Cmake命令之cmake_path
一、命令简介
该命令用来操作路径，仅仅是语义概念上的处理，并不会在文件系统上执行任何交互。路径可以不存在，甚至是当前系统不允许的路径。如果想要了解与文件系统交互的命令，参考file()。
注意：
cmake_path会将路径以CMake所运行的构建系统（例如，主机平台）的格式处理，而不是目标系统。在交叉编译过程中，如果路径包含的元素在CMake运行的系统上无法表示（例如路径是C:\\，但是系统并不是Windows），那么结果不可预测。
cmake_path分为以下几个子命令：分解、查询、修改、生成、转换、哈希。
该命令有如下约定:
-
<path-var>：变量名，如果作为命令中的输入，该变量必须存在并且是单个路径。 -
<input>：字符串字面值，可以包含一个路径、路径的一部分、以分号分割的多个路径。 -
<input>...：0或多个字符串字面的参数。 -
<out-var>：存储命令的结果。
路径结构和术语：
一个路径的组成如下（都是可选的，但是可能存在某些限制）：
root-name root-directory-separator (item-name directory-separator)\* filename
-
root-name：支持多个根路径的系统的（例如Windows）根路径名称，可选。例如C:或者//myserver等。 -
root-directory-separator：目录分隔符，如果存在表明路径是绝对路径，如果确实该分隔符，并且第一个部分不是root-name，是item-name，那么它是一个相对路径。 -
item-name：一个字符组成的序列，不能是目录分隔符，该名称可能标识一个文件、或一个目录，或软连接，或硬链接，有连个特殊的情况： 1）包含单个点.，表示当前目录 2）包含两个点..，表示父目录(item-name directory-separator)*表示0个或多个item-name，以目录分隔符分割，()*不是路径的一部分。 -
directory-separator：目录分隔符，即/，连续的多个/会被当成一个/处理，例如/usr/////lib与/usr/lib相同。 -
filename：文件名。如果路径不是以/结束，路径中的最后一个item-name就是文件名，它可以是文件、软硬链接、目录。文件名还可以包含扩展，默认情况下，扩展是从最左边的.分隔符开始，例如file.dat1.dat2，那么.dat1.dat2就是扩展名。filename除去扩展部分被称为stem，因此默认情况下file.dat1.dat2的stem就是file。可以使用LAST_ONLY关键字来指定扩展从filename最右边的.开始。例外情况是:1）如果第一个字符就是.，那么解析为无扩展，例如.profile；或2）如果是.或..同样认为没有扩展。
创建一个路径变量
可以使用set命令来创建一个路径，推荐用cmake_path(SET)，因为后者会自动转换路径为要求的格式。cmake_path(APPEND)是另一个适合通过将多个片段拼接成一个路径的子命令，下面三个命令都是创建同一个路径：
set(path1 "${CMAKE_CURRENT_SOURCE_DIR}/data")
cmake_path(SET path2 "${CMAKE_CURRENT_SOURCE_DIR}/data")
cmake_path(APPEND path3 "${CMAKE_CURRENT_SOURCE_DIR}" "data")
修改和生成子命令可以将结果存入输入路径变量中，也可以使用OUTPUT_VARIABLE指定输出路径的变量。其他子命令都强制将结果存储在<out-var>中。
规范化
一些子命令支持规范化一个路径，规范一个路径的算法如下：
-
如果路径为空，命令终止（即空路径规范化后仍是空路径）
-
替换多个连续的
/分隔符为单个/分隔符，例如//a/b///c规范化后为/a/b/c。 -
移除在分隔符后的单个
.，例如/a/./b/c/.规范化后为/a/b/c。 -
如果
item-name后面紧跟着是..，那么删除该item-name、..以及跟在他们之后的分隔符/，例如/a/b/../c规范化后为/a/c。 -
如果路径包含根路径，并且根路径后面紧跟的是
..，那么移除..和跟在它后面的/，因为根目录的父路径仍是它自己。例如/../a规范化后为/a. -
如果最后一个
item-name是..，那么移除..之后的分隔符/，例如../规范化后为..。 -
经过上述规范化后的路径仍为空，那么规范化后的路径为
.(./的规范化路径也是.).
二、子命令详解
2.1 分解
cmake_path(GET)子命令从一个路径中获取不同的部分。如果待获取的部分在路径中不存在，那么该子命令会将空字符串结果存于<out-var>中，例如只有在Windows中才存在root-name的概念，因此，如果CMake运行的系统非Windows，获取ROOT_NAME会得到一个空字符串。如果HAS_RELATIVE_PART子命令结果为false，那么获取PARENT_NAME的结果就是<path-var>，由于根路径的父路径是它自己，当HAS_RELATIVE_PART子命令结果为true时，表明<path-var>至少包含一个元素。
子命令形式为：
cmake_path(GET <path-var> ROOT_NAME <out-var>)cmake_path(GET <path-var> ROOT_DIRECTORY <out-var>)cmake_path(GET <path-var> ROOT_PATH <out-var>)cmake_path(GET <path-var> FILENAME <out-var>)cmake_path(GET <path-var> EXTENSION [LAST_ONLY] <out-var>)cmake_path(GET <path-var> STEM [LAST_ONLY] <out-var>)cmake_path(GET <path-var> RELATIVE_PART <out-var>)cmake_path(GET <path-var> PARENT_PATH <out-var>)
-
cmake_path(GET <path-var> [ROOT_NAME|ROOT_DIRECTORY|ROOT_PATH] <out-var>)：获取根名称、目录、路径，我的系统非Windows，因此ROOT_NAME为空。
set(path "/usr/bin")
cmake_path(GET path ROOT_NAME rootName)
cmake_path(GET path ROOT_DIRECTORY rootDirectory)
cmake_path(GET path ROOT_PATH rootPath)
message("root name is: ${rootName}")
message("root directory is: ${rootDirectory}")
message("root path is: ${rootPath}")
运行结果： root name is: root directory is: / root path is: /
-
cmake_path(GET <path-var> FILENAME <out-var>)：获取文件名。
set(path "/usr/bin")
cmake_path(GET path FILENAME filename)
message("filename is: ${filename}")
set(path "/a/b/")
cmake_path(GET path FILENAME filename)
message("filename is: ${filename}")
运行结果： filename is: bin filename is:
-
cmake_path(GET <path-var> [EXTENSION|STEM] <out-var>)：获取扩展或stem信息。默认情况下，扩展是从最左边的.分隔符开始，例如file.dat1.dat2，那么.dat1.dat2就是扩展名。filename除去扩展部分被称为stem，因此默认情况下file.dat1.dat2的stem就是file。可以使用LAST_ONLY关键字来指定扩展从filename最右边的.开始。
set(path "name.ext1.ext2")
# 默认扩展是取最左侧的.到尾部的部分
cmake_path(GET path EXTENSION fullExt)
cmake_path(GET path STEM fullStem)
message("full extension: ${fullExt}")
message("full stem: ${fullStem}")
# 指定LAST_ONLY后，扩展取的是最右侧的.到尾部的部分
cmake_path(GET path EXTENSION LAST_ONLY lastExt)
cmake_path(GET path STEM LAST_ONLY lastStem)
message("last extension: ${lastExt}")
message("last stem: ${lastStem}")
set(dotPath "/a/.")
set(dotDotPath "/a/..")
set(someMorePath "/a/.some.more")
# filename是.的特例：扩展为空
cmake_path(GET dotPath EXTENSION dotExt)
cmake_path(GET dotPath STEM dotStem)
# filename是..的特例：扩展为空
cmake_path(GET dotDotPath EXTENSION dotDotExt)
cmake_path(GET dotDotPath STEM dotDotStem)
cmake_path(GET someMorePath EXTENSION someMoreExt)
cmake_path(GET someMorePath STEM someMoreStem)
message("Dot extension is ${dotExt}")
message("Dot stem is ${dotStem}")
message("Dot-dot extension is ${dotDotExt}")
message("Dot-dot stem is ${dotDotStem}")
message(".some.more extension is ${someMoreExt}")
message(".some.more stem is ${someMoreStem}")
运行结果 full extension: .ext1.ext2 full stem: name last extension: .ext2 last stem: name.ext1 Dot extension is Dot stem is . Dot-dot extension is Dot-dot stem is . .some.more extension is .more .some.more stem is .some
-
cmake_path(GET <path-var> RELATIVE_PART <out-var>)：获取相对路径信息。
set(path "/usr/local/bin")
cmake_path(GET path RELATIVE_PART relativePath)
message("relative path: ${relativePath}")
set(path "usr/local/bin") # 如果本身就是相对路径，那么RELATIVE_PART子命令获取到的就是它本身
cmake_path(GET path RELATIVE_PART relativePath)
message("relative path: ${relativePath}")
set(path "/")
cmake_path(GET path RELATIVE_PART relativePath)
message("relative path: \"${relativePath}\"")
运行于
macOS下的结果 relative path: usr/local/bin relative path: usr/local/bin relative path: ""
-
cmake_path(GET <path-var> PARENT_PATH <out-var>)：获取父目录。
set(path "/usr/local/bin")
cmake_path(GET path PARENT_PATH parentPath)
message("parent path: ${parentPath}")
set(path "usr/local/bin")
cmake_path(GET path PARENT_PATH parentPath)
message("parent path: ${parentPath}")
set(path "usr/local/bin/") # 如果filename为空，则父目录就是自身
cmake_path(GET path PARENT_PATH parentPath)
message("parent path: ${parentPath}")
set(path "/")
cmake_path(GET path PARENT_PATH parentPath)
message("parent path: ${parentPath}")
运行结果 parent path: /usr/local parent path: usr/local parent path: usr/local/bin parent path: /
2.2 查询
查询分为三大类，形式分别为HAS_XXX、IS_XXX、COMPARE。
1、HAS_XXX查询子命令
每一个cmake_path(GET)子命令都对应一个HAS_XXX子命令，以便判定GET子命令获取的路径信息是否存在。HAS_XXX的子命令格式如下：
cmake_path(HAS_ROOT_NAME <path-var> <out-var>)cmake_path(HAS_ROOT_DIRECTORY <path-var> <out-var>)cmake_path(HAS_ROOT_PATH <path-var> <out-var>)cmake_path(HAS_FILENAME <path-var> <out-var>)cmake_path(HAS_EXTENSION <path-var> <out-var>)cmake_path(HAS_STEM <path-var> <out-var>)cmake_path(HAS_RELATIVE_PART <path-var> <out-var>)cmake_path(HAS_PARENT_PATH <path-var> <out-var>)
如果查询的路径信息存在，则<out-var>返回true，两个特例为：
-
对于
HAS_ROOT_PATH子命令，只有当root-name或root-directory其中一个为非空时返回true。 -
对于
HAS_PARENT_PATH子命令，根目录的父目录是根目录自身，因此除非路径仅包含filename，否则该子命令返回true。
set(path "/usr/local/bin/myfile.ext1.ext2")
cmake_path(HAS_ROOT_NAME path hrn)
cmake_path(HAS_ROOT_DIRECTORY path hrd)
cmake_path(HAS_ROOT_PATH path hrp)
cmake_path(HAS_FILENAME path hfn)
cmake_path(HAS_EXTENSION path hext)
cmake_path(HAS_STEM path hstem)
cmake_path(HAS_RELATIVE_PART path hrelative)
cmake_path(HAS_PARENT_PATH path hpp)
message("${path} has root name? --- ${hrn}")
message("${path} has root directory? --- ${hrd}")
message("${path} has root path? --- ${hrp}")
message("${path} has filename? --- ${hfn}")
message("${path} has extension? --- ${hext}")
message("${path} has stem? --- ${hstem}")
message("${path} has relative part? --- ${hrelative}")
message("${path} has parent path? --- ${hpp}")
运行结果 /usr/local/bin/myfile.ext1.ext2 has root name? --- OFF /usr/local/bin/myfile.ext1.ext2 has root directory? --- ON /usr/local/bin/myfile.ext1.ext2 has root path? --- ON /usr/local/bin/myfile.ext1.ext2 has filename? --- ON /usr/local/bin/myfile.ext1.ext2 has extension? --- ON /usr/local/bin/myfile.ext1.ext2 has stem? --- ON /usr/local/bin/myfile.ext1.ext2 has relative part? --- ON /usr/local/bin/myfile.ext1.ext2 has parent path? --- ON
2、IS_XXX查询子命令
IS_XXX查询子命令的格式如下：
cmake_path(IS_ABSOLUTE <path-var> <out-var>)cmake_path(IS_RELATIVE <path-var> <out-var>)cmake_path(IS_PREFIX <path-var> <input> [NORMALIZE] <out-var>)
-
cmake_path(IS_ABSOLUTE <path-var> <out-var>)：如果是绝对路径，<out-var>结果为true。绝对路径唯一标识一个文件的位置。在Windows下，绝对路径必须包含root-name和目录分隔符（例如C:/）。其他平台下，只需要目录分隔符即可（例如Linux下的/）。
set(path "/usr/local/bin")
cmake_path(IS_ABSOLUTE path result)
message("${path} is absolute? --- ${result}")
set(path "usr/local/bin")
cmake_path(IS_ABSOLUTE path result)
message("${path} is absolute? --- ${result}")
运行结果： /usr/local/bin is absolute? --- ON usr/local/bin is absolute? --- OFF
-
cmake_path(IS_RELATIVE <path-var> <out-var>)：判断路径是否是相对路径。
set(path "/usr/local/bin")
cmake_path(IS_RELATIVE path result)
message("${path} is relative? --- ${result}")
set(path "usr/local/bin")
cmake_path(IS_RELATIVE path result)
message("${path} is relative? --- ${result}")
运行结果： /usr/local/bin is relative? --- OFF usr/local/bin is relative? --- ON
-
cmake_path(IS_PREFIX <path-var> <input> [NORMALIZE] <out-var>)：判断路径<path-var>是否是<input>的前缀，如果指定了NORMALIZE选项，<path-var>和<input>在比较前会进行路径规范化。
set(path "/usr/local/bin")
cmake_path(IS_PREFIX path "/usr/local/bin/myfile" result)
message("if ${path} is prefix of /usr/local/bin/myfile? --- ${result}")
cmake_path(IS_PREFIX path "/usr/local" result)
message("if ${path} is prefix of /usr/local? --- ${result}")
set(path "/usr/../tmp/dir")
cmake_path(IS_PREFIX path "/tmp/dir/myfile" NORMALIZE result)
message("if ${path} is prefix of /tmp/dir/myfil? --- ${result}")
运行结果： if /usr/local/bin is prefix of /usr/local/bin/myfile? --- ON if /usr/local/bin is prefix of /usr/local? --- OFF if /usr/../tmp/dir is prefix of /tmp/dir/myfil? --- ON
3、COMPARE查询子命令
-
cmake_path(COMPARE <input1> EQUAL <input2> <out-var>) cmake_path(COMPARE <input1> NOT_EQUAL <input2> <out-var>)：比较两个路径是否相同或者不同，路径被当做字符串字面值进行比较，注意该子命令的两个输入路径都不会进行路径规范化。
cmake_path(COMPARE "/usr/local/bin" EQUAL "/usr/local/bin" cmpResult)
message("CMP result: ${cmpResult}")
cmake_path(COMPARE "/usr/local/bin/" NOT_EQUAL "/usr/local/bin" cmpResult)
message("CMP result: ${cmpResult}")
运行结果： CMP result: ON CMP result: ON
2.3 修改
修改包含设置、追加、移除、替换等子命令操作，具体格式如下：
cmake_path(SET <path-var> [NORMALIZE] <input>)
cmake_path(APPEND <path-var> [<input>...] [OUTPUT_VARIABLE <out-var>])
cmake_path(APPEND_STRING <path-var> [<input>...] [OUTPUT_VARIABLE <out-var>])
cmake_path(REMOVE_FILENAME <path-var> [OUTPUT_VARIABLE <out-var>])
cmake_path(REPLACE_FILENAME <path-var> <input> [OUTPUT_VARIABLE <out-var>])
cmake_path(REMOVE_EXTENSION <path-var> [LAST_ONLY] [OUTPUT_VARIABLE <out-var>])
cmake_path(REPLACE_EXTENSION <path-var> [LAST_ONLY] <input> [OUTPUT_VARIABLE <out-var>])
1、设置子命令
-
cmake_path(SET <path-var> [NORMALIZE] <input>)：将<input>赋值给<input-var>，注意<input>应该是一个变量（用$取变量符号），否则会当成一个字符串。如果指定了NORMALIZE关键字，那么在执行赋值前会先对路径进行规范化。
set(path "/usr/local/bin")
cmake_path(SET result ${path}) # 如果此处不是用${path}而是path，那么result的结果是"path"
message("${path} set to ${result}")
set(path "/usr/../local////bin/.")
cmake_path(SET result NORMALIZE ${path})
message("${path} normalized set to ${result}")
运行结果： /usr/local/bin set to /usr/local/bin /usr/../local////bin/. normalized set to /local/bin/
2、追加子命令
-
cmake_path(APPEND_STRING <path-var> [<input>...] [OUTPUT_VARIABLE <out-var>])：顺序遍历多个<input>路径，用/拼接后，结果存入<path-var>。在遍历过程中，根据<input>的具体内容，通过前面<input>计算得到的<input-var>可能会被丢弃。规则如下：
-
如果当前的
<input>是绝对路径，则使用<input>替换<path-var>，拼接过程结束。 -
<input>存在ROOT_NAME并且与<path-var>的ROOT_NAME不相同，那么使用<input>替换<path-var>，拼接过程结束。 -
如果
<input>的ROOT_DIRECTORY存在，那么移除<path-var>任意的ROOT_DIRECTORY和整个相对路径。 -
如果
<input>不存在ROOT_DIRECTORY且<path-var>存在filename，或<path-var>是非绝对路径且不存在ROOT_DIRECTORY，那么将/拼接到<path-var>。 -
将
<input>拼接到<path-var>，忽略ROOT_NAME。
cmake_path(APPEND result "/usr/local/bin" "test/myfile" "myfile2")
message("path result is : ${result}")
cmake_path(APPEND result1 "usr/local" "/tmp/myfile")
message("path result1 is : ${result1}")
cmake_path(APPEND result2 "usr/local/" "mydir1" "mydir2")
message("path result2 is : ${result2}")
运行结果： path result is : /usr/local/bin/test/myfile/myfile2 path result1 is : /tmp/myfile path result2 is : usr/local/mydir1/mydir2
-
cmake_path(APPEND_STRING <path-var> [<input>...] [OUTPUT_VARIABLE <out-var>])：将<input>拼接到path-var，不会额外添加任何目录分隔符。注意：该命令下<path-var>需要已经定义。
set(rs "")
cmake_path(APPEND_STRING rs "/usr/" "local" "bin")
message("APPEND_STRING: ${rs}")
运行结果： APPEND_STRING: /usr/localbin
-
cmake_path(REMOVE_FILENAME <path-var> [OUTPUT_VARIABLE <out-var>])：将路径中的filename移除（也就是通过GET ... FILENAME获取到的内容），移除后尾部的目录分隔符/会被保留。如果没有定义OUTPUT_VARIABLE，那么执行该子命令后，对<path-var>执行HAS_FILENAME会返回false。
set(path "/usr/local/bin/myfile")
cmake_path(REMOVE_FILENAME path)
message("For ${path} remove filename is: ${path}")
cmake_path(HAS_FILENAME path hfn)
message("For ${path} has filename? --- ${hfn}")
set(path "/usr/local/bin/myfile")
cmake_path(REMOVE_FILENAME path OUTPUT_VARIABLE output) # 有OUTPUT_VARIABLE，原始路径path不会被修改
message("For ${path} remove filename is: ${output}, orignal path is ${path}")
cmake_path(HAS_FILENAME path hfn)
message("For ${path} has filename? --- ${hfn}")
运行结果： For /usr/local/bin/ remove filename is: /usr/local/bin/ For /usr/local/bin/ has filename? --- OFF For /usr/local/bin/myfile remove filename is: /usr/local/bin/, orignal path is /usr/local/bin/myfile For /usr/local/bin/myfile has filename? --- ON
-
cmake_path(REPLACE_FILENAME <path-var> <input> [OUTPUT_VARIABLE <out-var>])：用<input>替代路径<path-var>中的filename，如果<path-var>中没有filename，那么<path-var>不会有任何变化（如果定义了OUTPUT_VARIABLE，也不会修改<path-var>），该命令等价于以下逻辑：
-
使用
cmake_path(HAS_FILENAME path has_filename)判断是否存在filename； -
第一步的
has_filename如果为真，那么依次执行cmake_path(REMOVE_FILENAME path)和cmake_path(APPEND path input)。
set(path "/usr/local/bin/myfile")
cmake_path(REPLACE_FILENAME path "replacefile" OUTPUT_VARIABLE output)
message("After replace ${path}, result is: ${output}")
# 等效的命令如下
set(path "/usr/local/bin/myfile")
cmake_path(HAS_FILENAME path has_filename)
if(has_filename)
cmake_path(REMOVE_FILENAME path OUTPUT_VARIABLE output)
cmake_path(APPEND output "replacefile")
message("After replace ${path} use combine commands, result is: ${output}")
endif()
运行结果： After replace /usr/local/bin/myfile, result is: /usr/local/bin/replacefile After replace /usr/local/bin/myfile use combine commands, result is: /usr/local/bin/replacefile
-
cmake_path(REMOVE_EXTENSION <path-var> [LAST_ONLY] [OUTPUT_VARIABLE <out-var>])：移除扩展名，可以使用LAST_ONLY指定从filename最右边的.开始移除。
set(path "/usr/local/bin/myfile.ext1.ext2")
cmake_path(REMOVE_EXTENSION path OUTPUT_VARIABLE output)
message("remove extersion of ${path} is ${output}")
cmake_path(REMOVE_EXTENSION path LAST_ONLY OUTPUT_VARIABLE output)
message("remove LAST_ONLY extersion of ${path} is ${output}")
运行结果： remove extersion of /usr/local/bin/myfile.ext1.ext2 is /usr/local/bin/myfile remove LAST_ONLY extersion of /usr/local/bin/myfile.ext1.ext2 is /usr/local/bin/myfile.ext1
-
cmake_path(REPLACE_EXTENSION <path-var> [LAST_ONLY] <input> [OUTPUT_VARIABLE <out-var>])：替换扩展名，可以使用LAST_ONLY指定从filename最右边的.开始替换。
set(path "/usr/local/bin/myfile.ext1.ext2")
cmake_path(REPLACE_EXTENSION path "dat1.dat2" OUTPUT_VARIABLE output)
message("use dat1.dat2 replace extersion of ${path} is ${output}")
cmake_path(REPLACE_EXTENSION path LAST_ONLY "dat1.dat2" OUTPUT_VARIABLE output)
message("user dat1.dat2 replace LAST_ONLY extersion of ${path} is ${output}")
运行结果： use dat1.dat2 replace extersion of /usr/local/bin/myfile.ext1.ext2 is /usr/local/bin/myfile.dat1.dat2 user dat1.dat2 replace LAST_ONLY extersion of /usr/local/bin/myfile.ext1.ext2 is /usr/local/bin/myfile.ext1.dat1.dat2
2.4 生成路径
分为规范化路径、产生相对路径、产生绝对路径三种。
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cmake_path(NORMAL_PATH <path-var> [OUTPUT_VARIABLE <out-var>])：规范化一个路径，具体规则参考本文前面索引处。具体例子不再赘述，参考其他例子中的规范化说明。 -
cmake_path(RELATIVE_PATH <path-var> [BASE_DIRECTORY <input>] [OUTPUT_VARIABLE <out-var>])：将路径<path-var>修改为相对于基路径（由BASE_DIRECTORY指定）的路径，若未指定BASE_DIRECTORY，那么默认的基路径从CMAKE_CURRENT_SOURCE_DIR变量中获取。简而言之，可以理解为从基路径切换到<path-var>路径需要经过的路径。
set(path "/usr/local/bin/myfile")
cmake_path(RELATIVE_PATH path BASE_DIRECTORY "/usr/local" OUTPUT_VARIABLE output)
message("the relative path ${path} for base /usr/local is ${output}")
cmake_path(RELATIVE_PATH path OUTPUT_VARIABLE output) # 未指定基路径，默认基路径为CMAKE_CURRENT_SOURCE_DIR变量存放的路径
message("the relative path ${path} for base ${CMAKE_CURRENT_SOURCE_DIR} is ${output}")
运行结果 the relative path /usr/local/bin/myfile for base /usr/local is bin/myfile the relative path /usr/local/bin/myfile for base /Users/XX1/XX2/XX3/XX4/XX5 is ../../../../../../usr/local/bin/myfile
-
cmake_path(ABSOLUTE_PATH <path-var> [BASE_DIRECTORY <input>] [NORMALIZE] [OUTPUT_VARIABLE <out-var>])：如果<path-var>是绝路径，那么结果就是<path-var>；如果<path-var>是相对路径，效果相当于将<path-var>拼接在基路径之后，当然，如果未指定基路径，则从CMAKE_CURRENT_SOURCE_DIR获取基路径。
set(path "/usr/local/bin/myfile") # 原路径是绝对路径，结果不变
cmake_path(ABSOLUTE_PATH path BASE_DIRECTORY "/usr/local" OUTPUT_VARIABLE output)
message("the absolute path ${path} for base /usr/local is ${output}")
cmake_path(ABSOLUTE_PATH path OUTPUT_VARIABLE output)
message("the absolute path ${path} for base ${CMAKE_CURRENT_SOURCE_DIR} is ${output}")
set(path "mydir/myfile") # 原路径是相对路径，拼接
cmake_path(ABSOLUTE_PATH path BASE_DIRECTORY "/usr/local" OUTPUT_VARIABLE output)
message("the absolute path ${path} for base /usr/local is ${output}")
cmake_path(ABSOLUTE_PATH path OUTPUT_VARIABLE output)
message("the absolute path ${path} for base ${CMAKE_CURRENT_SOURCE_DIR} is ${output}")
运行结果： the absolute path /usr/local/bin/myfile for base /usr/local is /usr/local/bin/myfile the absolute path /usr/local/bin/myfile for base /Users/XX1/XX2/XX3/XX4/XX5 is /usr/local/bin/myfile
the absolute path mydir/myfile for base /usr/local is /usr/local/mydir/myfile the absolute path mydir/myfile for base /Users/XX1/XX2/XX3/XX4/XX5 is /Users/XX1/XX2/XX3/XX4/XX5/mydir/myfile
2,5 原生转换
指的是转换成CMake所在的构建平台路径规范，因此这里的原生指交叉编译的构建平台，而不是目标平台。例如程序在windows下构建，在嵌入式设备上跑，那么原生指的是Windows平台。
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cmake_path(NATIVE_PATH <path-var> [NORMALIZE] <out-var>)：将路径转换成平台相关的斜杆（Windows下的右斜杠\，其他平台的左斜杠/）,还可以通过NORMALIZE在转换前将路径规范化。
set(path "/usr/local/bin/myfile")
cmake_path(NATIVE_PATH path output)
message("the native path of ${path} is: ${output}")
运行结果（macOS): the native path of /usr/local/bin/myfile is: /usr/local/bin/myfile
运行结果（Windows）: the native path of /usr/local/bin/myfile is: \usr\local\bin\myfile
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cmake_path(CONVERT <input> TO_CMAKE_PATH_LIST <out-var> [NORMALIZE])：将路径转换为cmake风格的路径，cmake风格的路径指的是以左斜杆/作为连接符，多个路径之间以;分隔。特别需要注意的是：CONVERT子命令的输入路径被当成字符串文本值处理，而不是当成一个变量进行解析，因此使用当<input>是多个路径时，不能直接使用${input}变量，而需要使用"${input}"确保得到的是一个字符串文本值。
set(path "/usr/local/bin/myfile" "/tmp/mydir") # cmake的set命令以`;`分隔多个值，${path}=/usr/local/bin/myfile;/tmp/mydir
cmake_path(CONVERT "${path}" TO_CMAKE_PATH_LIST output) # 注意要使用"${path}"而不是${path}变量
message("1: the native path ${path} to cmake-style: ${output}")
set(path "/usr/local/bin/myfile:/tmp/mydir") # 模拟以:分隔多个值，${path}="/usr/local/bin/myfile:/tmp/mydir"
cmake_path(CONVERT "${path}" TO_CMAKE_PATH_LIST output) # ${path}变量已经是一个字符串字面量，因此此处也可以用${path}
message("2: the native path ${path} to cmake-style: ${output}")
运行结果(macOS)(注意;和:的区别)： 1: the native path /usr/local/bin/myfile;/tmp/mydir to cmake-style: /usr/local/bin/myfile;/tmp/mydir 2: the native path /usr/local/bin/myfile:/tmp/mydir to cmake-style: /usr/local/bin/myfile;/tmp/mydir
运行结果(Windows)(注意;和:的区别)： 1: the native path /usr/local/bin/myfile;/tmp/mydir to cmake-style: /usr/local/bin/myfile:/tmp/mydir 2: the native path /usr/local/bin/myfile:/tmp/mydir to cmake-style: /usr/local/bin/myfile:/tmp/mydir
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cmake_path(CONVERT <input> TO_NATIVE_PATH_LIST <out-var> [NORMALIZE])：将cmake风格的路径转换为原生路径，例如Windows下的右斜杠\，其他平台的左斜杠/，当多个路径时候，Windows下的以;分隔，其他平台以:分隔。特别需要注意的是：CONVERT子命令的输入路径被当成字符串文本值处理，而不是当成一个变量进行解析，因此使用当<input>是多个路径时，不能直接使用${input}变量，而需要使用"${input}"确保得到的是一个字符串文本值。
set(path "/usr/local/bin/myfile" "/tmp/mydir") # cmake的set命令以`;`分隔多个值，${path}=/usr/local/bin/myfile;/tmp/mydir
cmake_path(CONVERT "${path}" TO_NATIVE_PATH_LIST output) # 注意要使用"${path}"而不是${path}变量
message("the cmake-style path ${path} to native path: ${output}")
运行结果(macOS)(注意;和:的区别)： the cmake-style path /usr/local/bin/myfile;/tmp/mydir to native path: /usr/local/bin/myfile:/tmp/mydir
运行结果(Windows)(注意;和:的区别)： the cmake-style path /usr/local/bin/myfile;/tmp/mydir to native path: \usr\local\bin\myfile;\tmp\mydir
2.6 哈希
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cmake_path(HASH <path-var> <out-var>)：计算路径的哈希值，哈希结果存放在<out-var>中。可用于比较两个路径是否相等（参考子命令COMPARE .. EQUAL，两个路径相等时，其哈希值也相等）。在计算哈希值之前，路径会被规范化。
set(path "/usr/local/bin/myfile")
cmake_path(HASH path hash_path)
cmake_path(APPEND path2 "/usr/local" "bin/myfile")
cmake_path(HASH path2 hash_path2)
message("path1:${path}, path1 hash:${hash_path}")
message("path2:${path2}, path2 hash:${hash_path2}")
运行结果： path1:/usr/local/bin/myfile, path1 hash:5a17be211871fbe path2:/usr/local/bin/myfile, path2 hash:5a17be211871fbe
