VMS Help
FORTRAN, Statements, STRUCTURE
*Conan The Librarian (sorry for the slow response - running on an old VAX)
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Indicates the beginning of the record structure declaration and
defines the name of the structure. Declaration format:
STRUCTURE [/str/][fnlist]
fdcl
[fdcl]
...
[fdcl]
END STRUCTURE
str Identifies a structure name, which is used in
following RECORD statements to refer to the
structure. A structure name is enclosed in slashes.
fnlist Identifies field names when used in a substructure
declaration.(Only allowed in nested structure
declarations.)
fdcl (Also called the declaration body.) Is any
declaration or combination of declarations of
substructures, unions, or typed data, or
PARAMETER statements.
Following RECORD statements use the structure name to refer to the
structure. A structure name must be unique among structure names,
but structures can share names with variables (scalar or array),
record fields, PARAMETER constants, and common blocks.
Structure declarations can be nested (contain one or more other
structure declarations). A structure name is required for the
structured declaration at the outermost level of nesting, and
optional for the other declarations nested in it. However, if you
wish to reference a nested structure in a RECORD statement in your
program, it must have a name.
Structure, field, and record names are all local to the defining
program unit. When records are passed as arguments, the fields
must match in type, order, and dimension.
Unlike type declaration statements, structure declarations do not
create variables. Structured variables (records) are created when
you use a RECORD statement containing the name of a previously
declared structure. The RECORD statement can be considered as a
kind of type statement. The difference is that aggregate items,
not single items, are being defined.
Within a structure declaration, the ordering of both the statements
and the field names within the statements is important because this
ordering determines the order of the fields in records.
In a structure declaration, each field offset is the sum of the
lengths of the previous fields. The length of the structure,
therefore, is the sum of the lengths of its fields. The structure
is packed; you must explicitly provide any alignment that is needed
by including, for example, unnamed fields of the appropriate
length.
In the following example, the declaration defines a structure named
DATE. This structure contains three scalar fields: DAY
(LOGICAL*1), MONTH (LOGICAL*1), and YEAR (INTEGER*2).
STRUCTURE /DATE/
LOGICAL*1 DAY, MONTH
INTEGER*2 YEAR
END STRUCTURE
The syntax of a type declaration within a record structure is
identical to that of a normal Fortran type declaration statement:
it includes a data type (for example, INTEGER), one or more names
of variables or arrays; and optionally, one or more data
initialization values.
The following rules and behavior apply to type declarations in
record structures:
o %FILL can be specified in place of a field name to leave space
in a record for purposes such as alignment. This creates an
unnamed field.
%FILL can have an array declarator; for example:
INTEGER %FILL (2,2)
Unnamed fields cannot be initialized. For example, the
following statement is invalid and generates an error message:
INTEGER*4 %FILL /1980/
o Initial values can be supplied in field declaration statements.
These initial values are supplied for all records that are
declared using this structure. Fields not initialized will
have undefined values when variables are declared by means of
RECORD statements. Unnamed fields cannot be initialized; they
are always undefined.
o Field names must always be given explicit data types. The
IMPLICIT statement has no effect on statements within a
structure declaration.
o All Fortran data types are allowed in field declarations.
o Any required array dimensions must be specified in the field
declaration statements. DIMENSION statements cannot be used to
define field names.
o Adjustable or assumed sized arrays and passed-length CHARACTER
declarations are not allowed in field declarations.
o Field names within the same declaration level must be unique,
but an inner structure declaration (substructure declaration)
can include field names used in an outer structure declaration
without conflict.
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2 - Substructure declarations
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A field within a structure can itself be a structured item composed
of other fields, other structures, or both. You can declare a
substructure in two ways:
o By nesting structure declarations within other structure or
union declarations (with the limitation that you cannot refer
to a structure inside itself at any level of nesting).
One or more field names must be defined in the STRUCTURE
statement for the substructure because all fields in a
structure must be named. In this case, the substructure is
being used as a field within a structure or union.
Field names within the same declaration nesting level must be
unique, but an inner structure declaration can include field
names used in an outer structure declaration without conflict.
%FILL can be specified in place of a field name to leave space
in a record for purposes such as alignment.
o By using a RECORD statement that specifies another previously
defined record structure, thereby including it in the structure
being declared.
A union declaration is a multistatement declaration defining a data
area that can be shared intermittently during program execution by
one or more fields or groups of fields. A union declaration must
be within a structure declaration. A union declaration is
initiated by a UNION statement and terminated by an END UNION
statement. Enclosed within these statements are two or more map
declarations, initiated and terminated by MAP and END MAP
statements. Each unique field or group of fields is defined by a
separate map declaration.
A union declaration takes the following form:
UNION
mdcl
[mdcl]
...
[mdcl]
END UNION
Where "mdcl" represents:
MAP
fdcl
[fdcl]
...
[fdcl]
END MAP
fdcl Is any declaration or combination of declarations
of substructures, unions, or type declarations.
As with normal Fortran type declarations, data can be initialized
in field declaration statements in union declarations. However, if
fields within multiple map declarations in a single union are
initialized, the data declarations are initialized in the order in
which the statements appear. As a result, only the final
initialization takes effect and all the preceding initializations
are overwritten.
The size of the shared area established for a union declaration is
the size of the largest map defined for that union. The size of a
map is the sum of the sizes of the fields declared within it.
As the variables or arrays declared in map fields in a union
declaration are assigned values during program execution, the
values are established in a record in the field shared with other
map fields in the union. The fields of only one of the map
declarations are defined within a union at any given point in the
execution of a program. However, if you overlay one variable with
another smaller variable, that portion of the initial variable is
retained that is not overlaid. Depending on the application, the
retained portion of an overlaid variable may or may not contain
meaningful data and can be used at a later point in the program.
Manipulating data using union declarations is similar to the effect
of using EQUIVALENCE statements. The difference is that data
entities specified within EQUIVALENCE statements are concurrently
associated with a common storage location and the data residing
there; with union declarations you can use one discrete storage
location to alternately contain a variety of fields (arrays or
variables).
With union declarations, only one map declaration within a union
declaration can be associated at any point in time with the storage
location that they share. Whenever a field within another map
declaration in the same union declaration is referenced in your
program, the fields in the prior map declaration become undefined
and are succeeded by the fields in the map declaration containing
the newly referenced field.
In the following example, the structure WORDS_LONG is defined.
This structure contains a union declaration defining two map
fields. The first map field consists of three INTEGER*2 variables
(WORD_0, WORD_1, and WORD_2), and the second, an INTEGER*4
variable, LONG:
STRUCTURE /WORDS_LONG/
UNION
MAP
INTEGER*2 WORD_0, WORD_1, WORD_2
END MAP
MAP
INTEGER*4 LONG
END MAP
END UNION
END STRUCTURE
PARAMETER statements: PARAMETER statements can appear in a
structure declaration, but cannot be given a data type within the
declaration block. Consider the following:
STRUCTURE /ABC/
INTEGER*4 P
PARAMETER (P=4)
REAL*4 F
END STRUCTURE
REAL*4 A(P)
In this example, the INTEGER*4 statement does not provide the data
type for PARAMETER constant P, but instead declares a record field
P in structure ABC. The following PARAMETER statement declares a
new, different symbol that is given the implicit data type for
identifiers beginning with the letter P.
Type declarations for PARAMETER symbolic names must precede the
PARAMETER statement and be outside of a STRUCTURE declaration, as
follows:
INTEGER*4 P
STRUCTURE /ABC/
PARAMETER (P=4)
REAL*4 F
END STRUCTURE
REAL*4 A(P)
For more information on PARAMETER statements, see STATEMENTS
PARAMETER in this Help file.
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