UNION and MAP (The GNU Fortran Compiler)

#### 5.1.23 `UNION` and `MAP` ¶

Unions are an old vendor extension which were commonly used with the nonstandard `STRUCTURE` and `RECORD` extensions. Use of `UNION` and `MAP` is automatically enabled with `-fdec-structure`.

A `UNION` declaration occurs within a structure; within the definition of each union is a number of `MAP` blocks. Each `MAP` shares storage with its sibling maps (in the same union), and the size of the union is the size of the largest map within it, just as with unions in C. The major difference is that component references do not indicate which union or map the component is in (the compiler gets to figure that out).

Here is a small example:

    structure /myunion/
    union
      map
        character(2) w0, w1, w2
      end map
      map
        character(6) long
      end map
    end union
    end structure

    record /myunion/ rec
    ! After this assignment...
    rec.long = 'hello!'

    ! The following is true:
    ! rec.w0 === 'he'
    ! rec.w1 === 'll'
    ! rec.w2 === 'o!'

The two maps share memory, and the size of the union is ultimately six bytes:

    0    1    2    3    4   5   6     Byte offset
    -------------------------------
    |    |    |    |    |    |    |
    -------------------------------

    ^    W0   ^    W1   ^    W2   ^
     \-------/ \-------/ \-------/

    ^             LONG            ^
     \---------------------------/

Following is an example mirroring the layout of an Intel x86_64 register:

    structure /reg/
      union ! U0                ! rax
        map
          character(16) rx
        end map
        map
          character(8) rh         ! rah
          union ! U1
            map
              character(8) rl     ! ral
            end map
            map
              character(8) ex     ! eax
            end map
            map
              character(4) eh     ! eah
              union ! U2
                map
                  character(4) el ! eal
                end map
                map
                  character(4) x  ! ax
                end map
                map
                  character(2) h  ! ah
                  character(2) l  ! al
                end map
              end union
            end map
          end union
        end map
      end union
    end structure
    record /reg/ a

    ! After this assignment...
    a.rx     =     'AAAAAAAA.BBB.C.D'

    ! The following is true:
    a.rx === 'AAAAAAAA.BBB.C.D'
    a.rh === 'AAAAAAAA'
    a.rl ===         '.BBB.C.D'
    a.ex ===         '.BBB.C.D'
    a.eh ===         '.BBB'
    a.el ===             '.C.D'
    a.x  ===             '.C.D'
    a.h  ===             '.C'
    a.l  ===               '.D'