# Getting undefined reference error despite of having the library specified in CMake

**URL:** https://discourse.cmake.org/t/getting-undefined-reference-error-despite-of-having-the-library-specified-in-cmake/1888
**Category:** Usage
**Created:** [September 22, 2020, 5:44pm UTC](https://discourse.cmake.org/t/getting-undefined-reference-error-despite-of-having-the-library-specified-in-cmake/1888 "2020-09-22T17:44:25Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![Coderx7](https://discourse.cmake.org/user_avatar/discourse.cmake.org/coderx7/32/837_2.png) [@Coderx7](https://discourse.cmake.org/u/Coderx7)
#### Post date: [September 22, 2020, 5:44pm UTC](https://discourse.cmake.org/t/getting-undefined-reference-error-despite-of-having-the-library-specified-in-cmake/1888/1 "2020-09-22T17:44:26Z")

</div>

Hello everyone,  
hope you are having a great time.  
I’m trying to create a test executable out of a library I made. Lets name them lib1 and lib2.  
lib1 gets built along with its test just fine. the lib2 is also built without any issues. however whenever I try to link lib2 with its test executable (i.e. a sample program that uses lib2) I get the following error :

```auto
usr/bin/ld: CMakeFiles/Lib2_Test.dir/Lib2_Test.cpp.o: in function `main':
Lib2_Test.cpp:(.text+0xf3): undefined reference to `Lib2::Lib2(std:: __cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__ cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, int)'
/usr/bin/ld: Lib2_Test.cpp:(.text+0x3f5): undefined reference to `Lib2::Evaluate(bool&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, float&, cv::Mat&, cv::Mat&, bool)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/Lib2_Test.dir/build.make:130: Lib2_Test] Error 1
make[1]: *** [CMakeFiles/Makefile2:76: CMakeFiles/Lib2_Test.dir/all] Error 2
make: *** [Makefile:130: all] Error 2

```

I tried viewing the headers using `readelf -d` and using the `ldd` command, both of the libs seems to be having all the necessary references. However lib1 doesn’t have any issues, while lib2 generates unreferenced related errors when being linked to an executable that uses it.  
I also tried to view the symbols defined in Lib2 using `nm -C ` and noticed its OK. here is the output : [https://paste.ee/p/b2Kk4](https://paste.ee/p/b2Kk4) .  
I also need to mention that these codes were originally developed in windows /visual studio and they build and work 100%. I also tried cmake and it ran smoothly. however in linux it seems different and it has given me lots of headaches so far. I’m not sure what I’m doing wrong.

Below are the cmakeLists I made for both of these libs and their test apps and later on I also included the output of `readelf -d` as well in case it matters.  
CMakelist.txt for `lib1`:

```cmake
cmake_minimum_required(VERSION 3.11)
project(Lib1)

set(CMAKE_CXX_STANDARD 17)

find_package(Torch REQUIRED)
find_package(OpenCV REQUIRED)
# I know this is not the best way, but bare with me!
add_definitions(-D_LIB1_BUILD_DLL) 

set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC" )

set(CMAKE_INSTALL_PREFIX /home/me/Desktop/LibtorchPort/built_stuff)

include_directories( /home/me/Desktop/LibtorchPort/Dependencies/include ${TORCH_INCLUDE_DIRS} ${OpenCV_INCLUDE_DIRS})
LINK_DIRECTORIES(/home/me/libtorch-cxx11-abi-shared-with-deps-1.6.0+cpu/libtorch/lib)

# http://dlib.net/examples/CMakeLists.txt.html
add_subdirectory(/home/me/dlib-19.21 /home/me/dlib-19.21/build)     

set(Lib1_SRC ./lib1.cpp)

add_library(lib1 SHARED ${Lib1_SRC})

# Link
target_link_libraries(lib1 ${TORCH_LIBRARIES})
target_link_libraries(lib1 ${OpenCV_LIBS})
target_link_libraries(lib1 dlib::dlib)

install(TARGETS lib1 LIBRARY DESTINATION lib)

```

This is the CMakeList.txt for `lib1_test`:

```cmake
cmake_minimum_required(VERSION 3.11)
project(Lib1_Test)
set(CMAKE_CXX_STANDARD 17)

find_package(Torch REQUIRED)
find_package(OpenCV REQUIRED)

set(CMAKE_INSTALL_PREFIX /home/me/Desktop/LibtorchPort/built_stuff)
include_directories( /home/me/Desktop/LibtorchPort/Dependencies/include ${TORCH_INCLUDE_DIRS} ${OpenCV_INCLUDE_DIRS})
 
# Link
add_executable(lib1_dynamic_test ./Lib1_Test.cpp)
target_link_directories(lib1_dynamic_test PRIVATE /home/me/Desktop/LibtorchPort/Lib1/build)

target_link_libraries(lib1_dynamic_test lib1 )
target_link_libraries(lib1_dynamic_test ${TORCH_LIBRARIES} )
target_link_libraries(lib1_dynamic_test ${OpenCV_LIBS})
install(TARGETS lib1_dynamic_test DESTINATION bin)

```

This is the CMakeList.txt for `lib2`:

```cmake
cmake_minimum_required(VERSION 3.11)
project(Lib2)

set(CMAKE_CXX_STANDARD 17)

find_package(Torch REQUIRED)
find_package(OpenCV REQUIRED)

add_definitions(-D_LIB2_BUILD_DLL) 

set(CMAKE_INSTALL_PREFIX /home/me/Desktop/LibtorchPort/built_stuff)

include_directories( /home/me/Desktop/LibtorchPort/Dependencies/include ${TORCH_INCLUDE_DIRS} ${OpenCV_INCLUDE_DIRS})

LINK_DIRECTORIES(/home/me/libtorch-cxx11-abi-shared-with-deps-1.6.0+cpu/libtorch/lib)
LINK_DIRECTORIES(/home/me/Desktop/LibtorchPort/Lib1/build)

set(LIB2_SRC ./lib2.cpp )

add_library(lib2_dynamic SHARED ${LIB2_SRC} )

target_link_directories(lib2_dynamic PRIVATE /home/me/Desktop/LibtorchPort/Lib1/build)

target_link_libraries(lib2_dynamic lib1)
target_link_libraries(lib2_dynamic ${TORCH_LIBRARIES})
target_link_libraries(lib2_dynamic ${OpenCV_LIBS})

install(TARGETS lib2_dynamic LIBRARY DESTINATION lib)

```

And finally here is the CMakeList for `lib2_test`:

```cmake
cmake_minimum_required(VERSION 3.11)
project(Lib2_Test)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_INSTALL_PREFIX /home/me/Desktop/LibtorchPort/built_stuff)

#find_package(Torch REQUIRED)
find_package(OpenCV REQUIRED)

include_directories( /home/me/Desktop/LibtorchPort/Dependencies/include ${OpenCV_INCLUDE_DIRS})
LINK_DIRECTORIES(/home/me/Desktop/LibtorchPort/Lib1/build)

add_executable(Lib2_Test ./lib2_test.cpp)

target_link_directories(Lib2_Test PRIVATE /home/me/Desktop/LibtorchPort/Lib2/build)
#target_link_directories(Lib2_Test PUBLIC /home/me/Desktop/LibtorchPort/Lib1/build)

#Link
target_link_libraries(Lib2_Test ${OpenCV_LIBS})
target_link_libraries(Lib2_Test lib2_dynamic)
install(TARGETS Lib2_Test DESTINATION bin)

```

running the `readelf -d lib1` this is the output I get:

```auto
Dynamic section at offset 0x908f8 contains 38 entries:
  Tag Type Name/Value
 0x0000000000000001 (NEEDED) Shared library: [libtorch.so]
 0x0000000000000001 (NEEDED) Shared library: [libc10.so]
 0x0000000000000001 (NEEDED) Shared library: [libtorch_cpu.so]
 0x0000000000000001 (NEEDED) Shared library: [libopencv_highgui.so.3.4]
 0x0000000000000001 (NEEDED) Shared library: [libopencv_imgproc.so.3.4]
 0x0000000000000001 (NEEDED) Shared library: [libopencv_core.so.3.4]
 0x0000000000000001 (NEEDED) Shared library: [libcblas.so.3]
 0x0000000000000001 (NEEDED) Shared library: [liblapack.so.3]
 0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6]
 0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
 0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
 0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
 0x0000000000000001 (NEEDED) Shared library: [ld-linux-x86-64.so.2]
 0x000000000000000e (SONAME) Library soname: [libLib1.so]
 0x000000000000001d (RUNPATH) Library runpath: [/home/me/anaconda3/lib/python3.8/site-packages/torch/lib:/home/me/libtorch-cxx11-abi-shared-with-deps-1.6.0+cpu/libtorch/lib:/usr/local/lib:]
 0x000000000000000c (INIT) 0x1a000
 0x000000000000000d (FINI) 0x57e00
 0x0000000000000019 (INIT_ARRAY) 0x90b28
 0x000000000000001b (INIT_ARRAYSZ) 32 (bytes)
 0x000000000000001a (FINI_ARRAY) 0x90b48
 0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
 0x000000006ffffef5 (GNU_HASH) 0x328
 0x0000000000000005 (STRTAB) 0x6840
 0x0000000000000006 (SYMTAB) 0x1758
 0x000000000000000a (STRSZ) 56053 (bytes)
 0x000000000000000b (SYMENT) 24 (bytes)
 0x0000000000000003 (PLTGOT) 0x92000
 0x0000000000000002 (PLTRELSZ) 8112 (bytes)
 0x0000000000000014 (PLTREL) RELA
 0x0000000000000017 (JMPREL) 0x17ff0
 0x0000000000000007 (RELA) 0x14b58
 0x0000000000000008 (RELASZ) 13464 (bytes)
 0x0000000000000009 (RELAENT) 24 (bytes)
 0x000000006ffffffe (VERNEED) 0x149f8
 0x000000006fffffff (VERNEEDNUM) 5
 0x000000006ffffff0 (VERSYM) 0x14336
 0x000000006ffffff9 (RELACOUNT) 6
 0x0000000000000000 (NULL) 0x0

```

and this is the output for `lib2`:

```auto
Dynamic section at offset 0x37ba0 contains 32 entries:
  Tag Type Name/Value
 0x0000000000000001 (NEEDED) Shared library: [libLib1.so]
 0x0000000000000001 (NEEDED) Shared library: [libtorch.so]
 0x0000000000000001 (NEEDED) Shared library: [libtorch_cpu.so]
 0x0000000000000001 (NEEDED) Shared library: [libopencv_core.so.3.4]
 0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6]
 0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
 0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
 0x000000000000000e (SONAME) Library soname: [libLib2_dynamic.so]
 0x000000000000001d (RUNPATH) Library runpath: [/home/me/anaconda3/lib/python3.8/site-packages/torch/lib:/home/me/libtorch-cxx11-abi-shared-with-deps-1.6.0+cpu/libtorch/lib:/home/me/Desktop/LibtorchPort/Lib1/build:/usr/local/lib:]
 0x000000000000000c (INIT) 0x1e000
 0x000000000000000d (FINI) 0x2ec10
 0x0000000000000019 (INIT_ARRAY) 0x38108
 0x000000000000001b (INIT_ARRAYSZ) 16 (bytes)
 0x000000000000001a (FINI_ARRAY) 0x38118
 0x000000000000001c (FINI_ARRAYSZ) 8 (bytes)
 0x000000006ffffef5 (GNU_HASH) 0x2f0
 0x0000000000000005 (STRTAB) 0x7d88
 0x0000000000000006 (SYMTAB) 0x1dd0
 0x000000000000000a (STRSZ) 62708 (bytes)
 0x000000000000000b (SYMENT) 24 (bytes)
 0x0000000000000003 (PLTGOT) 0x39000
 0x0000000000000002 (PLTRELSZ) 14784 (bytes)
 0x0000000000000014 (PLTREL) RELA
 0x0000000000000017 (JMPREL) 0x19e90
 0x0000000000000007 (RELA) 0x17b38
 0x0000000000000008 (RELASZ) 9048 (bytes)
 0x0000000000000009 (RELAENT) 24 (bytes)
 0x000000006ffffffe (VERNEED) 0x17a78
 0x000000006fffffff (VERNEEDNUM) 3
 0x000000006ffffff0 (VERSYM) 0x1727c
 0x000000006ffffff9 (RELACOUNT) 4
 0x0000000000000000 (NULL) 0x0

```

And yet the `lib1` builds and links just fine, while `lib2` that is depandant on `lib1`, has issues when being linked to its test or any other libs. I’m completely clueless at this point and have no idea what could have caused this. what is wrong here? What am I missing?

**Note 1:**  
This is the `lib2_test.cpp` : [https://paste.ee/p/pOgFk](https://paste.ee/p/pOgFk)  
and this is how the header file looks like :

```cpp
#ifndef Lib2_H
#define Lib2_H

/* If we are we on Windows, we want a single define for it.*/
#if !defined(_WIN32) && (defined( __WIN32__ ) || defined(WIN32) || defined( __MINGW32__ ))
#define _WIN32
#endif /* _WIN32 */

#if defined(_WIN32) && defined(_Lib2_BUILD_DLL)
/* We are building Lib2 as a Win32 DLL */
#define LIB2_API __declspec(dllexport)
#elif defined(_WIN32) && defined(Lib2_DLL)
/* We are calling Lib2 as a Win32 DLL */
#define LIB2_API __declspec(dllimport)
#elif defined( __GNUC__ ) && defined(_Lib2_BUILD_DLL)
/* We are building Lib2 as a shared / dynamic library */
#define LIB2_API __attribute__ ((visibility("default")))
#else
/* We are building or calling Lib2 as a static library */
#define LIB2_API
#endif

#include <string>
#include <vector>
#include <map>
#include <memory>
#include <opencv2/core.hpp>

enum class ValidationStatus
{
	None = -1,
	UnderValidation = 0,
	Validated = 1,
	Invalidated = 2,
	AnomalyDetected = 3,
	ToomuchAnonalyDetected = 4
};

typedef std::tuple<ValidationStatus, std::string, bool> Lib2Result;

class Lib2Impl;

class LIB2_API Lib2
{
private:
	std::shared_ptr<Lib2Impl> Lib2;

public:
	Lib2(std::string shape_predictor_path = "",	std::string eyeNet_path = "", int valid_presence_delay = 5,	int fpasPassed = 0);

	std::vector<Lib2Result> Run(std::map<std::string, bool>& validity_result, 
									   std::vector<std::tuple<std::string, float>>& ids,
									   std::vector<cv::Mat>& faces, 
									   cv::Mat& originalImage,
									   bool show_debug_info=false);

	Lib2Result Evaluate(bool& status, std::string& name, float& theta, cv::Mat& face_image, cv::Mat& originalImage, bool debug_info = true);
	~Lib2();

};

#endif // !Lib2_H

```

**Note 2:**  
I need to add this as well that , the project was built just fine in windows using visual studio and also cmake! however under Linux(ubuntu 20.04), I’m facing these issues.  
So this is not just calling different/wrong methods or with wrong signatures. this code should compile and link just fine but I’m doing something wrong here which I’m not sure what it is.

**Note 3:**  
The lib1 and lib2 are just made up name for the actual filenames (I just chose lib1 and lib2 to keep things simpler and replaced the names here, so if you see difference in cases , don’t mind it, the actual files are the same

Thank you all in advance

---

<div class="post-metadata">

### Author: ![Coderx7](https://discourse.cmake.org/user_avatar/discourse.cmake.org/coderx7/32/837_2.png) [@Coderx7](https://discourse.cmake.org/u/Coderx7)
#### Post date: [September 23, 2020, 2:50am UTC](https://discourse.cmake.org/t/getting-undefined-reference-error-despite-of-having-the-library-specified-in-cmake/1888/2 "2020-09-23T02:50:25Z")

</div>

## TLDR:

The thrid party lib (torch) was built using a `Pre-cxx11` ABI, and libs built with that couldn’t obviously be linked to the object that was using `cxx11` ABI!

## Long Explanation:

After hours of debugging of a code that worked just fine in windows both in Visual Studio and CMake while being a pain in the neck in linux I found the culprit!  
The libtorch is shipped with two types of build `Pre-cxx11` and `cxx11` ABIs!  
It was using the `Pre-cxx11` built libs that was shipped with its Python package (torch1.6cpu) and since Anaconda3 was in the Path and I was also using it to build the libs I faced this issue.  
What gave it away after all these hours was the weird arguments to the undefined methods which were : `std::__cxx11::basic_string` where it should have been simply `std::string`. I didnt expect this two to be different and thought thats a weird naming scheme the g++ is using until moments ago that made me say, lets give that a search maybe I can get something out it! lo and behold! this was the case :

> If you get linker errors about undefined references to symbols that  
> involve types in the std::\_\_cxx11 namespace or the tag [abi:cxx11]  
> then it probably indicates that you are trying to link together object  
> files that were compiled with different values for the  
> \_GLIBCXX\_USE\_CXX11\_ABI macro. This commonly happens when linking to a third-party library that was compiled with an older version of GCC. If  
> the third-party library cannot be rebuilt with the new ABI then you  
> will need to recompile your code with the old ABI.

[ref](https://stackoverflow.com/questions/33394934/converting-std-cxx11string-to-stdstring)  
To fix this I simply directly used the libtorch cxx11 in all libs creation and that just did it. meaning, unlike what is being shown in the Pytorch’s official documentaion,  
**DO NOT DO:**

```auto
cmake -DCMAKE_PREFIX_PATH="$(python -c 'import torch.utils; print(torch.utils.cmake_prefix_path)')" ..

```

By doing this all hell will break loose if you, like me expected the libs to have been built with `cxx11`! becasue they have not!

Note that this is not the case under windows! so if you are on linux, just grab and use the prebuilt libs with `CXX11` ABI! and avoid what ships with Pytorch!