CHARACTER types in f18¶
The f18 compiler and runtime support three kinds of the intrinsic
CHARACTER type of Fortran 2018.
The default (
CHARACTER(KIND=1)) holds 8-bit character codes;
CHARACTER(KIND=2) holds 16-bit character codes;
CHARACTER(KIND=4) holds 32-bit character codes.
We assume that code values 0 through 127 correspond to
the 7-bit ASCII character set (ISO-646) in every kind of
This is a valid assumption for Unicode (UCS == ISO/IEC-10646),
ISO-8859, and many legacy character sets and interchange formats.
CHARACTER data in memory and unformatted files are not in an
interchange representation (like UTF-8, Shift-JIS, EUC-JP, or a JIS X).
Each character’s code in memory occupies a 1-, 2-, or 4- byte
word and substrings can be indexed with simple arithmetic.
In formatted I/O, however,
CHARACTER data may be assumed to use
the UTF-8 variable-length encoding when it is selected with
CHARACTER(KIND=1) literal constants in Fortran source files,
Hollerith constants, and formatted I/O with
are not translated.
For the purposes of non-default-kind
CHARACTER constants in Fortran
source files, formatted I/O with
ENCODING='UTF-8' or non-default-kind
CHARACTER value, and conversions between kinds of
CHARACTER(KIND=1)is assumed to be ISO-8859-1 (Latin-1),
CHARACTER(KIND=2)is assumed to be UCS-2 (16-bit Unicode), and
CHARACTER(KIND=4)is assumed to be UCS-4 (full Unicode in a 32-bit word).
In particular, conversions between kinds are assumed to be simple zero-extensions or truncation, not table look-ups.
We might want to support one or more environment variables to change these
assumptions, especially for
KIND=1 users of ISO-8859 character sets
CHARACTER objects in Fortran may defer the specification
of their lengths until the time of their allocation or whole (non-substring)
Non-allocatable objects (and non-deferred-length allocatables) have
lengths that are fixed or assumed from an actual argument, or,
in the case of assumed-length
CHARACTER functions, their local
declaration in the calling scope.
The elements of
CHARACTER arrays have the same length.
Assignments to targets that are not deferred-length allocatables will truncate or pad the assigned value to the length of the left-hand side of the assignment.
Lengths and offsets that are used by or exposed to Fortran programs via
declarations, substring bounds, and the
LEN() intrinsic function are always
represented in units of characters, not bytes.
In generated code, assumed-length arguments, the runtime support library,
and in the
elem_len field of the interoperable descriptor
lengths are always in units of bytes.
The distinction matters only for kinds other than the default.
Fortran substrings are rather like subscript triplets into a hidden
“zero” dimension of a scalar
CHARACTER value, but they cannot have
Fortran has one
CHARACTER-valued intrinsic operator,
concatenates its operands (10.1.5.3).
The operands must have the same kind type parameter.
One or both of the operands may be arrays; if both are arrays, their
shapes must be identical.
The effective length of the result is the sum of the lengths of the
Parentheses may be ignored, so any
may be “flattened” into a single sequence of concatenations.
The result of
// may be used
as an operand to another concatenation,
as an operand of a
as an actual argument,
as the right-hand side of an assignment,
as the selector or case-expr of an
as a component of a structure or array constructor,
as the value of a named constant or initializer,
as the stop-code of a
as the value of a specifier of an I/O statement,
or as the value of a statement function.
The f18 compiler has a general (but slow) means of implementing concatenation and a specialized (fast) option to optimize the most common case.
In the most general case, the f18 compiler’s generated code and
runtime support library represent the result as a deferred-length allocatable
CHARACTER temporary scalar or array variable that is initialized
as a zero-length array by
and then progressively augmented in place by the values of each of the
operands of the concatenation sequence in turn with calls to
Conformability errors are fatal – Fortran has no means by which a program
may recover from them.
The result is then used as any other deferred-length allocatable
array or scalar would be, and finally deallocated like any other
The runtime routine
CharacterAssign() takes care of
truncating, padding, or replicating the value(s) assigned to the left-hand
side, as well as reallocating an nonconforming or deferred-length allocatable
left-hand side. It takes the descriptors of the left- and right-hand sides of
CHARACTER assignemnt as its arguments.
When the left-hand side of a
CHARACTER assignment is a deferred-length
allocatable and the right-hand side is a temporary, use of the runtime’s
MoveAlloc() subroutine instead can save an allocation and a copy.
CHARACTER(KIND=1) expressions evaluated as the right-hand sides of
assignments to independent substrings or whole variables that are not
deferred-length allocatables can be optimized into a sequence of
calls to the runtime support library that do not allocate temporary
CharacterAppend() copies data from the right-hand side value
to the remaining space, if any, in the left-hand side object, and returns
the new offset of the reduced remaining space.
It is essentially
memcpy(lhs + offset, rhs, min(lhsLength - offset, rhsLength)).
It does nothing when
offset > lhsLength.
void CharacterPad()adds any necessary trailing blank characters.