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(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.