SUBROUTINE
Purpose
The SUBROUTINE statement is the first statement of a subroutine subprogram.
Syntax
.------------. V | >>---+--------+-+--SUBROUTINE--name-----------------------------> '-prefix-' >--+-------------------------------+----------------------------> '-(--+---------------------+--)-' '-dummy_argument_list-' >--+----------------------------------------------------+------>< | (1) | '-BIND------(--C--+-----------------------------+--)-' '-, -NAME-- = --binding_label-'
- Fortran 2003
- prefix
- is one of the following:
- ELEMENTAL
- IMPURE
- MODULE
- PURE
- RECURSIVE Note: type_spec is not permitted as a prefix in a subroutine.
- name
- The name of the subroutine subprogram.
binding_label A scalar character constant expression.
Rules
At most one of each kind of prefix can be specified. You cannot specify both the RECURSIVE and ELEMENTAL prefix specifiers. You cannot specify both the PURE and IMPURE prefix specifiers.
The subroutine name cannot appear in any other statement in the scope of the subroutine, unless recursion has been specified.
- 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.
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.
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.
The BIND keyword implicitly or explicitly defines a binding label by which a procedure is accessed from the C programming language. A dummy argument cannot be zero-sized. A dummy argument for a procedure with the BIND attribute must have interoperable types and type parameters, and cannot have the ALLOCATABLE or POINTER attribute.
If the SUBROUTINE statement appears as part of an interface body that describes a dummy procedure, the NAME= specifier must not appear. An elemental procedure cannot have the BIND attribute.
- When you specify the MODULE prefix specifier for the SUBROUTINE statement of a module subprogram, the module subprogram is a separate module procedure.
- When you specify the MODULE prefix specifier for the SUBROUTINE statement of a nonabstract interface body, the interface body is a module procedure interface body.
The BIND attribute with the NAME= specifier is not allowed on an internal procedure.
Example 1
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
Example 2 (Fortran 2008)
MODULE m
! The MODULE prefix specifier is specified for the SUBROUTINE
! statement of a module procedure interface body.
INTERFACE
MODULE SUBROUTINE sub(arg)
INTEGER :: arg
END SUBROUTINE
END INTERFACE
END MODULE
SUBMODULE (m) n
CONTAINS
! The MODULE prefix specifier is specified for the SUBROUTINE
! statement of a separate module procedure.
MODULE SUBROUTINE sub(arg)
INTEGER :: arg
arg = 1
END SUBROUTINE
END SUBMODULE
Related information
- Function and subroutine subprograms
- Dummy arguments
- Recursion
- CALL
- ENTRY
- Statement Function
- BIND (Fortran 2003)
- RETURN
- Definition status of variables
- Pure procedures
- -qrecur option in the XL Fortran Compiler Reference
- Interoperability of procedures
- Modules
- Submodules (Fortran 2008)
- Separate module procedures (Fortran 2008)