6.1.21 UNION and MAP

Unions are an old vendor extension which were commonly used with the non-standard 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'

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https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gfortran/UNION-and-MAP.html