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(g77-300.info)Compiler Types

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Compiler Types
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   Fortran implementations have a fair amount of freedom given them by
the standard as far as how much storage space is used and how much
precision and range is offered by the various types such as
`LOGICAL(KIND=1)', `INTEGER(KIND=1)', `REAL(KIND=1)', `REAL(KIND=2)',
`COMPLEX(KIND=1)', and `CHARACTER'.  Further, many compilers offer
so-called `*N' notation, but the interpretation of N varies across
compilers and target architectures.

   The standard requires that `LOGICAL(KIND=1)', `INTEGER(KIND=1)', and
`REAL(KIND=1)' occupy the same amount of storage space, and that
`COMPLEX(KIND=1)' and `REAL(KIND=2)' take twice as much storage space
as `REAL(KIND=1)'.  Further, it requires that `COMPLEX(KIND=1)'
entities be ordered such that when a `COMPLEX(KIND=1)' variable is
storage-associated (such as via `EQUIVALENCE') with a two-element
`REAL(KIND=1)' array named `R', `R(1)' corresponds to the real element
and `R(2)' to the imaginary element of the `COMPLEX(KIND=1)' variable.

   (Few requirements as to precision or ranges of any of these are
placed on the implementation, nor is the relationship of storage sizes
of these types to the `CHARACTER' type specified, by the standard.)

   `g77' follows the above requirements, warning when compiling a
program requires placement of items in memory that contradict the
requirements of the target architecture.  (For example, a program can
require placement of a `REAL(KIND=2)' on a boundary that is not an even
multiple of its size, but still an even multiple of the size of a
`REAL(KIND=1)' variable.  On some target architectures, using the
canonical mapping of Fortran types to underlying architectural types,
such placement is prohibited by the machine definition or the
Application Binary Interface (ABI) in force for the configuration
defined for building `gcc' and `g77'.  `g77' warns about such
situations when it encounters them.)

   `g77' follows consistent rules for configuring the mapping between
Fortran types, including the `*N' notation, and the underlying
architectural types as accessed by a similarly-configured applicable
version of the `gcc' compiler.  These rules offer a widely portable,
consistent Fortran/C environment, although they might well conflict
with the expectations of users of Fortran compilers designed and
written for particular architectures.

   These rules are based on the configuration that is in force for the
version of `gcc' built in the same release as `g77' (and which was
therefore used to build both the `g77' compiler components and the
`libg2c' run-time library):

`REAL(KIND=1)'
     Same as `float' type.

`REAL(KIND=2)'
     Same as whatever floating-point type that is twice the size of a
     `float'--usually, this is a `double'.

`INTEGER(KIND=1)'
     Same as an integral type that is occupies the same amount of
     memory storage as `float'--usually, this is either an `int' or a
     `long int'.

`LOGICAL(KIND=1)'
     Same `gcc' type as `INTEGER(KIND=1)'.

`INTEGER(KIND=2)'
     Twice the size, and usually nearly twice the range, as
     `INTEGER(KIND=1)'--usually, this is either a `long int' or a `long
     long int'.

`LOGICAL(KIND=2)'
     Same `gcc' type as `INTEGER(KIND=2)'.

`INTEGER(KIND=3)'
     Same `gcc' type as signed `char'.

`LOGICAL(KIND=3)'
     Same `gcc' type as `INTEGER(KIND=3)'.

`INTEGER(KIND=6)'
     Twice the size, and usually nearly twice the range, as
     `INTEGER(KIND=3)'--usually, this is a `short'.

`LOGICAL(KIND=6)'
     Same `gcc' type as `INTEGER(KIND=6)'.

`COMPLEX(KIND=1)'
     Two `REAL(KIND=1)' scalars (one for the real part followed by one
     for the imaginary part).

`COMPLEX(KIND=2)'
     Two `REAL(KIND=2)' scalars.

`NUMERIC-TYPE*N'
     (Where NUMERIC-TYPE is any type other than `CHARACTER'.)  Same as
     whatever `gcc' type occupies N times the storage space of a `gcc'
     `char' item.

`DOUBLE PRECISION'
     Same as `REAL(KIND=2)'.

`DOUBLE COMPLEX'
     Same as `COMPLEX(KIND=2)'.

   Note that the above are proposed correspondences and might change in
future versions of `g77'--avoid writing code depending on them.

   Other types supported by `g77' are derived from gcc types such as
`char', `short', `int', `long int', `long long int', `long double', and
so on.  That is, whatever types `gcc' already supports, `g77' supports
now or probably will support in a future version.  The rules for the
`NUMERIC-TYPE*N' notation apply to these types, and new values for
`NUMERIC-TYPE(KIND=N)' will be assigned in a way that encourages
clarity, consistency, and portability.