g ++找不到标题，但我确实包含它们 [英] g++ can't find headers but I did include them

问题描述

我正在编译一个levelDB的小测试：

  #include< assert.h> 
 #includeleveldb / db.h
 
 using namespace std; 
 
 int main（）{
 leveldb :: DB * db; 
 leveldb ::选项选项; 
 options.create_if_missing = true; 
 leveldb :: Status status = leveldb :: DB :: Open（options，/ tmp / testdb，& db）; 
 assert（status.ok（））; 
 
返回1; 
 
 $ / code> 

以下是g ++命令：



  g ++ -I include / testLevelDB.cpp 
  

输出：



  /tmp/ccuBnfE7.o：函数`main'：
 testLevelDB.cpp :(。文本+ 0x14）：对`leveldb :: Options :: Options（）'的未定义引用
 testLevelDB.cpp :( .text + 0x57）：未定义对`leveldb :: DB :: Open的引用（leveldb :: Options const&;; std :: string const&; leveldb :: DB **）'
  

是带有levelDB头文件的。

解决方案

您需要告诉链接器链接到 leveldb 库，例如

  g ++ -I include / testLevelDB.cpp -lleveldb 
  code> 

但是如果库不在 / usr / lib $ LEVELDB_PATH 的某个路径中，则可以使用或 / usr / local / lib 你需要

  g ++ -I include -L $ LEVELDB_PATH testLevelDB.cpp -lleveldb 
  

-L 非常类似于 -I 但它告诉链接器在哪里查找库。

另外，由于您似乎是gcc世界的新手，请查看此 gcc介绍文档。

I am starting on c++ and already going wrong ...

I am trying to compile a small test of levelDB :

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

using namespace std;

int main() {
  leveldb::DB* db;
  leveldb::Options options;
  options.create_if_missing = true;
  leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
  assert(status.ok());

  return 1;
}

Here is the g++ command :

g++ -I include/ testLevelDB.cpp

Output:

/tmp/ccuBnfE7.o: In function `main':
testLevelDB.cpp:(.text+0x14): undefined reference to `leveldb::Options::Options()'
testLevelDB.cpp:(.text+0x57): undefined reference to `leveldb::DB::Open(leveldb::Options const&, std::string const&, leveldb::DB**)'

The include folder is the one with the levelDB headers.

解决方案

You need to tell the linker to link to the leveldb library such as

g++ -I include/ testLevelDB.cpp -lleveldb

But this won't work if the library is not in /usr/lib or /usr/local/lib for that case assuming the libleveldb.so exists in some path called $LEVELDB_PATH you need to do

g++ -I include -L $LEVELDB_PATH testLevelDB.cpp -lleveldb

-L is much like -I but it tells the linker where to looks for libraries.

Also since you seem to be new to gcc world, please have a look at this gcc intro document.

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