Language Reference

SUBROUTINE

Purpose

The SUBROUTINE statement is the first statement of a subroutine subprogram.

Format

   .------------.
   V            |
>>---+--------+-+--SUBROUTINE--name----------------------------->
     '-prefix-'
 
>--+-------------------------------+---------------------------><
   '-(--+---------------------+--)-'
        '-dummy_argument_list-'
 
 

prefix

is one of the following:

ELEMENTAL

PURE

RECURSIVE

Note:

type_spec is not permitted as a prefix in a subroutine.

name

is the name of the subroutine subprogram

Rules

At most one of each kind of prefix can be specified.

The subroutine name cannot appear in any other statement in the scope of the subroutine, unless recursion has been specified.

The RECURSIVE keyword must be specified if, directly or indirectly,

• The subroutine invokes itself.
• The subroutine invokes a procedure defined by an ENTRY statement in the same subprogram.
• An entry procedure in the same subprogram invokes itself.
• An entry procedure in the same subprogram invokes another entry procedure in the same subprogram.
• An entry procedure in the same subprogram invokes the subprogram defined by the SUBROUTINE statement.

If the RECURSIVE keyword is specified, the procedure interface is explicit within the subprogram.

+---------------------------------Fortran 95---------------------------------+

Using the PURE or ELEMENTAL prefix indicates that the subroutine may be invoked by the compiler in any order as it is free of side effects. For elemental procedures, the keyword ELEMENTAL must be specified. If the ELEMENTAL keyword is specified, the RECURSIVE keyword cannot be specified.

+-----------------------------End of Fortran 95------------------------------+

+-------------------------------IBM Extension--------------------------------+

XL Fortran allows three exceptions to PURE subroutine:

• Dummy arguments with an intent of OUT or INOUT can be modified.
• You can modify the association status of dummy arguments with the POINTER attribute, unless you have specified the pointer dummy argument as INTENT(IN).
• The value of dummy arguments with the POINTER attribute can be modified.

You can also call external procedures recursively when you specify the -qrecur compiler option, although XL Fortran disregards this option if the SUBROUTINE statement specifies the RECURSIVE keyword.

+----------------------------End of IBM Extension----------------------------+

Examples

RECURSIVE SUBROUTINE SUB(X,Y)
  INTEGER X,Y
  IF (X.LT.Y) THEN
    RETURN
  ELSE
    CALL SUB(X,Y+1)
  END IF
END SUBROUTINE SUB

Related Information

• Function and Subroutine Subprograms
• Dummy Arguments
• Recursion
• CALL
• ENTRY
• RETURN
• Definition Status of Variables
• Pure Procedures
• "-qrecur Option" in the User's Guide