SUBROUTINE

Statement: The initial statement of a subroutine subprogram. A subroutine subprogram is invoked in a CALL statement or by a defined assignment statement, and does not return a particular value.

Syntax

[keyword] SUBROUTINE name [([d-arg-list]) [lang-binding]]
     [specification-part]
     [execution-part]
[CONTAINS
     internal-subprogram-part]
END [SUBROUTINE [name]]

keyword
Is one of the following:

Keyword	Meaning
RECURSIVE	Permits direct recursion to occur.
PURE	Asserts that the procedure has no side effects.
ELEMENTAL	Restricted form of pure procedure that acts on one array element at a time.

name
Is the name of the subroutine.

d-arg-list
Is a list of one or more dummy arguments or alternate return specifiers (*).

lang-binding
Takes the following form:

BIND (C [, NAME=ext-name])

ext-name
Is a character scalar initialization expression that can be used to construct the external name.

specification-part
Is one or more specification statements, except for the following:

• INTENT (or its equivalent attribute)
• OPTIONAL (or its equivalent attribute)
• PUBLIC and PRIVATE (or their equivalent attributes)

An automatic object must not appear in a specification statement. If a SAVE statement is specified, it has no effect.

execution-part
Is one or more executable constructs or statements, except for ENTRY or RETURN statements.

internal-subprogram-part
Is one or more internal subprograms (defining internal procedures). The internal-subprogram-part is preceded by a CONTAINS statement.

Description

A subroutine is invoked by a CALL statement or defined assignment. When a subroutine is invoked, dummy arguments (if present) become associated with the corresponding actual arguments specified in the call.

Execution begins with the first executable construct or statement following the SUBROUTINE statement. Control returns to the calling program unit once the END statement (or a RETURN statement) is executed.

A subroutine subprogram cannot contain a FUNCTION statement, a BLOCK DATA statement, a PROGRAM statement, or another SUBROUTINE statement. ENTRY statements can be included to provide multiple entry points to the subprogram.

You need an interface block for a subroutine when:

• Calling arguments use argument keywords.
• Some arguments are optional.
• A dummy argument is an assumed-shape array, a pointer, or a target.
• The subroutine extends intrinsic assignment.
• The subroutine can be referenced by a generic name.
• The subroutine is in a dynamic-link library.

If the subroutine is in a DLL and is called from your program, use the option DLLEXPORT or DLLIMPORT, which you can specify with the ATTRIBUTES directive.

Note that if you specify lang-binding, you have to use the parentheses even if there are no arguments. For example, without lang-binding you can specify SUBROUTINE F but with lang-binding you have to specify SUBROUTINE F( ) BIND (C).

See Also

FUNCTION, INTERFACE, PURE, ELEMENTAL, CALL, RETURN, ENTRY, Argument Association, Program Units and Procedures, General Rules for Function and Subroutine Subprograms, Obsolescent and Deleted Language Features

Examples

The following example shows a subroutine:

Main Program          Subroutine
CALL HELLO_WORLD      SUBROUTINE HELLO_WORLD
...                   PRINT *, "Hello World"
END                   END SUBROUTINE

The following example uses alternate return specifiers to determine where control transfers on completion of the subroutine:

Main Program                          Subroutine
    CALL CHECK(A,B,*10,*20,C)             SUBROUTINE CHECK(X,Y,*,*,Q)
    TYPE *, 'VALUE LESS THAN ZERO'        ...
    GO TO 30                          50  IF (Z)  60,70,80
10  TYPE*, 'VALUE EQUALS ZERO'        60  RETURN
    GO TO 30                          70  RETURN 1
20  TYPE*, 'VALUE MORE THAN ZERO'     80  RETURN 2
30  CONTINUE                              END
    ...

The SUBROUTINE statement argument list contains two dummy alternate return arguments corresponding to the actual arguments *10 and *20 in the CALL statement argument list.

The value of Z determines the return, as follows:

• If Z < zero, a normal return occurs and control is transferred to the first executable statement following CALL CHECK in the main program.
  
  
• If Z = = zero, the return is to statement label 10 in the main program.
  
  
• If Z > zero, the return is to statement label 20 in the main program.

(An alternate return is an obsolescent feature in Fortran 90 and Fortran 95.)

The following shows another example:

     SUBROUTINE GetNum (num, unit)
     INTEGER num, unit
 10  READ (unit, '(I10)', ERR = 10) num
     END