为何CMake中未定义的MBEDTLS变量会被赋值为NOTFOUND？

我创建了一个空的CMake项目，并添加了以下代码：

message(WARNING "Non existent var = ${NON_EXISTENT_VAR}")
message(WARNING "MBEDTLS INCLUDE DIR = ${MBEDTLS_INCLUDE_DIR}")
message(WARNING "MBEDTLS_LIBRARY = ${MBEDTLS_LIBRARY}")
message(WARNING "MBEDX509_LIBRARY = ${MBEDTLS_LIBRARY}")
message(WARNING "MBEDCRYPTO_LIBRARY = ${MBEDTLS_LIBRARY}")

执行后得到如下输出：

[cmake] CMake Warning at CMakeLists.txt:40 (message):
[cmake]   Non existent var =
[cmake] 
[cmake] 
[cmake] CMake Warning at CMakeLists.txt:41 (message):
[cmake]   MBEDTLS INCLUDE DIR = MBEDTLS_INCLUDE_DIR-NOTFOUND
[cmake] 
[cmake] 
[cmake] CMake Warning at CMakeLists.txt:42 (message):
[cmake]   MBEDTLS_LIBRARY = MBEDTLS_LIBRARY-NOTFOUND
[cmake] 
[cmake] 
[cmake] CMake Warning at CMakeLists.txt:43 (message):
[cmake]   MBEDX509_LIBRARY = MBEDTLS_LIBRARY-NOTFOUND
[cmake] 
[cmake] 
[cmake] CMake Warning at CMakeLists.txt:44 (message):
[cmake]   MBEDCRYPTO_LIBRARY = MBEDTLS_LIBRARY-NOTFOUND

疑问解答 #

• MBEDTLS_LIBRARY这类变量不是CMake自动定义的。你看到的XXX-NOTFOUND是CMake的特殊标记：当调用find_package(MbedTLS)、find_library或find_path这类查找命令但未找到目标时，CMake会将对应变量设置为XXX-NOTFOUND，标记查找失败。
• 你的空项目里没手动调用查找命令，但可能是CMake内置模块、或者环境中的全局配置脚本触发了相关查找逻辑，导致这些变量被创建并赋值为NOTFOUND。
• CMake本身不会自动为常见软件包预定义变量，只有显式调用查找命令时才会创建对应变量。未被定义的变量（比如NON_EXISTENT_VAR）在输出时会是空值，这和被标记为NOTFOUND的变量有明显区别。
• 另外注意代码中的笔误：MBEDX509_LIBRARY和MBEDCRYPTO_LIBRARY都错误引用了${MBEDTLS_LIBRARY}，所以它们的输出和MBEDTLS_LIBRARY一致，这是代码写法错误导致的，并非变量本身的问题。

内容的提问来源于stack exchange，提问作者Zebrafish