1.2.1. Fortran 95 Language Standard

Fortran 95 continues the evolutionary model introduced in Fortran 90 by deleting several of the features marked as obsolescent in Fortran 90 and identifying a few new obsolescent features. For information on these features, see the Fortran Language Reference Manual, Volume 3.

Fortran 95 is a relatively minor evolution of standard Fortran, with the emphasis in this revision being upon correcting defects in the Fortran 90 standard. This new standard also provides interpretation for a number of questions that have arisen concerning Fortran 90 semantics and syntax. For example, the Fortran 95 SIGN(3i) intrinsic function behaves differently from the Fortran 90 SIGN(3i) function if the second argument is negative real zero.

In addition to corrections and clarifications, Fortran 95 contains several extensions to Fortran 90. The major extensions are as follows:

• The FORALL statement and construct.

• PURE and ELEMENTAL procedures.

• Pointer initialization and structure default initialization.

• Additional intrinsic procedures. A Fortran 90 program may have a different interpretation under the Fortran 95 standard if it invokes an external procedure that has the same name as one of the new standard intrinsic procedures unless that procedure is specified in an EXTERNAL statement or an interface body.

The Fortran 95 standard specifies the following information:

• Syntax of Fortran statements and forms for Fortran programs

• Semantics of Fortran statements and Fortran programs

• Specifications for correct input data

• Appearance of standard output data

The standard does not specify the following information:

• The way in which each Fortran compiler is written

• Operating system facilities defining the computing system

• Methods used to transfer data to and from peripheral storage devices and the nature of the peripheral devices

• Behavior of vendor extensions

• Size and complexity of a Fortran program and its data

• Hardware or firmware used to run the program

• The way values are represented and the way numeric values are computed

• Physical representation of data

• Characteristics of tapes, disks, and various storage media

1.2.2. Program Conformance

A program conforms to the standard if all the statements are syntactically correct, execution of the program causes no violations of the standard (such as dividing by zero), and all the input data is in the correct form.

The compiler extensions to the Fortran standard appear in notes throughout the text.

1.2.3. Processor Conformance

In the Fortran standard, the term processor means the combination of a Fortran compiler and the computing system that executes the code. A processor conforms to the standard if it compiles and executes programs that conform to the standard, provided that the Fortran program is not too large or complex for the computer system in question.

Options on the Cray Fortran command line can direct the compiler to flag nonstandard usage. For more information on the command line, see the Cray Fortran Compiler Commands and Directives Reference Manual or the ftn(1) man page. When the option is in effect, the compiler prints messages for extensions to the standard that are used in the program. As required by the standard, the compiler also flags the following items and provides the reason that the item is being flagged:

• Obsolescent features

• Deleted features

• Kind type parameters not supported

• Violations of any syntax rules and the accompanying constraints

• Characters not permitted by the processor

• Illegal source form

• Violations of the scope rules for names, labels, operators, and assignment symbols

The Cray Fortran Compiler includes extensions to the Fortran standard. Because the compiler processes programs according to the standard, it is considered to be a standard-conforming processor. When the option to note deviations from the Fortran standard is in effect, extensions to the standard are flagged with ANSI messages when detected at compile time.

1.2.4. Portability

One of the main purposes of a standard is to describe how to write portable programs. However, some things are standard-conforming but not portable; for example, a program that computes a very large number like 10²⁵⁰. Certain computing systems will not accommodate a number this large. Such a number could be part of a standard-conforming program, but it might not run on all systems; therefore, it might not be portable. Another example is a program that uses a deeper nesting of control constructs than is allowed by a particular compiler.