g ++未定义的引用，尽管符号存在于* .so文件中 [英] g++ undefined reference although symbol is present in *.so file

问题描述

我发现了一些类似的问题（例如这， 或 gnss-sdr 核心），如下所示：

  $ nm libgnss_system_parameters_dyn.so | c ++ filt | grep Gps_Eph 
  

包含符号 Gps_Ephemeris :: Gps_Ephemeris ），应该是一个构造函数。

我写了一些最小的代码：

  #include< iostream> 
 #include< core / system_parameters / gps_ephemeris.h> 
 
 int main（int argc，const char * argv []）
 {
 Gps_Ephemeris ge; 
返回0; 
} 
  

我编译的内容：

  g ++ main.cpp -std = c ++ 0x -I some_include_path -L some_lib_path -l gnss_system_parameters_dyn` 
  

链接器会抱怨：

  /tmp/ccHCvldG.o：In函数`main'：
 main.cpp :(。text + 0x33）：未定义引用到`Gps_Ephemeris :: Gps_Ephemeris（）'
 collect2：错误：ld返回1退出状态
  

我也尝试了 cmake ，但它生成的行类似于（它只是添加了 -rdynamic 在链接之前），并且它仍然生成完全相同的链接器错误。

请注意，库和我的最小代码正在使用相同的编译器（g ++ - 5）进行编译，具有完全相同的标志和相同的c ++ 0x标准。

---

解决Maxim Egorushkin的答案：

  nm --demangle --defined-only --extern - 仅限libgnss_sys tem_parameters.so | grep Gps_Eph 
  

不输出任何内容。但是，符号是在静态库（即 * .a库）中定义的：

  00000000000006b0 T Gps_Ephemeris :: Gps_Ephemeris（）
 00000000000006b0 T Gps_Ephemeris :: Gps_Ephemeris（）
  

知道它们都是由 cmake 生成的，如下所示：

  add_library（lib_name SHARED $ {sources_etc }）＃为* .so 
 add_library（lib_name_2 $ {sources_etc}）＃为* .a 
  

这些库中包含/定义的符号应该没有区别，对吧？我在 add_library 的 cmake 文档中没有注意到任何内容。我错过了什么明显的东西？

解决方案

c $ c>出口符号为 nm --demangle --dynamic --defined-only --extern-only< lib.so> | grep< symbol> 。

没有 - 仅定义您的命令也会显示 undefined p>

如果没有 - extern-only ，它还会显示内部链接符号链接。

看起来你需要链接另一个库，因为 Gps_Ephemeris :: Gps_Ephermeris（）链接 libgnss_system_parameters_dyn.so 。开始一个好的方法就是图书馆的文件和例子。

I found a number of similar questions (e.g. this, that or this), but none of them helped me solve my problem. I have a *.so file (from the core of gnss-sdr) that, as indicated by:

$nm libgnss_system_parameters_dyn.so  | c++filt  |grep Gps_Eph

contains the symbol Gps_Ephemeris::Gps_Ephemeris(), which is supposed to be a constructor.

I've written some minimal code:

#include <iostream>
#include <core/system_parameters/gps_ephemeris.h>

int main(int argc,const char* argv[])
{
    Gps_Ephemeris ge;
    return 0; 
}

which I compile with:

g++  main.cpp -std=c++0x -I some_include_path -L some_lib_path -l gnss_system_parameters_dyn`

The linker then complains:

/tmp/ccHCvldG.o: In function `main':
main.cpp:(.text+0x33): undefined reference to `Gps_Ephemeris::Gps_Ephemeris()'
collect2: error: ld returned 1 exit status

I also tried cmake, but the line it generated was similar to that (it just added -rdynamic before linking), and it still generated the exact same linker error.

Note that both the library and my minimal code are being compiled with the same compiler (g++-5), with the exact same flags and the same c++0x standard.

---

Addressing the answer by Maxim Egorushkin, the line:

nm --demangle --defined-only --extern-only libgnss_system_parameters.so  |grep Gps_Eph

doesn't output anything. However, the symbol is defined in the static library (i.e. the *.a library):

00000000000006b0 T Gps_Ephemeris::Gps_Ephemeris()
00000000000006b0 T Gps_Ephemeris::Gps_Ephemeris()

Knowing that both are generated by cmake, in the following way:

add_library(lib_name SHARED ${sources_etc}) #for the *.so
add_library(lib_name_2 ${sources_etc}) #for the *.a

there should be no difference in symbols contained/defined in those libraries, right? I didn't notice anything in cmake's documentation on add_library. Am I missing something obvious?

解决方案

The pedantically correct way to check that a .so exports a symbol is nm --demangle --dynamic --defined-only --extern-only <lib.so> | grep <symbol>.

Without --defined-only your command also shows undefined symbols.

Without --extern-only it also shows symbols with internal linkage which are unavailable for linking.

It looks like you need to link another library because Gps_Ephemeris::Gps_Ephermeris() is not resolved by linking libgnss_system_parameters_dyn.so. A good way to start is that library's documentation and examples.

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