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/GNU Fortran 7

5.1 KIND Type Parameters

The KIND type parameters supported by GNU Fortran for the primitive data types are:

INTEGER

1, 2, 4, 8*, 16*, default: 4**

LOGICAL

1, 2, 4, 8*, 16*, default: 4**

REAL

4, 8, 10*, 16*, default: 4***

COMPLEX

4, 8, 10*, 16*, default: 4***

DOUBLE PRECISION

4, 8, 10*, 16*, default: 8***

CHARACTER

1, 4, default: 1

* not available on all systems ** unless -fdefault-integer-8 is used *** unless -fdefault-real-8 is used (see Fortran Dialect Options)

The KIND value matches the storage size in bytes, except for COMPLEX where the storage size is twice as much (or both real and imaginary part are a real value of the given size). It is recommended to use the SELECTED_CHAR_KIND, SELECTED_INT_KIND and SELECTED_REAL_KIND intrinsics or the INT8, INT16, INT32, INT64, REAL32, REAL64, and REAL128 parameters of the ISO_FORTRAN_ENV module instead of the concrete values. The available kind parameters can be found in the constant arrays CHARACTER_KINDS, INTEGER_KINDS, LOGICAL_KINDS and REAL_KINDS in the ISO_FORTRAN_ENV module. For C interoperability, the kind parameters of the ISO_C_BINDING module should be used.

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https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gfortran/KIND-Type-Parameters.html