Make a variable as a global variable within a function

New to Julia
20

Umm… actually I’m not familiar with macros.
I think that it is enough to solve the below problem:

function _my_func(a, b)
    c = 2*a
    b = b + 1
    a + b
end

function my_func(a)
    b = a
    @log _my_func(a, b)
end

Then, the definition of my_func should be

function my_func(a)
    b = a
    # the content of `_my_func`. It must not change the above `b`... how?
    c = 2*a
    b = b + 1
    a+b
    #
end

Do you have any idea?

EDIT:
Ah, generate a new function _my_func__LOG__ and run it may work.
But I don’t still know how to generate _my_func__LOG__ as the following.
This question is continued at the next question.

function my_func(a)
    b = a
    # the content of `_my_func`. It must not change the above `b`... how?
    function _my_func__LOG__(a, b)
        c = 2*a
        b = b + 1
        a+b
    end
    _my_func__LOG__(a, b)
    #
end