gfortran.texi

理解和实践操作系统的一本好书

@samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
then library run-time errors cause core dumps. To disable the core
dumps, set the variable to @samp{n}, @samp{N}, @samp{0}. Default
is not to core dump unless the @option{-fdump-core} compile option
was used.

@node GFORTRAN_ERROR_BACKTRACE
@section @env{GFORTRAN_ERROR_BACKTRACE}---Show backtrace on run-time errors

If the @env{GFORTRAN_ERROR_BACKTRACE} variable is set to
@samp{y}, @samp{Y} or @samp{1} (only the first letter is relevant)
then a backtrace is printed when a run-time error occurs.
To disable the backtracing, set the variable to
@samp{n}, @samp{N}, @samp{0}. Default is not to print a backtrace
unless the @option{-fbacktrace} compile option
was used.

@c =====================================================================
@c PART II: LANGUAGE REFERENCE
@c =====================================================================

@tex
\part{II}{Language Reference}
@end tex

@c ---------------------------------------------------------------------
@c Fortran 2003 Status
@c ---------------------------------------------------------------------

@node Fortran 2003 status
@chapter Fortran 2003 Status

Although GNU Fortran focuses on implementing the Fortran 95
standard for the time being, a few Fortran 2003 features are currently
available.

@itemize
@item 
Intrinsics @code{command_argument_count}, @code{get_command},
@code{get_command_argument}, @code{get_environment_variable}, and
@code{move_alloc}.

@item 
@cindex array, constructors
@cindex @code{[...]}
Array constructors using square brackets. That is, @code{[...]} rather
than @code{(/.../)}.

@item
@cindex @code{FLUSH} statement
@cindex statement, @code{FLUSH}
@code{FLUSH} statement.

@item
@cindex @code{IOMSG=} specifier
@code{IOMSG=} specifier for I/O statements.

@item
@cindex @code{ENUM} statement
@cindex @code{ENUMERATOR} statement
@cindex statement, @code{ENUM}
@cindex statement, @code{ENUMERATOR}
@opindex @code{fshort-enums}
Support for the declaration of enumeration constants via the
@code{ENUM} and @code{ENUMERATOR} statements.  Interoperability with
@command{gcc} is guaranteed also for the case where the
@command{-fshort-enums} command line option is given.

@item
@cindex TR 15581
TR 15581:
@itemize
@item
@cindex @code{ALLOCATABLE} dummy arguments
@code{ALLOCATABLE} dummy arguments.
@item
@cindex @code{ALLOCATABLE} function results
@code{ALLOCATABLE} function results
@item
@cindex @code{ALLOCATABLE} components of derived types
@code{ALLOCATABLE} components of derived types
@end itemize

@item
@cindex @code{STREAM} I/O
@cindex @code{ACCESS='STREAM'} I/O
The @code{OPEN} statement supports the @code{ACCESS='STREAM'} specifier,
allowing I/O without any record structure.

@item
Namelist input/output for internal files.

@item
@cindex @code{PROTECTED} statement
@cindex statement, @code{PROTECTED}
The @code{PROTECTED} statement and attribute.

@item
@cindex @code{VALUE} statement
@cindex statement, @code{VALUE}
The @code{VALUE} statement and attribute.

@item
@cindex @code{VOLATILE} statement
@cindex statement, @code{VOLATILE}
The @code{VOLATILE} statement and attribute.

@item
@cindex @code{IMPORT} statement
@cindex statement, @code{IMPORT}
The @code{IMPORT} statement, allowing to import
host-associated derived types.

@item
@cindex @code{USE, INTRINSIC} statement
@cindex statement, @code{USE, INTRINSIC}
@cindex @code{ISO_FORTRAN_ENV} statement
@cindex statement, @code{ISO_FORTRAN_ENV}
@code{USE} statement with @code{INTRINSIC} and @code{NON_INTRINSIC}
attribute; supported intrinsic modules: @code{ISO_FORTRAN_ENV},
@code{OMP_LIB} and @code{OMP_LIB_KINDS}.

@item
Renaming of operators in the @code{USE} statement.

@item
@cindex ISO C Bindings
Interoperability with C (ISO C Bindings)

@item
BOZ as argument of INT, REAL, DBLE and CMPLX.

@end itemize


@c ---------------------------------------------------------------------
@c Extensions
@c ---------------------------------------------------------------------

@c Maybe this chapter should be merged with the 'Standards' section,
@c whenever that is written :-)

@node Extensions
@chapter Extensions
@cindex extensions

The two sections below detail the extensions to standard Fortran that are
implemented in GNU Fortran, as well as some of the popular or
historically important extensions that are not (or not yet) implemented.
For the latter case, we explain the alternatives available to GNU Fortran
users, including replacement by standard-conforming code or GNU
extensions.

@menu
* Extensions implemented in GNU Fortran::
* Extensions not implemented in GNU Fortran::
@end menu


@node Extensions implemented in GNU Fortran
@section Extensions implemented in GNU Fortran
@cindex extensions, implemented

GNU Fortran implements a number of extensions over standard
Fortran. This chapter contains information on their syntax and
meaning.  There are currently two categories of GNU Fortran
extensions, those that provide functionality beyond that provided
by any standard, and those that are supported by GNU Fortran
purely for backward compatibility with legacy compilers.  By default,
@option{-std=gnu} allows the compiler to accept both types of
extensions, but to warn about the use of the latter.  Specifying
either @option{-std=f95} or @option{-std=f2003} disables both types
of extensions, and @option{-std=legacy} allows both without warning.

@menu
* Old-style kind specifications::
* Old-style variable initialization::
* Extensions to namelist::
* X format descriptor without count field::
* Commas in FORMAT specifications::
* Missing period in FORMAT specifications::
* I/O item lists::
* BOZ literal constants::
* Real array indices::
* Unary operators::
* Implicitly convert LOGICAL and INTEGER values::
* Hollerith constants support::
* Cray pointers::
* CONVERT specifier::
* OpenMP::
* Argument list functions::
@end menu

@node Old-style kind specifications
@subsection Old-style kind specifications
@cindex kind, old-style

GNU Fortran allows old-style kind specifications in declarations. These
look like:
@smallexample
      TYPESPEC*size x,y,z
@end smallexample
@noindent
where @code{TYPESPEC} is a basic type (@code{INTEGER}, @code{REAL},
etc.), and where @code{size} is a byte count corresponding to the
storage size of a valid kind for that type.  (For @code{COMPLEX}
variables, @code{size} is the total size of the real and imaginary
parts.)  The statement then declares @code{x}, @code{y} and @code{z} to
be of type @code{TYPESPEC} with the appropriate kind.  This is
equivalent to the standard-conforming declaration
@smallexample
      TYPESPEC(k) x,y,z
@end smallexample
@noindent
where @code{k} is equal to @code{size} for most types, but is equal to
@code{size/2} for the @code{COMPLEX} type.

@node Old-style variable initialization
@subsection Old-style variable initialization

GNU Fortran allows old-style initialization of variables of the
form:
@smallexample
      INTEGER i/1/,j/2/
      REAL x(2,2) /3*0.,1./
@end smallexample
The syntax for the initializers is as for the @code{DATA} statement, but
unlike in a @code{DATA} statement, an initializer only applies to the
variable immediately preceding the initialization.  In other words,
something like @code{INTEGER I,J/2,3/} is not valid.  This style of
initialization is only allowed in declarations without double colons
(@code{::}); the double colons were introduced in Fortran 90, which also
introduced a standard syntax for initializing variables in type
declarations.

Examples of standard-conforming code equivalent to the above example
are:
@smallexample
! Fortran 90
      INTEGER :: i = 1, j = 2
      REAL :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
! Fortran 77
      INTEGER i, j
      REAL x(2,2)
      DATA i/1/, j/2/, x/3*0.,1./
@end smallexample

Note that variables which are explicitly initialized in declarations
or in @code{DATA} statements automatically acquire the @code{SAVE}
attribute.

@node Extensions to namelist
@subsection Extensions to namelist
@cindex Namelist

GNU Fortran fully supports the Fortran 95 standard for namelist I/O
including array qualifiers, substrings and fully qualified derived types.
The output from a namelist write is compatible with namelist read.  The
output has all names in upper case and indentation to column 1 after the
namelist name.  Two extensions are permitted:

Old-style use of @samp{$} instead of @samp{&}
@smallexample
$MYNML
 X(:)%Y(2) = 1.0 2.0 3.0
 CH(1:4) = "abcd"
$END
@end smallexample

It should be noted that the default terminator is @samp{/} rather than
@samp{&END}.

Querying of the namelist when inputting from stdin. After at least
one space, entering @samp{?} sends to stdout the namelist name and the names of
the variables in the namelist:
@smallexample
 ?

&mynml
 x
 x%y
 ch
&end
@end smallexample

Entering @samp{=?} outputs the namelist to stdout, as if
@code{WRITE(*,NML = mynml)} had been called:
@smallexample
=?

&MYNML
 X(1)%Y=  0.000000    ,  1.000000    ,  0.000000    ,
 X(2)%Y=  0.000000    ,  2.000000    ,  0.000000    ,
 X(3)%Y=  0.000000    ,  3.000000    ,  0.000000    ,
 CH=abcd,  /
@end smallexample

To aid this dialog, when input is from stdin, errors send their
messages to stderr and execution continues, even if @code{IOSTAT} is set.

@code{PRINT} namelist is permitted.  This causes an error if
@option{-std=f95} is used.
@smallexample
PROGRAM test_print
  REAL, dimension (4)  ::  x = (/1.0, 2.0, 3.0, 4.0/)
  NAMELIST /mynml/ x
  PRINT mynml
END PROGRAM test_print
@end smallexample

Expanded namelist reads are permitted.  This causes an error if 
@option{-std=f95} is used.  In the following example, the first element
of the array will be given the value 0.00 and the two succeeding
elements will be given the values 1.00 and 2.00.
@smallexample
&MYNML
  X(1,1) = 0.00 , 1.00 , 2.00
/
@end smallexample

@node X format descriptor without count field
@subsection @code{X} format descriptor without count field

To support legacy codes, GNU Fortran permits the count field of the
@code{X} edit descriptor in @code{FORMAT} statements to be omitted.
When omitted, the count is implicitly assumed to be one.

@smallexample
       PRINT 10, 2, 3
10     FORMAT (I1, X, I1)
@end smallexample

@node Commas in FORMAT specifications
@subsection Commas in @code{FORMAT} specifications

To support legacy codes, GNU Fortran allows the comma separator
to be omitted immediately before and after character string edit
descriptors in @code{FORMAT} statements.

@smallexample
       PRINT 10, 2, 3
10     FORMAT ('FOO='I1' BAR='I2)
@end smallexample


@node Missing period in FORMAT specifications
@subsection Missing period in @code{FORMAT} specifications

To support legacy codes, GNU Fortran allows missing periods in format
specifications if and only if @option{-std=legacy} is given on the
command line.  This is considered non-conforming code and is
discouraged.

@smallexample
       REAL :: value
       READ(*,10) value
10     FORMAT ('F4')
@end smallexample

@node I/O item lists
@subsection I/O item lists
@cindex I/O item lists

To support legacy codes, GNU Fortran allows the input item list
of the @code{READ} statement, and the output item lists of the
@code{WRITE} and @code{PRINT} statements, to start with a comma.