[cmake command notes]find_ path

Compiling basalt when ,cmake Tips need to be 3.4.0 Version of Eigen, And in find_package Added in exact Parameters , So it can't be changed .
However, I use apt Installed eigen yes 3.3.7 Version of . So download and install 3.4.0 Version of Eigen, however cmake Still can't find this version Eigen.
see basalt The compiled file found that he was using FindEigen3.cmake File search Eigen Library , That is to say CMakelists.txt of use find_package() Find library , then findpackage Use module Pattern to find , see CMake Search method
So just modify FindEigen3.cmake Files can be searched , stay FindEigen3.cmake There is a use in the document find_path.
There will be find_path The official document of the command is translated as follows
Link to the original text —CMake3.23.1—find_path

find_path

The compact command signature is ：

find_path (<VAR> name1 [path1 path2 ...])

The generalized command signature is

find_path (
          <VAR>
          name | NAMES name1 [name2 ...]
          [HINTS [path | ENV var]... ]
          [PATHS [path | ENV var]... ]
          [PATH_SUFFIXES suffix1 [suffix2 ...]]
          [DOC "cache documentation string"]
          [NO_CACHE]
          [REQUIRED]
          [NO_DEFAULT_PATH]
          [NO_PACKAGE_ROOT_PATH]
          [NO_CMAKE_PATH]
          [NO_CMAKE_ENVIRONMENT_PATH]
          [NO_SYSTEM_ENVIRONMENT_PATH]
          [NO_CMAKE_SYSTEM_PATH]
          [CMAKE_FIND_ROOT_PATH_BOTH |
           ONLY_CMAKE_FIND_ROOT_PATH |
           NO_CMAKE_FIND_ROOT_PATH]
         )

The first thing you need to know is Command line syntax parameters <>, [], | What do you mean by them
This command is used to find a directory , This directory contains the named file( It should be in the command parameter name、NAMES These parameters specify the file ), If you choose NO_CACHE Parameters , that <VAR> The specified variable is created to hold the search results of the command . Find success , Then the search results will be saved to <VAR> in , If not found , Then the result will be saved as <VAR>-NOTFOUND.

Options include
NAMES
Here is either directly write name, Or write NAMES name1 [name2 …] It means . Is the file item to be found .

HINTS, PATHS
In addition to the default location , Specify other directories to search .ENV var Sub options read the path from the system environment variable .

PATH_SUFFIXS
Specify additional subdirectories to check under each directory .(otherwise considered I don't know what it means )

NO_DEFAULT_PATH
Do not find the default path

You know that the above parameters are almost complete

Take the following command for example , This is to find EIGEN3 The location of this library ,signature_of_eigen3_matrix_library yes eigen3 A file in this library , The directory where this file is located is the directory where the library is located , Therefore use find_path Find this file and save the results to EIGEN3_INCLUDE_DIR_NAMES.
HINTS Provides a search path , It's a variable. EIGEN_INCLUDE_DIR_HINTS Provided .
NO_DEFAULT_PATH Indicates that the default path is not searched , such as /usr/local/include、/usr/include Wait, path .

  find_path(EIGEN3_INCLUDE_DIR NAMES signature_of_eigen3_matrix_library
      HINTS ${EIGEN_INCLUDE_DIR_HINTS}
      NO_DEFAULT_PATH
    )