so库调用异常：Linux: C++ lookup error – undefined symbol

编译生成动态链接库后，调用时出现：

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws on git:lichunhong/dev x [18:54:05] C:127
$ rosrun path_plan PathPlanSimulation
/home/lichunhong/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws/devel/lib/path_plan/PathPlanSimulation:
 symbol lookup error: /home/lichunhong/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws/src/pathPlan/lib/libpathplan.so: 
 undefined symbol: _ZN12ninebot_algo10AprAlgoLog9instance_E

即 symbol lookup error: libpathplan.so: undefined symbol: _ZN12ninebot_algo10AprAlgoLog9instance_E

出现这种问题时，往往是链接时出现了问题，下面分3步解决

（1）使用file 命令查看 so库的架构，看看是否与平台一致

可以看到，当前so库架构为x86-64，可以在GNU/Linux平台下使用。平台与架构一致

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/motion_planner/bin on git:dev x [18:47:54] 
$ file libpathplan.so 
libpathplan.so: ELF 64-bit LSB shared object, x86-64, version 1 (GNU/Linux), dynamically linked, 
BuildID[sha1]=32ae641e73c547376df20ca94746fbf5507de415, not stripped

接下来，需要定位一下 undefined symbol的具体信息

(2)通过 ldd -r xxx.so 命令查看so库链接状态和错误信息

ldd命令，可以查看对应的可执行文件或库文件依赖哪些库，但可执行文件或库文件要求与操作系统的编译器类型相同，即电脑是X86的GCC编译器，那么无法通过ldd命令查看ARM交叉编译器编译出来的可执行文件或库文件。

如果想在Ubuntu等Linux宿主机上查看ARM交叉编译好的可执行程序和库文件的相关依赖关系，可以通过以下命令：
readelf -d xxx.so | grep NEEDED

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws/src/pathPlan/lib on git:lichunhong/dev x [18:57:19] 
$ ldd -r libpathplan.so
	linux-vdso.so.1 =>  (0x00007ffec1bd8000)
	libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f186cc0a000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f186c901000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f186c6eb000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f186c321000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f186d27a000)
undefined symbol: pthread_create	(./libpathplan.so)
undefined symbol: _ZN12ninebot_algo10AprAlgoLog9instance_E	(./libpathplan.so)
undefined symbol: _ZN2cv3maxERKNS_3MatES2_	(./libpathplan.so)
undefined symbol: _ZN12ninebot_algo10AprAlgoLog8WriteLogE10LEVEL_TYPEPKcS3_z	(./libpathplan.so)
undefined symbol: _ZN2cv6dilateERKNS_11_InputArrayERKNS_12_OutputArrayES2_NS_6Point_IiEEiiRKNS_7Scalar_IdEE	(./libpathplan.so)
undefined symbol: _ZN2cvgtERKNS_3MatEd	(./libpathplan.so)
undefined symbol: _ZN2cv8fastFreeEPv	(./libpathplan.so)
undefined symbol: _ZN2cv3Mat5setToERKNS_11_InputArrayES3_	(./libpathplan.so)
undefined symbol: _ZN12ninebot_algo10AprAlgoLog9instance_E	(./libpathplan.so)

可以看到有好多 undefined symbol ，其中就有提到的 _ZN12ninebot_algo10AprAlgoLog9instance_E 错误

(3) 使用 c++filt symbol 定位错误在那个C++文件中

从上面的undefined symbol中，通过cc++filt ，可以定位到大多是opencv的问题

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws/src/pathPlan/lib on git:lichunhong/dev x [19:04:26] C:1
$ c++filt _ZN2cv7waitKeyEi
cv::waitKey(int)

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/effective_robotics_programming_with_ros-master/catkin_ws/src/pathPlan/lib on git:lichunhong/dev x [19:04:31] 
$ c++filt _ZN2cv3maxERKNS_3MatES2_
cv::max(cv::Mat const&, cv::Mat const&)

是不是忘记添加opencv的链接库了呢，查看项目的cmakelists，果然没有添加。在cmakelists最后，添加如下代码

    target_link_libraries(pathplan
            ${OpenCV_LIBS}
            )

好的，重新编译生成so库，使用 ldd 命令查看是否还有 undefined symbol

$ ldd -r libpathplan.so           
undefined symbol: _ZN12ninebot_algo10AprAlgoLog9instance_E	(./libpathplan.so)

嗯，只剩一个了，继续

# lichunhong @ lichunhong-ThinkPad-T470p in ~/Documents/src/motion_planner/bin on git:dev x [19:11:52] 
$ c++filt _ZN12ninebot_algo10AprAlgoLog9instance_E
ninebot_algo::AprAlgoLog::instance_

原来是log文件错误？查看cmakelists ，在add_library中漏掉了log文件依赖，加上

${PROJECT_SOURCE_DIR}/third_party/ninebot_log/ninebot_log.cpp

add_library{} 整体如下：

add_library(pathplan
        SHARED
        ${AprMotionPlanner_src}
        ${DWA_SRC}
        ${THETA_STAR_SRC}
	    ${PROJECT_SOURCE_DIR}/third_party/ninebot_log/ninebot_log.cpp
        )

这次生成的so库可以运行了。