAKE_VERBOSE_MAKEFILE TRUE)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -v -stdlib=libstdc++")
  

然后，这里是错误：

    /Applications/CLion.app/Contents/bin/cmake/bin/cmake -E cmake_link_script CMakeFiles/ldb.dir/link.txt --verbose=1
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++    -v -stdlib=libstdc++ -g -Wl,-search_paths_first -Wl,-headerpad_max_install_names  CMakeFiles/ldb.dir/main.cpp.o  -o ldb /usr/local/lib/libleveldb.a -lpthread 
    Apple LLVM version 8.1.0 (clang-802.0.41)
    Target: x86_64-apple-darwin16.5.0
    Thread model: posix
    InstalledDir:/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
    clang: warning: libstdc++ is deprecated; move to libc++ [-Wdeprecated]
    "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/libLTO.dylib -dynamic -arch x86_64 -macosx_version_min 10.12.0 -o ldb -search_paths_first -headerpad_max_install_names CMakeFiles/ldb.dir/main.cpp.o /usr/local/lib/libleveldb.a -lpthread -lstdc++ -lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0/lib/darwin/libclang_rt.osx.a
    Undefined symbols for architecture x86_64:
      "snappy::RawCompress(char const*, unsigned long, char*, unsigned long*)", referenced from:
          leveldb::TableBuilder::WriteBlock(leveldb::BlockBuilder*, leveldb::BlockHandle*) in libleveldb.a(table_builder.o)
      "snappy::RawUncompress(char const*, unsigned long, char*)", referenced from:
          leveldb::ReadBlock(leveldb::RandomAccessFile*, leveldb::ReadOptions const&, leveldb::BlockHandle const&, leveldb::BlockContents*) in libleveldb.a(format.o)
      "snappy::MaxCompressedLength(unsigned long)", referenced from:
          leveldb::TableBuilder::WriteBlock(leveldb::BlockBuilder*, leveldb::BlockHandle*) in libleveldb.a(table_builder.o)
      "snappy::GetUncompressedLength(char const*, unsigned long, unsigned long*)", referenced from:
          leveldb::ReadBlock(leveldb::RandomAccessFile*, leveldb::ReadOptions const&, leveldb::BlockHandle const&, leveldb::BlockContents*) in libleveldb.a(format.o)
    ld: symbol(s) not found for architecture x86_64
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    make[2]: *** [ldb] Error 1
    make[1]: *** [CMakeFiles/ldb.dir/all] Error 2
    make: *** [all] Error 2

它可以在CentOS服务器上运行，但是不能在我的Mac上运行，这是什么问题？

如果我手动编译它。gcc-4.8 main.cpp -L. -I./Users/apple/CLionProjects/leveldb/include -lpthread -lleveldb -lstdc++收到警告：section "__textcoal_nt" is deprecated，但是如果我将gcc-4.8替换为gcc-5，则会出现错误：

    Undefined symbols for architecture x86_64:
    "leveldb::DB::Open(leveldb::Options const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, leveldb::DB**)", referenced from:
          _main in ccWAzDAN.o
    ld: symbol(s) not found for architecture x86_64
    collect2: error: ld returned 1 exit status

为什么不同？

Answer 1

通常应使用find_package。但是在此答案出现时，使用find_package无法检测到levelDB。这是信息https://github.com/google/leveldb/issues/641

考虑对LevelDB使用externalproject_add：https://github.com/ohhmm/openmind/blob/f1459119e8d36d6cc6edc776b2f242edaf51fecf/CMakeLists.txt#L292

或者只是将leveldb与pthread一起添加到TARGET_LINK_LIBRARIES。这适用于posix，但不适用于Windows。