How to set the global variable in a function for c

问题:

I'm writing a CMakeLists.txt to generate files and compile the generated files. I create a function to add some file path strings to a global list variable.

My CMakeLists.txt:

set(source_list "nothing")
function(test file_path)
    list(APPEND source_list ${file_path})
endfunction(test)
test(abc.txt)
test(def.txt)
message("At last, the source_list is:\"${source_list}\"")

The cmake output:

At last, the source_list is:"nothing"

Someone suggested that to use macro instead of function, but I do need use local variable, so I need to use the function instead of macro.

How can I correctly set the global variable source_list in the function test()? Can't cmake do it in a simple and normal way?

回答1:

You need to use set instead of list to affect the variable in the parent scope.

So replace your list command with:

set(source_list ${source_list} ${file_path} PARENT_SCOPE)

回答2:

PARENT_SCOPE is only for parent, it won't work if you have other non-parent script that want to see it as well.

You need cache for the true "global-like" variable. In your case, use:

SET(source_list  "${source_list}" CACHE INTERNAL "source_list")

回答3:

Another approach is to use global properties. Once you set it:

set_property(GLOBAL PROPERTY source_list_property "${source_list}")

you can read it from everywhere:

get_property(source_list GLOBAL PROPERTY source_list_property)

I used in examples above the different names for property (source_list_property) and for variable (source_list). Maybe it is better to use the same name. But point is to use a property as global variables, and not about naming.

Such global properties aren't in cache.

回答4:

Building upon Maxim Suslov's answer, the following code worked for a similar problem I faced:

set_property(GLOBAL PROPERTY source_list)
function(add_source)
    get_property(tmp GLOBAL PROPERTY source_list)
    foreach(arg ${ARGV})
        set(tmp "${tmp} ${arg}")
    endforeach()
    set_property(GLOBAL PROPERTY source_list "${tmp}")
endfunction(add_source)

add_source(a_file)
add_source(b_file c_file)

get_property(local_prop GLOBAL PROPERTY source_list)
message("list: ${local_prop}")

Function add_source can be called from inside any sub-directory.