8. The GNU Fortran Language

GNU Fortran supports a variety of extensions to, and dialects of, the Fortran language. Its primary base is the ANSI FORTRAN 77 standard, currently available on the network at http://www.fortran.com/fortran/F77_std/rjcnf0001.html or as monolithic text at http://www.fortran.com/fortran/F77_std/f77_std.html. It offers some extensions that are popular among users of UNIX f77 and f2c compilers, some that are popular among users of other compilers (such as Digital products), some that are popular among users of the newer Fortran 90 standard, and some that are introduced by GNU Fortran.

(If you need a text on Fortran, a few freely available electronic references have pointers from http://www.fortran.com/fortran/Books/. There is a `cooperative net project', User Notes on Fortran Programming at ftp://vms.huji.ac.il/fortran/ and mirrors elsewhere; some of this material might not apply specifically to g77.)

Part of what defines a particular implementation of a Fortran system, such as g77, is the particular characteristics of how it supports types, constants, and so on. Much of this is left up to the implementation by the various Fortran standards and accepted practice in the industry.

The GNU Fortran language is described below. Much of the material is organized along the same lines as the ANSI FORTRAN 77 standard itself.

See section 9. Other Dialects, for information on features g77 supports that are not part of the GNU Fortran language.

Note: This portion of the documentation definitely needs a lot of work!

Relationship to the ANSI FORTRAN 77 standard:

8.1 Direction of Language Development		Where GNU Fortran is headed.
8.2 ANSI FORTRAN 77 Standard Support		Degree of support for the standard.

Extensions to the ANSI FORTRAN 77 standard:

8.3 Conformance
8.4 Notation Used in This Chapter
8.5 Fortran Terms and Concepts
8.6 Characters, Lines, and Execution Sequence
8.7 Data Types and Constants
8.8 Expressions
8.9 Specification Statements
8.10 Control Statements
8.11 Functions and Subroutines
8.12 Scope and Classes of Symbolic Names
8.13 I/O
8.14 Fortran 90 Features

8.1 Direction of Language Development

The purpose of the following description of the GNU Fortran language is to promote wide portability of GNU Fortran programs.

GNU Fortran is an evolving language, due to the fact that g77 itself is in beta test. Some current features of the language might later be redefined as dialects of Fortran supported by g77 when better ways to express these features are added to g77, for example. Such features would still be supported by g77, but would be available only when one or more command-line options were used.

The GNU Fortran language is distinct from the GNU Fortran compilation system (g77).

For example, g77 supports various dialects of Fortran--in a sense, these are languages other than GNU Fortran--though its primary purpose is to support the GNU Fortran language, which also is described in its documentation and by its implementation.

On the other hand, non-GNU compilers might offer support for the GNU Fortran language, and are encouraged to do so.

Currently, the GNU Fortran language is a fairly fuzzy object. It represents something of a cross between what g77 accepts when compiling using the prevailing defaults and what this document describes as being part of the language.

Future versions of g77 are expected to clarify the definition of the language in the documentation. Often, this will mean adding new features to the language, in the form of both new documentation and new support in g77. However, it might occasionally mean removing a feature from the language itself to "dialect" status. In such a case, the documentation would be adjusted to reflect the change, and g77 itself would likely be changed to require one or more command-line options to continue supporting the feature.

The development of the GNU Fortran language is intended to strike a balance between:

• Serving as a mostly-upwards-compatible language from the de facto UNIX Fortran dialect as supported by f77.

• Offering new, well-designed language features. Attributes of such features include not making existing code any harder to read (for those who might be unaware that the new features are not in use) and not making state-of-the-art compilers take longer to issue diagnostics, among others.

• Supporting existing, well-written code without gratuitously rejecting non-standard constructs, regardless of the origin of the code (its dialect).

• Offering default behavior and command-line options to reduce and, where reasonable, eliminate the need for programmers to make any modifications to code that already works in existing production environments.

• Diagnosing constructs that have different meanings in different systems, languages, and dialects, while offering clear, less ambiguous ways to express each of the different meanings so programmers can change their code appropriately.

One of the biggest practical challenges for the developers of the GNU Fortran language is meeting the sometimes contradictory demands of the above items.

For example, a feature might be widely used in one popular environment, but the exact same code that utilizes that feature might not work as expected--perhaps it might mean something entirely different--in another popular environment.

Traditionally, Fortran compilers--even portable ones--have solved this problem by simply offering the appropriate feature to users of the respective systems. This approach treats users of various Fortran systems and dialects as remote "islands", or camps, of programmers, and assume that these camps rarely come into contact with each other (or, especially, with each other's code).

Project GNU takes a radically different approach to software and language design, in that it assumes that users of GNU software do not necessarily care what kind of underlying system they are using, regardless of whether they are using software (at the user-interface level) or writing it (for example, writing Fortran or C code).

As such, GNU users rarely need consider just what kind of underlying hardware (or, in many cases, operating system) they are using at any particular time. They can use and write software designed for a general-purpose, widely portable, heterogenous environment--the GNU environment.

In line with this philosophy, GNU Fortran must evolve into a product that is widely ported and portable not only in the sense that it can be successfully built, installed, and run by users, but in the larger sense that its users can use it in the same way, and expect largely the same behaviors from it, regardless of the kind of system they are using at any particular time.

This approach constrains the solutions g77 can use to resolve conflicts between various camps of Fortran users. If these two camps disagree about what a particular construct should mean, g77 cannot simply be changed to treat that particular construct as having one meaning without comment (such as a warning), lest the users expecting it to have the other meaning are unpleasantly surprised that their code misbehaves when executed.

The use of the ASCII backslash character in character constants is an excellent (and still somewhat unresolved) example of this kind of controversy. See section 16.5.1 Backslash in Constants. Other examples are likely to arise in the future, as g77 developers strive to improve its ability to accept an ever-wider variety of existing Fortran code without requiring significant modifications to said code.

Development of GNU Fortran is further constrained by the desire to avoid requiring programmers to change their code. This is important because it allows programmers, administrators, and others to more faithfully evaluate and validate g77 (as an overall product and as new versions are distributed) without having to support multiple versions of their programs so that they continue to work the same way on their existing systems (non-GNU perhaps, but possibly also earlier versions of g77).

8.2 ANSI FORTRAN 77 Standard Support

GNU Fortran supports ANSI FORTRAN 77 with the following caveats. In summary, the only ANSI FORTRAN 77 features g77 doesn't support are those that are probably rarely used in actual code, some of which are explicitly disallowed by the Fortran 90 standard.

8.2.1 No Passing External Assumed-length		CHAR*(*) CFUNC restriction.
8.2.2 No Passing Dummy Assumed-length		CHAR*(*) CFUNC restriction.
8.2.3 No Pathological Implied-DO		No `((..., I=...), I=...)'.
8.2.4 No Useless Implied-DO		No `(A, I=1, 1)'.

8.2.1 No Passing External Assumed-length

g77 disallows passing of an external procedure as an actual argument if the procedure's type is declared CHARACTER*(*). For example:

CHARACTER*(*) CFUNC EXTERNAL CFUNC CALL FOO(CFUNC) END

It isn't clear whether the standard considers this conforming.

8.2.2 No Passing Dummy Assumed-length

g77 disallows passing of a dummy procedure as an actual argument if the procedure's type is declared CHARACTER*(*).

SUBROUTINE BAR(CFUNC) CHARACTER*(*) CFUNC EXTERNAL CFUNC CALL FOO(CFUNC) END

It isn't clear whether the standard considers this conforming.

8.2.3 No Pathological Implied-DO

The DO variable for an implied-DO construct in a DATA statement may not be used as the DO variable for an outer implied-DO construct. For example, this fragment is disallowed by g77:

DATA ((A(I, I), I= 1, 10), I= 1, 10) /.../

This also is disallowed by Fortran 90, as it offers no additional capabilities and would have a variety of possible meanings.

Note that it is very unlikely that any production Fortran code tries to use this unsupported construct.

8.2.4 No Useless Implied-DO

An array element initializer in an implied-DO construct in a DATA statement must contain at least one reference to the DO variables of each outer implied-DO construct. For example, this fragment is disallowed by g77:

DATA (A, I= 1, 1) /1./

This also is disallowed by Fortran 90, as FORTRAN 77's more permissive requirements offer no additional capabilities. However, g77 doesn't necessarily diagnose all cases where this requirement is not met.

Note that it is very unlikely that any production Fortran code tries to use this unsupported construct.

8.3 Conformance

(The following information augments or overrides the information in Section 1.4 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 1 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

The definition of the GNU Fortran language is akin to that of the ANSI FORTRAN 77 language in that it does not generally require conforming implementations to diagnose cases where programs do not conform to the language.

However, g77 as a compiler is being developed in a way that is intended to enable it to diagnose such cases in an easy-to-understand manner.

A program that conforms to the GNU Fortran language should, when compiled, linked, and executed using a properly installed g77 system, perform as described by the GNU Fortran language definition. Reasons for different behavior include, among others:

• Use of resources (memory--heap, stack, and so on; disk space; CPU time; etc.) exceeds those of the system.

• Range and/or precision of calculations required by the program exceeds that of the system.

• Excessive reliance on behaviors that are system-dependent (non-portable Fortran code).

• Bugs in the program.

• Bug in g77.

• Bugs in the system.

Despite these "loopholes", the availability of a clear specification of the language of programs submitted to g77, as this document is intended to provide, is considered an important aspect of providing a robust, clean, predictable Fortran implementation.

The definition of the GNU Fortran language, while having no special legal status, can therefore be viewed as a sort of contract, or agreement. This agreement says, in essence, "if you write a program in this language, and run it in an environment (such as a g77 system) that supports this language, the program should behave in a largely predictable way".

8.4 Notation Used in This Chapter

(The following information augments or overrides the information in Section 1.5 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 1 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

In this chapter, "must" denotes a requirement, "may" denotes permission, and "must not" and "may not" denote prohibition. Terms such as "might", "should", and "can" generally add little or nothing in the way of weight to the GNU Fortran language itself, but are used to explain or illustrate the language.

For example:

``The FROBNITZ statement must precede all executable statements in a program unit, and may not specify any dummy arguments. It may specify local or common variables and arrays. Its use should be limited to portions of the program designed to be non-portable and system-specific, because it might cause the containing program unit to behave quite differently on different systems.''

Insofar as the GNU Fortran language is specified, the requirements and permissions denoted by the above sample statement are limited to the placement of the statement and the kinds of things it may specify. The rest of the statement--the content regarding non-portable portions of the program and the differing behavior of program units containing the FROBNITZ statement--does not pertain the GNU Fortran language itself. That content offers advice and warnings about the FROBNITZ statement.

Remember: The GNU Fortran language definition specifies both what constitutes a valid GNU Fortran program and how, given such a program, a valid GNU Fortran implementation is to interpret that program.

It is not incumbent upon a valid GNU Fortran implementation to behave in any particular way, any consistent way, or any predictable way when it is asked to interpret input that is not a valid GNU Fortran program.

Such input is said to have undefined behavior when interpreted by a valid GNU Fortran implementation, though an implementation may choose to specify behaviors for some cases of inputs that are not valid GNU Fortran programs.

Other notation used herein is that of the GNU texinfo format, which is used to generate printed hardcopy, on-line hypertext (Info), and on-line HTML versions, all from a single source document. This notation is used as follows:

• Keywords defined by the GNU Fortran language are shown in uppercase, as in: COMMON, INTEGER, and BLOCK DATA.

  Note that, in practice, many Fortran programs are written in lowercase--uppercase is used in this manual as a means to readily distinguish keywords and sample Fortran-related text from the prose in this document.

• Portions of actual sample program, input, or output text look like this: `Actual program text'.

  Generally, uppercase is used for all Fortran-specific and Fortran-related text, though this does not always include literal text within Fortran code.

  For example: `PRINT *, 'My name is Bob''.

• A metasyntactic variable--that is, a name used in this document to serve as a placeholder for whatever text is used by the user or programmer--appears as shown in the following example:

  "The INTEGER ivar statement specifies that ivar is a variable or array of type INTEGER."

  In the above example, any valid text may be substituted for the metasyntactic variable ivar to make the statement apply to a specific instance, as long as the same text is substituted for both occurrences of ivar.

• Ellipses ("...") are used to indicate further text that is either unimportant or expanded upon further, elsewhere.

• Names of data types are in the style of Fortran 90, in most cases.

  See section 8.7.1.3 Kind Notation, for information on the relationship between Fortran 90 nomenclature (such as INTEGER(KIND=1)) and the more traditional, less portably concise nomenclature (such as INTEGER*4).

8.5 Fortran Terms and Concepts

(The following information augments or overrides the information in Chapter 2 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 2 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.5.1 Syntactic Items
8.5.2 Statements, Comments, and Lines
8.5.3 Scope of Symbolic Names and Statement Labels

8.5.1 Syntactic Items

(Corresponds to Section 2.2 of ANSI X3.9-1978 FORTRAN 77.)

In GNU Fortran, a symbolic name is at least one character long, and has no arbitrary upper limit on length. However, names of entities requiring external linkage (such as external functions, external subroutines, and COMMON areas) might be restricted to some arbitrary length by the system. Such a restriction is no more constrained than that of one through six characters.

Underscores (`_') are accepted in symbol names after the first character (which must be a letter).

8.5.2 Statements, Comments, and Lines

(Corresponds to Section 2.3 of ANSI X3.9-1978 FORTRAN 77.)

Use of an exclamation point (`!') to begin a trailing comment (a comment that extends to the end of the same source line) is permitted under the following conditions:

• The exclamation point does not appear in column 6. Otherwise, it is treated as an indicator of a continuation line.

• The exclamation point appears outside a character or Hollerith constant. Otherwise, the exclamation point is considered part of the constant.

• The exclamation point appears to the left of any other possible trailing comment. That is, a trailing comment may contain exclamation points in their commentary text.

Use of a semicolon (`;') as a statement separator is permitted under the following conditions:

• The semicolon appears outside a character or Hollerith constant. Otherwise, the semicolon is considered part of the constant.

• The semicolon appears to the left of a trailing comment. Otherwise, the semicolon is considered part of that comment.

• Neither a logical IF statement nor a non-construct WHERE statement (a Fortran 90 feature) may be followed (in the same, possibly continued, line) by a semicolon used as a statement separator.

  This restriction avoids the confusion that can result when reading a line such as:

  IF (VALIDP) CALL FOO; CALL BAR

  Some readers might think the `CALL BAR' is executed only if `VALIDP' is .TRUE., while others might assume its execution is unconditional.

  (At present, g77 does not diagnose code that violates this restriction.)

8.5.3 Scope of Symbolic Names and Statement Labels

(Corresponds to Section 2.9 of ANSI X3.9-1978 FORTRAN 77.)

Included in the list of entities that have a scope of a program unit are construct names (a Fortran 90 feature). See section 8.10.3 Construct Names, for more information.

8.6 Characters, Lines, and Execution Sequence

(The following information augments or overrides the information in Chapter 3 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 3 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.6.1 GNU Fortran Character Set
8.6.2 Lines
8.6.3 Continuation Line
8.6.4 Statements
8.6.5 Statement Labels
8.6.6 Order of Statements and Lines
8.6.7 Including Source Text
8.6.8 Cpp-style directives

8.6.1 GNU Fortran Character Set

(Corresponds to Section 3.1 of ANSI X3.9-1978 FORTRAN 77.)

Letters include uppercase letters (the twenty-six characters of the English alphabet) and lowercase letters (their lowercase equivalent). Generally, lowercase letters may be used in place of uppercase letters, though in character and Hollerith constants, they are distinct.

Special characters include:

• Semicolon (`;')

• Exclamation point (`!')

• Double quote (`"')

• Backslash (`\')

• Question mark (`?')

• Hash mark (`#')

• Ampersand (`&')

• Percent sign (`%')

• Underscore (`_')

• Open angle (`<')

• Close angle (`>')

• The FORTRAN 77 special characters (SPC, `=', `+', `-', `*', `/', `(', `)', `,', `.', `$', `'', and `:')

Note that this document refers to SPC as space, while X3.9-1978 FORTRAN 77 refers to it as blank.

8.6.2 Lines

(Corresponds to Section 3.2 of ANSI X3.9-1978 FORTRAN 77.)

The way a Fortran compiler views source files depends entirely on the implementation choices made for the compiler, since those choices are explicitly left to the implementation by the published Fortran standards.

The GNU Fortran language mandates a view applicable to UNIX-like text files--files that are made up of an arbitrary number of lines, each with an arbitrary number of characters (sometimes called stream-based files).

This view does not apply to types of files that are specified as having a particular number of characters on every single line (sometimes referred to as record-based files).

Because a "line in a program unit is a sequence of 72 characters", to quote X3.9-1978, the GNU Fortran language specifies that a stream-based text file is translated to GNU Fortran lines as follows:

• A newline in the file is the character that represents the end of a line of text to the underlying system. For example, on ASCII-based systems, a newline is the NL character, which has ASCII value 10 (decimal).

• Each newline in the file serves to end the line of text that precedes it (and that does not contain a newline).

• The end-of-file marker (EOF) also serves to end the line of text that precedes it (and that does not contain a newline).

• Any line of text that is shorter than 72 characters is padded to that length with spaces (called "blanks" in the standard).

• Any line of text that is longer than 72 characters is truncated to that length, but the truncated remainder must consist entirely of spaces.

• Characters other than newline and the GNU Fortran character set are invalid.

For the purposes of the remainder of this description of the GNU Fortran language, the translation described above has already taken place, unless otherwise specified.

The result of the above translation is that the source file appears, in terms of the remainder of this description of the GNU Fortran language, as if it had an arbitrary number of 72-character lines, each character being among the GNU Fortran character set.

For example, if the source file itself has two newlines in a row, the second newline becomes, after the above translation, a single line containing 72 spaces.

8.6.3 Continuation Line

(Corresponds to Section 3.2.3 of ANSI X3.9-1978 FORTRAN 77.)

A continuation line is any line that both

• Contains a continuation character, and

• Contains only spaces in columns 1 through 5

A continuation character is any character of the GNU Fortran character set other than space (SPC) or zero (`0') in column 6, or a digit (`0' through `9') in column 7 through 72 of a line that has only spaces to the left of that digit.

The continuation character is ignored as far as the content of the statement is concerned.

The GNU Fortran language places no limit on the number of continuation lines in a statement. In practice, the limit depends on a variety of factors, such as available memory, statement content, and so on, but no GNU Fortran system may impose an arbitrary limit.

8.6.4 Statements

(Corresponds to Section 3.3 of ANSI X3.9-1978 FORTRAN 77.)

Statements may be written using an arbitrary number of continuation lines.

Statements may be separated using the semicolon (`;'), except that the logical IF and non-construct WHERE statements may not be separated from subsequent statements using only a semicolon as statement separator.

The END PROGRAM, END SUBROUTINE, END FUNCTION, and END BLOCK DATA statements are alternatives to the END statement. These alternatives may be written as normal statements--they are not subject to the restrictions of the END statement.

However, no statement other than END may have an initial line that appears to be an END statement--even END PROGRAM, for example, must not be written as:

END &PROGRAM

8.6.5 Statement Labels

(Corresponds to Section 3.4 of ANSI X3.9-1978 FORTRAN 77.)

A statement separated from its predecessor via a semicolon may be labeled as follows:

• The semicolon is followed by the label for the statement, which in turn follows the label.

• The label must be no more than five digits in length.

• The first digit of the label for the statement is not the first non-space character on a line. Otherwise, that character is treated as a continuation character.

A statement may have only one label defined for it.

8.6.6 Order of Statements and Lines

(Corresponds to Section 3.5 of ANSI X3.9-1978 FORTRAN 77.)

Generally, DATA statements may precede executable statements. However, specification statements pertaining to any entities initialized by a DATA statement must precede that DATA statement. For example, after `DATA I/1/', `INTEGER I' is not permitted, but `INTEGER J' is permitted.

The last line of a program unit may be an END statement, or may be:

• An END PROGRAM statement, if the program unit is a main program.

• An END SUBROUTINE statement, if the program unit is a subroutine.

• An END FUNCTION statement, if the program unit is a function.

• An END BLOCK DATA statement, if the program unit is a block data.

8.6.7 Including Source Text

Additional source text may be included in the processing of the source file via the INCLUDE directive:

INCLUDE filename

The source text to be included is identified by filename, which is a literal GNU Fortran character constant. The meaning and interpretation of filename depends on the implementation, but typically is a filename.

(g77 treats it as a filename that it searches for in the current directory and/or directories specified via the `-I' command-line option.)

The effect of the INCLUDE directive is as if the included text directly replaced the directive in the source file prior to interpretation of the program. Included text may itself use INCLUDE. The depth of nested INCLUDE references depends on the implementation, but typically is a positive integer.

This virtual replacement treats the statements and INCLUDE directives in the included text as syntactically distinct from those in the including text.

Therefore, the first non-comment line of the included text must not be a continuation line. The included text must therefore have, after the non-comment lines, either an initial line (statement), an INCLUDE directive, or nothing (the end of the included text).

Similarly, the including text may end the INCLUDE directive with a semicolon or the end of the line, but it cannot follow an INCLUDE directive at the end of its line with a continuation line. Thus, the last statement in an included text may not be continued.

Any statements between two INCLUDE directives on the same line are treated as if they appeared in between the respective included texts. For example:

INCLUDE 'A'; PRINT *, 'B'; INCLUDE 'C'; END PROGRAM

If the text included by `INCLUDE 'A'' constitutes a `PRINT *, 'A'' statement and the text included by `INCLUDE 'C'' constitutes a `PRINT *, 'C'' statement, then the output of the above sample program would be

A B C

(with suitable allowances for how an implementation defines its handling of output).

Included text must not include itself directly or indirectly, regardless of whether the filename used to reference the text is the same.

Note that INCLUDE is not a statement. As such, it is neither a non-executable or executable statement. However, if the text it includes constitutes one or more executable statements, then the placement of INCLUDE is subject to effectively the same restrictions as those on executable statements.

An INCLUDE directive may be continued across multiple lines as if it were a statement. This permits long names to be used for filename.

8.6.8 Cpp-style directives

cpp output-style # directives (see section `C Preprocessor Output' in The C Preprocessor) are recognized by the compiler even when the preprocessor isn't run on the input (as it is when compiling `.F' files). (Note the distinction between these cpp # output directives and #line input directives.)

8.7 Data Types and Constants

(The following information augments or overrides the information in Chapter 4 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 4 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

To more concisely express the appropriate types for entities, this document uses the more concise Fortran 90 nomenclature such as INTEGER(KIND=1) instead of the more traditional, but less portably concise, byte-size-based nomenclature such as INTEGER*4, wherever reasonable.

When referring to generic types--in contexts where the specific precision and range of a type are not important--this document uses the generic type names INTEGER, LOGICAL, REAL, COMPLEX, and CHARACTER.

In some cases, the context requires specification of a particular type. This document uses the `KIND=' notation to accomplish this throughout, sometimes supplying the more traditional notation for clarification, though the traditional notation might not work the same way on all GNU Fortran implementations.

Use of `KIND=' makes this document more concise because g77 is able to define values for `KIND=' that have the same meanings on all systems, due to the way the Fortran 90 standard specifies these values are to be used.

(In particular, that standard permits an implementation to arbitrarily assign nonnegative values. There are four distinct sets of assignments: one to the CHARACTER type; one to the INTEGER type; one to the LOGICAL type; and the fourth to both the REAL and COMPLEX types. Implementations are free to assign these values in any order, leave gaps in the ordering of assignments, and assign more than one value to a representation.)

This makes `KIND=' values superior to the values used in non-standard statements such as `INTEGER*4', because the meanings of the values in those statements vary from machine to machine, compiler to compiler, even operating system to operating system.

However, use of `KIND=' is not generally recommended when writing portable code (unless, for example, the code is going to be compiled only via g77, which is a widely ported compiler). GNU Fortran does not yet have adequate language constructs to permit use of `KIND=' in a fashion that would make the code portable to Fortran 90 implementations; and, this construct is known to not be accepted by many popular FORTRAN 77 implementations, so it cannot be used in code that is to be ported to those.

The distinction here is that this document is able to use specific values for `KIND=' to concisely document the types of various operations and operands.

A Fortran program should use the FORTRAN 77 designations for the appropriate GNU Fortran types--such as INTEGER for INTEGER(KIND=1), REAL for REAL(KIND=1), and DOUBLE COMPLEX for COMPLEX(KIND=2)---and, where no such designations exist, make use of appropriate techniques (preprocessor macros, parameters, and so on) to specify the types in a fashion that may be easily adjusted to suit each particular implementation to which the program is ported. (These types generally won't need to be adjusted for ports of g77.)

Further details regarding GNU Fortran data types and constants are provided below.

8.7.1 Data Types
8.7.2 Constants
8.7.3 Integer Type
8.7.4 Character Type

8.7.1 Data Types

(Corresponds to Section 4.1 of ANSI X3.9-1978 FORTRAN 77.)

GNU Fortran supports these types:

1. Integer (generic type INTEGER)

2. Real (generic type REAL)

3. Double precision

4. Complex (generic type COMPLEX)

5. Logical (generic type LOGICAL)

6. Character (generic type CHARACTER)

7. Double Complex

(The types numbered 1 through 6 above are standard FORTRAN 77 types.)

The generic types shown above are referred to in this document using only their generic type names. Such references usually indicate that any specific type (kind) of that generic type is valid.

For example, a context described in this document as accepting the COMPLEX type also is likely to accept the DOUBLE COMPLEX type.

The GNU Fortran language supports three ways to specify a specific kind of a generic type.

8.7.1.1 Double Notation		As in DOUBLE COMPLEX.
8.7.1.2 Star Notation		As in INTEGER*4.
8.7.1.3 Kind Notation		As in INTEGER(KIND=1).

8.7.1.1 Double Notation

The GNU Fortran language supports two uses of the keyword DOUBLE to specify a specific kind of type:

• DOUBLE PRECISION, equivalent to REAL(KIND=2)

• DOUBLE COMPLEX, equivalent to COMPLEX(KIND=2)

Use one of the above forms where a type name is valid.

While use of this notation is popular, it doesn't scale well in a language or dialect rich in intrinsic types, as is the case for the GNU Fortran language (especially planned future versions of it).

After all, one rarely sees type names such as `DOUBLE INTEGER', `QUADRUPLE REAL', or `QUARTER INTEGER'. Instead, INTEGER*8, REAL*16, and INTEGER*1 often are substituted for these, respectively, even though they do not always have the same meanings on all systems. (And, the fact that `DOUBLE REAL' does not exist as such is an inconsistency.)

Therefore, this document uses "double notation" only on occasion for the benefit of those readers who are accustomed to it.

8.7.1.2 Star Notation

The following notation specifies the storage size for a type:

generic-type*n

generic-type must be a generic type--one of INTEGER, REAL, COMPLEX, LOGICAL, or CHARACTER. n must be one or more digits comprising a decimal integer number greater than zero.

Use the above form where a type name is valid.

The `*n' notation specifies that the amount of storage occupied by variables and array elements of that type is n times the storage occupied by a CHARACTER*1 variable.

This notation might indicate a different degree of precision and/or range for such variables and array elements, and the functions that return values of types using this notation. It does not limit the precision or range of values of that type in any particular way--use explicit code to do that.

Further, the GNU Fortran language requires no particular values for n to be supported by an implementation via the `*n' notation. g77 supports INTEGER*1 (as INTEGER(KIND=3)) on all systems, for example, but not all implementations are required to do so, and g77 is known to not support REAL*1 on most (or all) systems.

As a result, except for generic-type of CHARACTER, uses of this notation should be limited to isolated portions of a program that are intended to handle system-specific tasks and are expected to be non-portable.

(Standard FORTRAN 77 supports the `*n' notation for only CHARACTER, where it signifies not only the amount of storage occupied, but the number of characters in entities of that type. However, almost all Fortran compilers have supported this notation for generic types, though with a variety of meanings for n.)

Specifications of types using the `*n' notation always are interpreted as specifications of the appropriate types described in this document using the `KIND=n' notation, described below.

While use of this notation is popular, it doesn't serve well in the context of a widely portable dialect of Fortran, such as the GNU Fortran language.

For example, even on one particular machine, two or more popular Fortran compilers might well disagree on the size of a type declared INTEGER*2 or REAL*16. Certainly there is known to be disagreement over such things among Fortran compilers on different systems.

Further, this notation offers no elegant way to specify sizes that are not even multiples of the "byte size" typically designated by INTEGER*1. Use of "absurd" values (such as INTEGER*1000) would certainly be possible, but would perhaps be stretching the original intent of this notation beyond the breaking point in terms of widespread readability of documentation and code making use of it.

Therefore, this document uses "star notation" only on occasion for the benefit of those readers who are accustomed to it.

8.7.1.3 Kind Notation

The following notation specifies the kind-type selector of a type:

generic-type(KIND=n)

Use the above form where a type name is valid.

generic-type must be a generic type--one of INTEGER, REAL, COMPLEX, LOGICAL, or CHARACTER. n must be an integer initialization expression that is a positive, nonzero value.

Programmers are discouraged from writing these values directly into their code. Future versions of the GNU Fortran language will offer facilities that will make the writing of code portable to g77 and Fortran 90 implementations simpler.

However, writing code that ports to existing FORTRAN 77 implementations depends on avoiding the `KIND=' construct.

The `KIND=' construct is thus useful in the context of GNU Fortran for two reasons:

• It provides a means to specify a type in a fashion that is portable across all GNU Fortran implementations (though not other FORTRAN 77 and Fortran 90 implementations).

• It provides a sort of Rosetta stone for this document to use to concisely describe the types of various operations and operands.

The values of n in the GNU Fortran language are assigned using a scheme that:

• Attempts to maximize the ability of readers of this document to quickly familiarize themselves with assignments for popular types

• Provides a unique value for each specific desired meaning

• Provides a means to automatically assign new values so they have a "natural" relationship to existing values, if appropriate, or, if no such relationship exists, will not interfere with future values assigned on the basis of such relationships

• Avoids using values that are similar to values used in the existing, popular `*n' notation, to prevent readers from expecting that these implied correspondences work on all GNU Fortran implementations

The assignment system accomplishes this by assigning to each "fundamental meaning" of a specific type a unique prime number. Combinations of fundamental meanings--for example, a type that is two times the size of some other type--are assigned values of n that are the products of the values for those fundamental meanings.

A prime value of n is never given more than one fundamental meaning, to avoid situations where some code or system cannot reasonably provide those meanings in the form of a single type.

The values of n assigned so far are:

KIND=0

This value is reserved for future use.

The planned future use is for this value to designate, explicitly, context-sensitive kind-type selection. For example, the expression `1D0 * 0.1_0' would be equivalent to `1D0 * 0.1D0'.

KIND=1

This corresponds to the default types for REAL, INTEGER, LOGICAL, COMPLEX, and CHARACTER, as appropriate.

These are the "default" types described in the Fortran 90 standard, though that standard does not assign any particular `KIND=' value to these types.

(Typically, these are REAL*4, INTEGER*4, LOGICAL*4, and COMPLEX*8.)

KIND=2

This corresponds to types that occupy twice as much storage as the default types. REAL(KIND=2) is DOUBLE PRECISION (typically REAL*8), COMPLEX(KIND=2) is DOUBLE COMPLEX (typically COMPLEX*16),

These are the "double precision" types described in the Fortran 90 standard, though that standard does not assign any particular `KIND=' value to these types.

n of 4 thus corresponds to types that occupy four times as much storage as the default types, n of 8 to types that occupy eight times as much storage, and so on.

The INTEGER(KIND=2) and LOGICAL(KIND=2) types are not necessarily supported by every GNU Fortran implementation.

KIND=3

This corresponds to types that occupy as much storage as the default CHARACTER type, which is the same effective type as CHARACTER(KIND=1) (making that type effectively the same as CHARACTER(KIND=3)).

(Typically, these are INTEGER*1 and LOGICAL*1.)

n of 6 thus corresponds to types that occupy twice as much storage as the n=3 types, n of 12 to types that occupy four times as much storage, and so on.

These are not necessarily supported by every GNU Fortran implementation.

KIND=5

This corresponds to types that occupy half the storage as the default (n=1) types.

(Typically, these are INTEGER*2 and LOGICAL*2.)

n of 25 thus corresponds to types that occupy one-quarter as much storage as the default types.

These are not necessarily supported by every GNU Fortran implementation.

KIND=7

This is valid only as INTEGER(KIND=7) and denotes the INTEGER type that has the smallest storage size that holds a pointer on the system.

A pointer representable by this type is capable of uniquely addressing a CHARACTER*1 variable, array, array element, or substring.

(Typically this is equivalent to INTEGER*4 or, on 64-bit systems, INTEGER*8. In a compatible C implementation, it typically would be the same size and semantics of the C type void *.)

Note that these are proposed correspondences and might change in future versions of g77---avoid writing code depending on them while g77, and therefore the GNU Fortran language it defines, is in beta testing.

Values not specified in the above list are reserved to future versions of the GNU Fortran language.

Implementation-dependent meanings will be assigned new, unique prime numbers so as to not interfere with other implementation-dependent meanings, and offer the possibility of increasing the portability of code depending on such types by offering support for them in other GNU Fortran implementations.

Other meanings that might be given unique values are:

• Types that make use of only half their storage size for representing precision and range.

  For example, some compilers offer options that cause INTEGER types to occupy the amount of storage that would be needed for INTEGER(KIND=2) types, but the range remains that of INTEGER(KIND=1).

• The IEEE single floating-point type.

• Types with a specific bit pattern (endianness), such as the little-endian form of INTEGER(KIND=1). These could permit, conceptually, use of portable code and implementations on data files written by existing systems.

Future prime numbers should be given meanings in as incremental a fashion as possible, to allow for flexibility and expressiveness in combining types.

For example, instead of defining a prime number for little-endian IEEE doubles, one prime number might be assigned the meaning "little-endian", another the meaning "IEEE double", and the value of n for a little-endian IEEE double would thus naturally be the product of those two respective assigned values. (It could even be reasonable to have IEEE values result from the products of prime values denoting exponent and fraction sizes and meanings, hidden bit usage, availability and representations of special values such as subnormals, infinities, and Not-A-Numbers (NaNs), and so on.)

This assignment mechanism, while not inherently required for future versions of the GNU Fortran language, is worth using because it could ease management of the "space" of supported types much easier in the long run.

The above approach suggests a mechanism for specifying inheritance of intrinsic (built-in) types for an entire, widely portable product line. It is certainly reasonable that, unlike programmers of other languages offering inheritance mechanisms that employ verbose names for classes and subclasses, along with graphical browsers to elucidate the relationships, Fortran programmers would employ a mechanism that works by multiplying prime numbers together and finding the prime factors of such products.

Most of the advantages for the above scheme have been explained above. One disadvantage is that it could lead to the defining, by the GNU Fortran language, of some fairly large prime numbers. This could lead to the GNU Fortran language being declared "munitions" by the United States Department of Defense.

8.7.2 Constants

(Corresponds to Section 4.2 of ANSI X3.9-1978 FORTRAN 77.)

A typeless constant has one of the following forms:

'binary-digits'B 'octal-digits'O 'hexadecimal-digits'Z 'hexadecimal-digits'X

binary-digits, octal-digits, and hexadecimal-digits are nonempty strings of characters in the set `01', `01234567', and `0123456789ABCDEFabcdef', respectively. (The value for `A' (and `a') is 10, for `B' and `b' is 11, and so on.)

A prefix-radix constant, such as `Z'ABCD'', can optionally be treated as typeless. See section Options Controlling Fortran Dialect, for information on the `-ftypeless-boz' option.

Typeless constants have values that depend on the context in which they are used.

All other constants, called typed constants, are interpreted--converted to internal form--according to their inherent type. Thus, context is never a determining factor for the type, and hence the interpretation, of a typed constant. (All constants in the ANSI FORTRAN 77 language are typed constants.)

For example, `1' is always type INTEGER(KIND=1) in GNU Fortran (called default INTEGER in Fortran 90), `9.435784839284958' is always type REAL(KIND=1) (even if the additional precision specified is lost, and even when used in a REAL(KIND=2) context), `1E0' is always type REAL(KIND=2), and `1D0' is always type REAL(KIND=2).

8.7.3 Integer Type

(Corresponds to Section 4.3 of ANSI X3.9-1978 FORTRAN 77.)

An integer constant also may have one of the following forms:

B'binary-digits' O'octal-digits' Z'hexadecimal-digits' X'hexadecimal-digits'

binary-digits, octal-digits, and hexadecimal-digits are nonempty strings of characters in the set `01', `01234567', and `0123456789ABCDEFabcdef', respectively. (The value for `A' (and `a') is 10, for `B' and `b' is 11, and so on.)

8.7.4 Character Type

(Corresponds to Section 4.8 of ANSI X3.9-1978 FORTRAN 77.)

A character constant may be delimited by a pair of double quotes (`"') instead of apostrophes. In this case, an apostrophe within the constant represents a single apostrophe, while a double quote is represented in the source text of the constant by two consecutive double quotes with no intervening spaces.

A character constant may be empty (have a length of zero).

A character constant may include a substring specification, The value of such a constant is the value of the substring--for example, the value of `'hello'(3:5)' is the same as the value of `'llo''.

8.8 Expressions

(The following information augments or overrides the information in Chapter 6 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 6 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.8.1 The %LOC() Construct

8.8.1 The %LOC() Construct

%LOC(arg)

The %LOC() construct is an expression that yields the value of the location of its argument, arg, in memory. The size of the type of the expression depends on the system--typically, it is equivalent to either INTEGER(KIND=1) or INTEGER(KIND=2), though it is actually type INTEGER(KIND=7).

The argument to %LOC() must be suitable as the left-hand side of an assignment statement. That is, it may not be a general expression involving operators such as addition, subtraction, and so on, nor may it be a constant.

Use of %LOC() is recommended only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions that deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

Do not depend on %LOC() returning a pointer that can be safely used to define (change) the argument. While this might work in some circumstances, it is hard to predict whether it will continue to work when a program (that works using this unsafe behavior) is recompiled using different command-line options or a different version of g77.

Generally, %LOC() is safe when used as an argument to a procedure that makes use of the value of the corresponding dummy argument only during its activation, and only when such use is restricted to referencing (reading) the value of the argument to %LOC().

Implementation Note: Currently, g77 passes arguments (those not passed using a construct such as %VAL()) by reference or descriptor, depending on the type of the actual argument. Thus, given `INTEGER I', `CALL FOO(I)' would seem to mean the same thing as `CALL FOO(%VAL(%LOC(I)))', and in fact might compile to identical code.

However, `CALL FOO(%VAL(%LOC(I)))' emphatically means "pass, by value, the address of `I' in memory". While `CALL FOO(I)' might use that same approach in a particular version of g77, another version or compiler might choose a different implementation, such as copy-in/copy-out, to effect the desired behavior--and which will therefore not necessarily compile to the same code as would `CALL FOO(%VAL(%LOC(I)))' using the same version or compiler.

See section 14. Debugging and Interfacing, for detailed information on how this particular version of g77 implements various constructs.

8.9 Specification Statements

(The following information augments or overrides the information in Chapter 8 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 8 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.9.1 NAMELIST Statement
8.9.2 DOUBLE COMPLEX Statement

8.9.1 NAMELIST Statement

The NAMELIST statement, and related I/O constructs, are supported by the GNU Fortran language in essentially the same way as they are by f2c.

This follows Fortran 90 with the restriction that on NAMELIST input, subscripts must have the form

subscript [ : subscript [ : stride]]

i.e.

&xx x(1:3,8:10:2)=1,2,3,4,5,6/

is allowed, but not, say,

&xx x(:3,8::2)=1,2,3,4,5,6/

As an extension of the Fortran 90 form, $ and $END may be used in place of & and / in NAMELIST input, so that

$&xx x(1:3,8:10:2)=1,2,3,4,5,6 $end

could be used instead of the example above.

8.9.2 DOUBLE COMPLEX Statement

DOUBLE COMPLEX is a type-statement (and type) that specifies the type COMPLEX(KIND=2) in GNU Fortran.

8.10 Control Statements

(The following information augments or overrides the information in Chapter 11 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 11 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.10.1 DO WHILE
8.10.2 END DO
8.10.3 Construct Names
8.10.4 The CYCLE and EXIT Statements

8.10.1 DO WHILE

The DO WHILE statement, a feature of both the MIL-STD 1753 and Fortran 90 standards, is provided by the GNU Fortran language. The Fortran 90 "do forever" statement comprising just DO is also supported.

8.10.2 END DO

The END DO statement is provided by the GNU Fortran language.

This statement is used in one of two ways:

• The Fortran 90 meaning, in which it specifies the termination point of a single DO loop started with a DO statement that specifies no termination label.

• The MIL-STD 1753 meaning, in which it specifies the termination point of one or more DO loops, all of which start with a DO statement that specify the label defined for the END DO statement.

  This kind of END DO statement is merely a synonym for CONTINUE, except it is permitted only when the statement is labeled and a target of one or more labeled DO loops.

  It is expected that this use of END DO will be removed from the GNU Fortran language in the future, though it is likely that it will long be supported by g77 as a dialect form.

8.10.3 Construct Names

The GNU Fortran language supports construct names as defined by the Fortran 90 standard. These names are local to the program unit and are defined as follows:

construct-name: block-statement

Here, construct-name is the construct name itself; its definition is connoted by the single colon (`:'); and block-statement is an IF, DO, or SELECT CASE statement that begins a block.

A block that is given a construct name must also specify the same construct name in its termination statement:

END block construct-name

Here, block must be IF, DO, or SELECT, as appropriate.

8.10.4 The CYCLE and EXIT Statements

The CYCLE and EXIT statements specify that the remaining statements in the current iteration of a particular active (enclosing) DO loop are to be skipped.

CYCLE specifies that these statements are skipped, but the END DO statement that marks the end of the DO loop be executed--that is, the next iteration, if any, is to be started. If the statement marking the end of the DO loop is not END DO---in other words, if the loop is not a block DO---the CYCLE statement does not execute that statement, but does start the next iteration (if any).

EXIT specifies that the loop specified by the DO construct is terminated.

The DO loop affected by CYCLE and EXIT is the innermost enclosing DO loop when the following forms are used:

CYCLE EXIT

Otherwise, the following forms specify the construct name of the pertinent DO loop:

CYCLE construct-name EXIT construct-name

CYCLE and EXIT can be viewed as glorified GO TO statements. However, they cannot be easily thought of as GO TO statements in obscure cases involving FORTRAN 77 loops. For example:

DO 10 I = 1, 5 DO 10 J = 1, 5 IF (J .EQ. 5) EXIT DO 10 K = 1, 5 IF (K .EQ. 3) CYCLE 10 PRINT *, 'I=', I, ' J=', J, ' K=', K 20 CONTINUE

In particular, neither the EXIT nor CYCLE statements above are equivalent to a GO TO statement to either label `10' or `20'.

To understand the effect of CYCLE and EXIT in the above fragment, it is helpful to first translate it to its equivalent using only block DO loops:

DO I = 1, 5 DO J = 1, 5 IF (J .EQ. 5) EXIT DO K = 1, 5 IF (K .EQ. 3) CYCLE 10 PRINT *, 'I=', I, ' J=', J, ' K=', K END DO END DO END DO 20 CONTINUE

Adding new labels allows translation of CYCLE and EXIT to GO TO so they may be more easily understood by programmers accustomed to FORTRAN coding:

DO I = 1, 5 DO J = 1, 5 IF (J .EQ. 5) GOTO 18 DO K = 1, 5 IF (K .EQ. 3) GO TO 12 10 PRINT *, 'I=', I, ' J=', J, ' K=', K 12 END DO END DO 18 END DO 20 CONTINUE

Thus, the CYCLE statement in the innermost loop skips over the PRINT statement as it begins the next iteration of the loop, while the EXIT statement in the middle loop ends that loop but not the outermost loop.

8.11 Functions and Subroutines

(The following information augments or overrides the information in Chapter 15 of ANSI X3.9-1978 FORTRAN 77 in specifying the GNU Fortran language. Chapter 15 of that document otherwise serves as the basis for the relevant aspects of GNU Fortran.)

8.11.1 The %VAL() Construct
8.11.2 The %REF() Construct
8.11.3 The %DESCR() Construct
8.11.4 Generics and Specifics
8.11.5 REAL() and AIMAG() of Complex
8.11.6 CMPLX() of DOUBLE PRECISION
8.11.7 MIL-STD 1753 Support
8.11.8 f77/f2c Intrinsics
8.11.9 Table of Intrinsic Functions

8.11.1 The %VAL() Construct

%VAL(arg)

The %VAL() construct specifies that an argument, arg, is to be passed by value, instead of by reference or descriptor.

%VAL() is restricted to actual arguments in invocations of external procedures.

Use of %VAL() is recommended only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions the deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

Implementation Note: Currently, g77 passes all arguments either by reference or by descriptor.

Thus, use of %VAL() tends to be restricted to cases where the called procedure is written in a language other than Fortran that supports call-by-value semantics. (C is an example of such a language.)

See section Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.

8.11.2 The %REF() Construct

%REF(arg)

The %REF() construct specifies that an argument, arg, is to be passed by reference, instead of by value or descriptor.

%REF() is restricted to actual arguments in invocations of external procedures.

Use of %REF() is recommended only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions the deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

Do not depend on %REF() supplying a pointer to the procedure being invoked. While that is a likely implementation choice, other implementation choices are available that preserve Fortran pass-by-reference semantics without passing a pointer to the argument, arg. (For example, a copy-in/copy-out implementation.)

Implementation Note: Currently, g77 passes all arguments (other than variables and arrays of type CHARACTER) by reference. Future versions of, or dialects supported by, g77 might not pass CHARACTER functions by reference.

Thus, use of %REF() tends to be restricted to cases where arg is type CHARACTER but the called procedure accesses it via a means other than the method used for Fortran CHARACTER arguments.

See section Procedures (SUBROUTINE and FUNCTION), for detailed information on how this particular version of g77 passes arguments to procedures.

8.11.3 The %DESCR() Construct

%DESCR(arg)

The %DESCR() construct specifies that an argument, arg, is to be passed by descriptor, instead of by value or reference.

%DESCR() is restricted to actual arguments in invocations of external procedures.

Use of %DESCR() is recommended only for code that is accessing facilities outside of GNU Fortran, such as operating system or windowing facilities. It is best to constrain such uses to isolated portions of a program--portions the deal specifically and exclusively with low-level, system-dependent facilities. Such portions might well provide a portable interface for use by the program as a whole, but are themselves not portable, and should be thoroughly tested each time they are rebuilt using a new compiler or version of a compiler.

Do not depend on %DESCR() supplying a pointer and/or a length passed by value to the procedure being invoked. While that is a likely implementation choice, other implementation choices are available that preserve the pass-by-reference semantics without passing a pointer to the argument, arg. (For example, a copy-in/copy-out implementation.) And, future versions of g77 might change the way descriptors are implemented, such as passing a single argument pointing to a record containing the pointer/length information instead of passing that same information via two arguments as it currently does.

Implementation Note: Currently, g77 passes all variables and arrays of type CHARACTER by descriptor. Future versions of, or dialects supported by, g77 might pass CHARACTER functions by descriptor as well.

Thus, use of %DESCR() tends to be restricted to cases where arg is not type CHARACTER but the called procedure accesses it via a means similar to the method used for Fortran CHARACTER arguments.

See section Procedures (SUBROUTINE and F