A subroutine subprogram is invoked in a CALL statement or by a defined assignment statement, and does not return a particular value.
The SUBROUTINE statement is the initial statement of a subroutine subprogram. It takes the following form:
|RECURSIVE||Permits direct recursion to occur (see Section 22.214.171.124).|
|PURE||Asserts that the procedure has no side effects (see Section 126.96.36.199).|
|ELEMENTAL||Restricted form of pure procedure that acts on one array element at a time (see Section 188.8.131.52).|
You can also specify one of the High Performance Fortran (HPF) keywords (see Section 8.2). These keywords are functional only in parallel programs on Tru64 UNIX systems.
Rules and Behavior
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.
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:
(An alternate return is an obsolescent feature in Fortran 90 and Fortran 95.)
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