leveldb编译及demo

由于服务器依赖比较老旧，因此需要重新安装各种依赖

leveldb依赖安装

1 2	sudo yum update sudo yum install cmake gcc g++

如果cmake无法安装到3.9以上的版本，可以从官网下载，然后直接解压到项目目录下，之后直接使用绝对路径或相对路径调用cmake，gcc和g++推荐使用7.5

snappy编译

git clone https://github.com/google/snappy.git
cd snappy
git submodule update --init
git checkout 1.1.9
mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Release ../
make

leveldb编译

使用--recurse-submodules下载subgit中的内容，使用git tag查看tag，选择最新的release，最后使用git checkout切换到最新的release版本上

git clone --recurse-submodules https://github.com/google/leveldb.git
cd leveldb
git tag
git checkout 1.23

1 2	mkdir -p build && cd build cmake -DCMAKE_BUILD_TYPE=Release .. && cmake --build .

执行测试检查编译是否成功

1	./db_bench

snappy支持

修改leveldb/CMakeLists.txt, 在check_library_exists(snappy snappy_compress "" HAVE_SNAPPY)语句前后添加依赖

其中/home/someone/project/snappy和/home/someone/project/snappy/build需要替换成自己的snappy源码目录和build目录
添加后形式如下：

# 之前添加依赖
link_directories("/home/someone/project/snappy/build")
include_directories("/home/someone/project/snappy" "/home/someone/project/snappy/build")
check_library_exists(snappy snappy_compress "" HAVE_SNAPPY)
# 之后强制开启HAVE_SNAPPY
set(HAVE_SNAPPY ON)

重新编译和测试

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mkdir -p build && cd build
cmake -DCMAKE_BUILD_TYPE=Release .. && cmake --build .
./db_bench

静态编译leveldb

因为服务器环境问题，需要静态编译leveldb, 尤其是静态编译libgcc和libg++

在check_library_exists(snappy snappy_compress "" HAVE_SNAPPY)语句前后添加link_libraries标签配置所有的目标静态链接gcc和g++

1	link_libraries(-static-libgcc -static-libstdc++)

执行ldd db_bench可以看到已经不再依赖gcc和g++了

执行strings /lib64/libstdc++.so.6 | grep GLIBCXX可以查看目前支持的版本

可能会遇到如下报错：

找不到成员

这种情况是由于gcc、g++版本太低导致的，需要升级版本

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leveldb/third_party/googletest/googletest/include/gtest/gtest-matchers.h:414:12: error: ‘is_trivially_copy_constructible’ is not a member of ‘std’
            std::is_trivially_copy_constructible<M>::value &&
            ^

使用如下命令检查版本

1 2	gcc -v g++ -v

/bin/c++ /bin/cc缺失

这种情况是软链缺失导致的，报错如下

CMake Error at CMakeLists.txt:7 (project):
  The CMAKE_CXX_COMPILER:

    /bin/c++

  is not a full path to an existing compiler tool.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

此时需要手动恢复软链，使用which gcc和which g++确定安装位置，然后使用ln设置软链

1 2	ln -s /usr/local/bin/g++ /bin/c++ ln -s gcc /bin/cc

/lib64/libstdc++.so.6: version `GLIBCXX_3.4.22’ not found

建议使用静态链接

demo

编写一个简单的测试demo, 放在与leveldb根目录同级的目录下

#include <iostream>
#include <string>
#include <assert.h>
#include "leveldb/db.h"

using namespace std;

int main(void)
{
    leveldb::DB *db;
    leveldb::Options options;
    options.create_if_missing = true;

    // open
    leveldb::Status status = leveldb::DB::Open(options, "testdb", &db);
    assert(status.ok());

    string key = "name";
    string value = "shane";

    cout << "write:" "("<< key << ")" "=>" "(" << value << ")" << endl;
    // write
    status = db->Put(leveldb::WriteOptions(), key, value);
    assert(status.ok());

    // read
    status = db->Get(leveldb::ReadOptions(), key, &value);
    assert(status.ok());

    cout << "read:" "("<< key << ")" "=>" "(" << value << ")" << endl;

    // delete
    status = db->Delete(leveldb::WriteOptions(), key);
    assert(status.ok());
    cout << "delete:" "("<< key << ")"  << status.ToString() << endl;

    status = db->Get(leveldb::ReadOptions(), key, &value);
    if (!status.ok()) {
        cout << "read error: " << status.ToString() << "("<< key << ")"  << endl;
    } else {
        cout << "read:" "("<< key << ")" "=>" "(" << value << ")" << endl;
    }

    //close
    delete db;
    
    return 0;
}

执行命令, 编译leveldb需要使用-Ileveldb/include引入头文件，-Lleveldb/build设置链库地址, -lleveldb -lpthread 指定要链接的库, -o leveldbtest 指定输出名字

1 2	g++ leveldbtest.cc -Ileveldb/include -Lleveldb/build -lleveldb -lpthread -o leveldbtest ./leveldbtest

运行结果如下

write:(name)=>(shane)
read:(name)=>(shane)
delete:(name)OK
read error: NotFound: (name)