1. 找不到g2o库！在CMakeLists.txt中使用指令 message(STATUS "${G2O_FOUND}") 打印结果为NO。
问题描述：
CMakeLists.txt 中采用如下命令寻找g2o库。
# g2o
find_package( G2O REQUIRED )
include_directories( ${G2O_INCLUDE_DIRS} )
科普时间：find_package() 会在模块路径中寻找FindNAME.cmake文件，再经由此文件寻找库所有文件，包括头文件，静态库(*.a)，动态库(*.so)，这种查找方法称为模块模式。更多详情参考此链接。
由于我把g2o安装在 /usr/local/g2o 路径中，不在FindNAME.cmake的默认查找路径中，因此一直找不到。
解决方法：
在CMakeLists.txt中进行修改，如下所示，
set(CMAKE_PREFIX_PATH "/usr/local/g2o") find_package( G2O REQUIRED ) message(STATUS "${G2O_INCLUDE_DIR}") include_directories( ${G2O_INCLUDE_DIR} )
同时在FindG2O.cmake中，将 NO_DEFAULT_PATH 注释掉。如下所示，
FIND_PATH(G2O_INCLUDE_DIR g2o/core/base_vertex.h ${G2O_ROOT}/include $ENV{G2O_ROOT}/include /usr/local/include ... #NO_DEFAULT_PATH )
如果指定了NO_DEFAULT_PATH选项，所有NO_*选项都会被激活，导致 set(CMAKE_PREFIX_PATH "/usr/local/g2o") 设置的路径被跳过。
2. 运行make进行编译时，出现如下报错，
/usr/bin/ld: cannot find -lg2o_core /usr/bin/ld: cannot find -lg2o_stuff /usr/bin/ld: cannot find -lg2o_types_slam3d
问题描述：
编译执行文件时使用 target_link_libraries() 链接上述三个动态库文件，在 /usr/local/g2o/lib 中可以找到这些文件，从报错可以判断应该是找不到正确的链接路径。
解决方法：
在CMakeList.txt中添加如下命令行，设置动态库所在的绝对路径。
link_directories("/usr/local/g2o/lib")
关于在cmake中添加头文件目录，链接动态库、静态库的操作参考这个链接。
3. 运行make编译代码，报错如下，
/home/gordon/kalibr_ws/devel/lib/libcholmod.a(cholmod_super_numeric.o): In function `cholmod_super_numeric': cholmod_super_numeric.c:(.text+0xe78): undefined reference to `dsyrk_' cholmod_super_numeric.c:(.text+0xf76): undefined reference to `dgemm_' cholmod_super_numeric.c:(.text+0x129c): undefined reference to `dpotrf_' cholmod_super_numeric.c:(.text+0x13a2): undefined reference to `dtrsm_' cholmod_super_numeric.c:(.text+0x1dd6): undefined reference to `zherk_' cholmod_super_numeric.c:(.text+0x1ed4): undefined reference to `zgemm_' cholmod_super_numeric.c:(.text+0x2286): undefined reference to `zpotrf_' cholmod_super_numeric.c:(.text+0x23b2): undefined reference to `ztrsm_' cholmod_super_numeric.c:(.text+0x2dcb): undefined reference to `zherk_' cholmod_super_numeric.c:(.text+0x2ec9): undefined reference to `zgemm_' cholmod_super_numeric.c:(.text+0x325c): undefined reference to `zpotrf_' cholmod_super_numeric.c:(.text+0x337c): undefined reference to `ztrsm_' /home/gordon/kalibr_ws/devel/lib/libcholmod.a(cholmod_super_solve.o): In function `cholmod_super_lsolve': cholmod_super_solve.c:(.text+0x548): undefined reference to `ztrsm_' cholmod_super_solve.c:(.text+0x5f2): undefined reference to `zgemm_' cholmod_super_solve.c:(.text+0x876): undefined reference to `dtrsm_' cholmod_super_solve.c:(.text+0x91d): undefined reference to `dgemm_' cholmod_super_solve.c:(.text+0xad0): undefined reference to `dtrsv_' cholmod_super_solve.c:(.text+0xb5c): undefined reference to `dgemv_' cholmod_super_solve.c:(.text+0xcc6): undefined reference to `ztrsv_' cholmod_super_solve.c:(.text+0xd55): undefined reference to `zgemv_' /home/gordon/kalibr_ws/devel/lib/libcholmod.a(cholmod_super_solve.o): In function `cholmod_super_ltsolve': cholmod_super_solve.c:(.text+0x133b): undefined reference to `zgemm_' cholmod_super_solve.c:(.text+0x13c9): undefined reference to `ztrsm_' cholmod_super_solve.c:(.text+0x169a): undefined reference to `dgemm_' cholmod_super_solve.c:(.text+0x1721): undefined reference to `dtrsm_' cholmod_super_solve.c:(.text+0x18bf): undefined reference to `dgemv_' cholmod_super_solve.c:(.text+0x1912): undefined reference to `dtrsv_' cholmod_super_solve.c:(.text+0x1a9d): undefined reference to `zgemv_' cholmod_super_solve.c:(.text+0x1af7): undefined reference to `ztrsv_'
问题描述：
由报错第一行可以判定该报错与Cholmod库有关。无法正确链接Cholmod库。
查看cmake输出内容，如下所示，
Found CHOLMOD: /home/gordon/kalibr_ws/devel/include/suitesparse
可以确定寻找cholmod库的结果是suitesparse库（目前不清楚这两个库的关系，先将它们等同起来）。我之前在kalibr_ws中安装了suitesparse库，同时，g2o的readme.md也让我在根目录下安装了suitesparse库。想到kalibr_ws是采用catkin_build编译的，其架构跟cmake必定不一致，因此尝试换成根目录下的库。
解决方法：
在 ~/.bashrc 中去除kalibr_ws的环境变量，重启终端。可通过 echo $PATH 命令查看该环境变量是否被移除。或者，暂时将kalibr_ws中的devel文件夹暂时移除。使用 cmake 重新链接库文件，得到如下结果，
Found CHOLMOD: /usr/include/suitesparse
至此即可编译成功。