Cmake / set command

One 、 Introduce

set Command can set common variables 、 cache line 、 Environment variables are the values of three variables , Corresponding to the following three command formats ：

set(<variable> <value>... [PARENT_SCOPE]) # Set common variables 
 
set(<variable> <value>... CACHE <type> <docstring> [FORCE]) # Set cache entries 
 
set(ENV{<variable>} [<value>]) # Set the environment variable 

When setting normal variables ,set Value <value>... Indicates that the variable can be set 0 One or more values , When setting multiple values （ Greater than 2 individual ）, Multiple values will pass Semicolon connector Connect to a real value and assign it to the variable , When setting 0 When it's worth , It's actually changing the variable to an unset state , Equivalent to calling unset command .

Two 、 Use

1） Set the variable to a given value

cmake_minimum_required (VERSION 3.10.2)
 
project (set_test)
 
set (normal_var a)
 
message (">>> value = ${normal_var}")

Print

2） Set the variable to multiple given values

cmake_minimum_required (VERSION 3.10.2)
 
project (set_test)
 
set (normal_var a b c)
 
message (">>> value = ${normal_var}")

  Print

3） Set the variable to null

cmake_minimum_required (VERSION 3.10.2)
 
project (set_test)
 
set (normal_var a b c)
 
message (">>> value = ${normal_var}")
 
set (normal_var) #  Set the variable to null 
 
message (">>> value = ${normal_var}")

  Print  

 4） If in a function ,set Use options PARENT_SCOPE, Then the scope of the variable can only be passed to the function calling it .

scene 1： In function （ The example is test_fn） Internal use set Defining variables , With options PARENT_SCOPE, Then in another function （ caller , The example is test_fn_parent） Call this function inside .
result ： The variable is defined in the caller function .
Conclusion ： Options PARENT_SCOPE Pass the variable to the upper layer and call the function

# CMakeLists.txt
cmake_minimum_required (VERSION 3.10.2)
project (set_test)
 
function (test_fn arg1)
    set (normal_var_in_fn ${arg1} PARENT_SCOPE)
    message (">>> in function, value = ${normal_var_in_fn}")
endfunction (test_fn)
 
function (test_fn_parent arg1)
    test_fn (${arg1})
    message (">>> in parent function, value = ${normal_var_in_fn}")
endfunction (test_fn_parent)
 
test_fn_parent (hello)

  Print

scene 2： Use options inside functions PARENT_SCOPE Defining variables , Use this variable outside the function .

result ： The variable is empty .
Conclusion ： Variables defined in functions , Call... Outside the function , Cannot find definition of variable

# CMakeLists.txt
 
cmake_minimum_required (VERSION 3.10.2)
 
project (set_test)
 
function (test_fn arg1)
    set (normal_var_in_fn ${arg1} PARENT_SCOPE)
endfunction (test_fn)
 
test_fn(hello)
message (">>> in directory, value = ${normal_var_fn}")

Print

scene 3： Use... Directly within a function set The variable of , With options PARENT_SCOPE
result ： The variable is empty .
Conclusion ： Use options inside functions PARENT_SCOPE Variables defined , It is also undefined in this function  

# CMakeLists.txt
cmake_minimum_required (VERSION 3.10.2)
project (set_test)
 
 function (test_fn arg1)
    set (normal_var_in_fn ${arg1} PARENT_SCOPE)
    message (">>> in function, value = ${normal_var_fn}")
endfunction (test_fn)
 
test_fn (hello)
message (">>> in directory, value = ${normal_var_fn}")

Print

scene 4： Within the function , First use set Command defines the variable , Reuse the tape PARENT_SCOPE Option defines the variable .

result ： The value of the variable in the function is No PARENT_SCOPE Option set Command defined .
Conclusion ： Options PARENT_SCOPE The defined variable scope is in the upper function , The variables of the current function must be used without options PARENT_SCOPE Definition .

# CMakeLists.txt
 
cmake_minimum_required (VERSION 3.10.2)
 
project (set_test)
 
function (test_fn arg1)
    set (normal_var_in_fn nohello)
    set (normal_var_in_fn ${arg1} PARENT_SCOPE)
    message (">>> in function, value = ${normal_var_in_fn}")
endfunction (test_fn)
 
test_fn (hello)
 

Print

3、 ... and 、 Set cache entries

Command format ：

set(<variable> <value>... CACHE <type> <docstring> [FORCE])

Meaning of order ：
Cache entries  variable  Set to value  <value>..., Unless the user makes a setting or uses an option FORCE, By default, the value of the cache entry is not overwritten . Cache entries can be accessed through CMAKE Of GUI Interface  add entry  Button to add . The essence of cache entries is variables that can be passed across levels , Similar to global variables .

Cache entry <type> There are mainly the following categories ：

• BOOL： Boolean value  ON/OFF,CMAKE Of GUI The interface will provide a check box for such cache entries .
• FILEPATH： File path ,CMAKE Of GUI The interface will provide a file selection box for such cache entries .
• PATH： Directory path ,CMAKE Of GUI The interface will provide a directory selection box for such cache entries .
• STRING / STRINGS： Text row ,CMAKE Of GUI The interface will provide a text box for such cache entries （ Corresponding  STRING） Or drop-down selection box （ Corresponding  STRINGS）.
• INTERNAL： Text row , But only for internal , No external presentation . It is mainly used to store variables during operation , So use this  type  It means using  FORCE.

Several considerations for caching entries ：

• If the variable is not previously defined or used  FORCE  Options , The cache entry will be assigned directly .
• Can be used in cmake The use of the build is through -D  Option to assign a value to the cache entry , such CMakeLists.txt Internal  set The command only adds types to cache entries .
• If the variable type is directory or file path , adopt -D  If only the relative path is passed in by the option , that  set  The current working directory will be added before the relative path to become an absolute path （ If it is already an absolute path, it will not process ）.

# CMakeLists.txt
cmake_minimum_required (VERSION 3.10.2)
project (set_test)
 
set (cache_entry_val ON OFF CACHE BOOL "choose ON to enable")
message (">>> value = ${cache_entry_val}")
 
set (cache_entry_val2 ON CACHE BOOL "choose ON to enable" FORCE)
message (">>> value2 = ${cache_entry_val2}")
 
set (cache_entry_val3 ON)
set (cache_entry_val3 OFF CACHE BOOL "choose ON to enable")
message (">>> value3 = ${cache_entry_val3}")
 
set (cache_entry_input OFF CACHE BOOL "choose ON to enable")
message (">>> value4 = ${cache_entry_input}")
 
set (mypath "test" CACHE FILEPATH "choose a file path")
message (">>> value5 = ${mypath}")

  Print

Four . Set the environment variable

   Command format ：set(ENV{<variable>} [<value>])
   Meaning of order ： Set the environment variable to the value <value>（ Note that there is no ...）, Then use $ENV{<variable>} You'll get a new value .cmake The environment variables in can refer to ： environment variable .
   Several considerations for setting environment variables ：
1） The environment variable set by this command is only in the current cmake Entry into force , It will not affect the caller's environment variables , It also does not affect the system environment variables .
2） If <value> The value is empty or ENV{<variable>} There are no parameters after , The command will clear the value of the current environment variable .
3）<value> The parameters after are ignored .

# CMakeLists.txt
cmake_minimum_required (VERSION 3.10.2)
project (set_test)
 
message (">>> value = $ENV{CMAKE_PREFIX_PATH}")
 
set (ENV{CMAKE_PREFIX_PATH} "/test/sub")
message (">>> value = $ENV{CMAKE_PREFIX_PATH}")
 
set (ENV{CMAKE_PREFIX_PATH})
message (">>> value = $ENV{CMAKE_PREFIX_PATH}")
 
set (ENV{CMAKE_PREFIX_PATH} "/test/top/") 
message (">>> value = $ENV{CMAKE_PREFIX_PATH}")
 
set (ENV{CMAKE_PREFIX_PATH} "") 
message (">>> value = $ENV{CMAKE_PREFIX_PATH}")

Print  

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