/GNU Fortran 10


Record structures are a pre-Fortran-90 vendor extension to create user-defined aggregate data types. Support for record structures in GNU Fortran can be enabled with the -fdec-structure compile flag. If you have a choice, you should instead use Fortran 90’s “derived types”, which have a different syntax.

In many cases, record structures can easily be converted to derived types. To convert, replace STRUCTURE /structure-name/ by TYPE type-name. Additionally, replace RECORD /structure-name/ by TYPE(type-name). Finally, in the component access, replace the period (.) by the percent sign (%).

Here is an example of code using the non portable record structure syntax:

! Declaring a structure named ``item'' and containing three fields:
! an integer ID, an description string and a floating-point price.
  CHARACTER(LEN=200) description
  REAL price

! Define two variables, an single record of type ``item''
! named ``pear'', and an array of items named ``store_catalog''
RECORD /item/ pear, store_catalog(100)

! We can directly access the fields of both variables
pear.id = 92316
pear.description = "juicy D'Anjou pear"
pear.price = 0.15
store_catalog(7).id = 7831
store_catalog(7).description = "milk bottle"
store_catalog(7).price = 1.2

! We can also manipulate the whole structure
store_catalog(12) = pear
print *, store_catalog(12)

This code can easily be rewritten in the Fortran 90 syntax as following:

! ``STRUCTURE /name/ ... END STRUCTURE'' becomes
! ``TYPE name ... END TYPE''
TYPE item
  CHARACTER(LEN=200) description
  REAL price

! ``RECORD /name/ variable'' becomes ``TYPE(name) variable''
TYPE(item) pear, store_catalog(100)

! Instead of using a dot (.) to access fields of a record, the
! standard syntax uses a percent sign (%)
pear%id = 92316
pear%description = "juicy D'Anjou pear"
pear%price = 0.15
store_catalog(7)%id = 7831
store_catalog(7)%description = "milk bottle"
store_catalog(7)%price = 1.2

! Assignments of a whole variable do not change
store_catalog(12) = pear
print *, store_catalog(12)

GNU Fortran implements STRUCTURES like derived types with the following rules and exceptions:

  • Structures act like derived types with the SEQUENCE attribute. Otherwise they may contain no specifiers.
  • Structures may contain a special field with the name %FILL. This will create an anonymous component which cannot be accessed but occupies space just as if a component of the same type was declared in its place, useful for alignment purposes. As an example, the following structure will consist of at least sixteen bytes:
    structure /padded/
      character(4) start
      character(8) %FILL
      character(4) end
    end structure
  • Structures may share names with other symbols. For example, the following is invalid for derived types, but valid for structures:
    structure /header/
      ! ...
    end structure
    record /header/ header
  • Structure types may be declared nested within another parent structure. The syntax is:
    structure /type-name/
        structure [/<type-name>/] <field-list>

    The type name may be ommitted, in which case the structure type itself is anonymous, and other structures of the same type cannot be instantiated. The following shows some examples:

    structure /appointment/
      ! nested structure definition: app_time is an array of two 'time'
      structure /time/ app_time (2)
        integer(1) hour, minute
      end structure
      character(10) memo
    end structure
    ! The 'time' structure is still usable
    record /time/ now
    now = time(5, 30)
    structure /appointment/
      ! anonymous nested structure definition
      structure start, end
        integer(1) hour, minute
      end structure
      character(10) memo
    end structure
  • Structures may contain UNION blocks. For more detail see the section on UNION and MAP.
  • Structures support old-style initialization of components, like those described in Old-style variable initialization. For array initializers, an initializer may contain a repeat specification of the form <literal-integer> * <constant-initializer>. The value of the integer indicates the number of times to repeat the constant initializer when expanding the initializer list.

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