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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.