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.\" ========================================================================

.\"

.IX Title "GFORTRAN 1"

.TH GFORTRAN 1 "2007-10-07" "gcc-4.2.2" "GNU"

.SH "NAME"

gfortran \- GNU Fortran compiler

.SH "SYNOPSIS"

.IX Header "SYNOPSIS"

gfortran [\fB\-c\fR|\fB\-S\fR|\fB\-E\fR]

         [\fB\-g\fR] [\fB\-pg\fR] [\fB\-O\fR\fIlevel\fR]

         [\fB\-W\fR\fIwarn\fR...] [\fB\-pedantic\fR]

         [\fB\-I\fR\fIdir\fR...] [\fB\-L\fR\fIdir\fR...]

         [\fB\-D\fR\fImacro\fR[=\fIdefn\fR]...] [\fB\-U\fR\fImacro\fR]

         [\fB\-f\fR\fIoption\fR...]

         [\fB\-m\fR\fImachine-option\fR...]

         [\fB\-o\fR \fIoutfile\fR] \fIinfile\fR...

.PP

Only the most useful options are listed here; see below for the

remainder.

.SH "DESCRIPTION"

.IX Header "DESCRIPTION"

The \fBgfortran\fR command supports all the options supported by the

\&\fBgcc\fR command.  Only options specific to \s-1GNU\s0 Fortran are documented here.

.PP

All \s-1GCC\s0 and \s-1GNU\s0 Fortran options

are accepted both by \fBgfortran\fR and by \fBgcc\fR

(as well as any other drivers built at the same time,

such as \fBg++\fR),

since adding \s-1GNU\s0 Fortran to the \s-1GCC\s0 distribution

enables acceptance of \s-1GNU\s0 Fortran options

by all of the relevant drivers.

.PP

In some cases, options have positive and negative forms;

the negative form of \fB\-ffoo\fR would be \fB\-fno\-foo\fR.

This manual documents only one of these two forms, whichever

one is not the default.

.SH "OPTIONS"

.IX Header "OPTIONS"

Here is a summary of all the options specific to \s-1GNU\s0 Fortran, grouped

by type.  Explanations are in the following sections.

.IP "\fIFortran Language Options\fR" 4

.IX Item "Fortran Language Options"

\&\fB\-fall\-intrinsics  \-ffree\-form  \-fno\-fixed\-form

\&\-fdollar\-ok  \-fimplicit\-none  \-fmax\-identifier\-length

\&\-std=\fR\fIstd\fR \fB\-fd\-lines\-as\-code  \-fd\-lines\-as\-comments

\&\-ffixed\-line\-length\-\fR\fIn\fR  \fB\-ffixed\-line\-length\-none

\&\-ffree\-line\-length\-\fR\fIn\fR  \fB\-ffree\-line\-length\-none

\&\-fdefault\-double\-8  \-fdefault\-integer\-8  \-fdefault\-real\-8

\&\-fcray\-pointer  \-fopenmp  \-frange\-check \-fno\-backslash\fR

.IP "\fIError and Warning Options\fR" 4

.IX Item "Error and Warning Options"

\&\fB\-fmax\-errors=\fR\fIn\fR

\&\fB\-fsyntax\-only  \-pedantic  \-pedantic\-errors

\&\-Wall  \-Waliasing  \-Wampersand  \-Wcharacter\-truncation  \-Wconversion

\&\-Wimplicit\-interface  \-Wline\-truncation  \-Wnonstd\-intrinsics  \-Wsurprising

\&\-Wno\-tabs  \-Wunderflow\fR

.IP "\fIDebugging Options\fR" 4

.IX Item "Debugging Options"

\&\fB\-fdump\-parse\-tree  \-ffpe\-trap=\fR\fIlist\fR

.IP "\fIDirectory Options\fR" 4

.IX Item "Directory Options"

\&\fB\-I\fR\fIdir\fR  \fB\-J\fR\fIdir\fR  \fB\-M\fR\fIdir\fR

.IP "\fIRuntime Options\fR" 4

.IX Item "Runtime Options"

\&\fB\-fconvert=\fR\fIconversion\fR  \fB\-frecord\-marker=\fR\fIlength\fR

\&\fB\-fmax\-subrecord\-length=\fR\fIlength\fR

.IP "\fICode Generation Options\fR" 4

.IX Item "Code Generation Options"

\&\fB\-fno\-automatic  \-ff2c  \-fno\-underscoring

\&\-fsecond\-underscore

\&\-fbounds\-check  \-fmax\-stack\-var\-size=\fR\fIn\fR

\&\fB\-fpack\-derived  \-frepack\-arrays  \-fshort\-enums\fR

.Sh "Options controlling Fortran dialect"

.IX Subsection "Options controlling Fortran dialect"

The following options control the details of the Fortran dialect

accepted by the compiler:

.IP "\fB\-ffree\-form\fR" 4

.IX Item "-ffree-form"

.PD 0

.IP "\fB\-ffixed\-form\fR" 4

.IX Item "-ffixed-form"

.PD

Specify the layout used by the source file.  The free form layout

was introduced in Fortran 90.  Fixed form was traditionally used in

older Fortran programs.  When neither option is specified, the source

form is determined by the file extension.

.IP "\fB\-fall\-intrinsics\fR" 4

.IX Item "-fall-intrinsics"

Accept all of the intrinsic procedures provided in libgfortran

without regard to the setting of \fB\-std\fR.  In particular,

this option can be quite useful with \fB\-std=f95\fR.  Additionally,

\&\fBgfortran\fR will ignore \fB\-Wnonstd\-intrinsics\fR.

.IP "\fB\-fd\-lines\-as\-code\fR" 4

.IX Item "-fd-lines-as-code"

.PD 0

.IP "\fB\-fd\-lines\-as\-comments\fR" 4

.IX Item "-fd-lines-as-comments"

.PD

Enable special treatment for lines beginning with \f(CW\*(C`d\*(C'\fR or \f(CW\*(C`D\*(C'\fR

in fixed form sources.  If the \fB\-fd\-lines\-as\-code\fR option is

given they are treated as if the first column contained a blank.  If the

\&\fB\-fd\-lines\-as\-comments\fR option is given, they are treated as

comment lines.

.IP "\fB\-fdefault\-double\-8\fR" 4

.IX Item "-fdefault-double-8"

Set the \f(CW\*(C`DOUBLE PRECISION\*(C'\fR type to an 8 byte wide type.

.IP "\fB\-fdefault\-integer\-8\fR" 4

.IX Item "-fdefault-integer-8"

Set the default integer and logical types to an 8 byte wide type.

Do nothing if this is already the default.

.IP "\fB\-fdefault\-real\-8\fR" 4

.IX Item "-fdefault-real-8"

Set the default real type to an 8 byte wide type.

Do nothing if this is already the default.

.IP "\fB\-fdollar\-ok\fR" 4

.IX Item "-fdollar-ok"

Allow \fB$\fR as a valid character in a symbol name.

.IP "\fB\-fno\-backslash\fR" 4

.IX Item "-fno-backslash"

Change the interpretation of backslashes in string literals from

\&\*(L"C\-style\*(R" escape characters to a single backslash character.

.IP "\fB\-ffixed\-line\-length\-\fR\fIn\fR" 4

.IX Item "-ffixed-line-length-n"

Set column after which characters are ignored in typical fixed-form

lines in the source file, and through which spaces are assumed (as

if padded to that length) after the ends of short fixed-form lines.

.Sp

Popular values for \fIn\fR include 72 (the

standard and the default), 80 (card image), and 132 (corresponding

to \*(L"extended\-source\*(R" options in some popular compilers).

\&\fIn\fR may also be \fBnone\fR, meaning that the entire line is meaningful

and that continued character constants never have implicit spaces appended

to them to fill out the line.

\&\fB\-ffixed\-line\-length\-0\fR means the same thing as

\&\fB\-ffixed\-line\-length\-none\fR.

.IP "\fB\-ffree\-line\-length\-\fR\fIn\fR" 4

.IX Item "-ffree-line-length-n"

Set column after which characters are ignored in typical free-form

lines in the source file. The default value is 132.

\&\fIn\fR may be \fBnone\fR, meaning that the entire line is meaningful.

\&\fB\-ffree\-line\-length\-0\fR means the same thing as

\&\fB\-ffree\-line\-length\-none\fR.

.IP "\fB\-fmax\-identifier\-length=\fR\fIn\fR" 4

.IX Item "-fmax-identifier-length=n"

Specify the maximum allowed identifier length. Typical values are

31 (Fortran 95) and 63 (Fortran 2003).

.IP "\fB\-fimplicit\-none\fR" 4

.IX Item "-fimplicit-none"

Specify that no implicit typing is allowed, unless overridden by explicit

\&\f(CW\*(C`IMPLICIT\*(C'\fR statements.  This is the equivalent of adding

\&\f(CW\*(C`implicit none\*(C'\fR to the start of every procedure.

.IP "\fB\-fcray\-pointer\fR" 4

.IX Item "-fcray-pointer"

Enable the Cray pointer extension, which provides C\-like pointer

functionality.

.IP "\fB\-fopenmp\fR" 4

.IX Item "-fopenmp"

Enable the OpenMP extensions.  This includes OpenMP \f(CW\*(C`!$omp\*(C'\fR directives

in free form

and \f(CW\*(C`c$omp\*(C'\fR, \f(CW*$omp\fR and \f(CW\*(C`!$omp\*(C'\fR directives in fixed form,

\&\f(CW\*(C`!$\*(C'\fR conditional compilation sentinels in free form

and \f(CW\*(C`c$\*(C'\fR, \f(CW\*(C`*$\*(C'\fR and \f(CW\*(C`!$\*(C'\fR sentinels in fixed form,

and when linking arranges for the OpenMP runtime library to be linked

in.

.IP "\fB\-frange\-check\fR" 4

.IX Item "-frange-check"

Enable range checking on results of simplification of constant

expressions during compilation.  For example, by default, \s-1GNU\s0 Fortran

will give an overflow error at compile time when simplifying \f(CW\*(C`a =

EXP(1000)\*(C'\fR. With \fB\-fno\-range\-check\fR, no error will be given and

the variable \f(CW\*(C`a\*(C'\fR will be assigned the value \f(CW\*(C`+Infinity\*(C'\fR.

Similarly, \f(CW\*(C`DATA i/Z'FFFFFFFF'/\*(C'\fR will result in an integer overflow

on most systems, but with \fB\-fno\-range\-check\fR the value will

\&\*(L"wrap around\*(R" and \f(CW\*(C`i\*(C'\fR will be initialized to \-1 instead.

.IP "\fB\-std=\fR\fIstd\fR" 4

.IX Item "-std=std"

Specify the standard to which the program is expected to conform, which

may be one of \fBf95\fR, \fBf2003\fR, \fBgnu\fR, or \fBlegacy\fR.

The default value for \fIstd\fR is \fBgnu\fR, which specifies a

superset of the Fortran 95 standard that includes all of the extensions

supported by \s-1GNU\s0 Fortran, although warnings will be given for obsolete

extensions not recommended for use in new code.  The \fBlegacy\fR value

is equivalent but without the warnings for obsolete extensions, and may

be useful for old non-standard programs.  The \fBf95\fR and

\&\fBf2003\fR values specify strict conformance to the Fortran 95 and

Fortran 2003 standards, respectively; errors are given for all

extensions beyond the relevant language standard, and warnings are given

for the Fortran 77 features that are permitted but obsolescent in later

standards.

.Sh "Options to request or suppress errors and warnings"

.IX Subsection "Options to request or suppress errors and warnings"

Errors are diagnostic messages that report that the \s-1GNU\s0 Fortran compiler

cannot compile the relevant piece of source code.  The compiler will

continue to process the program in an attempt to report further errors

to aid in debugging, but will not produce any compiled output.

.PP

Warnings are diagnostic messages that report constructions which

are not inherently erroneous but which are risky or suggest there is

likely to be a bug in the program.  Unless \fB\-Werror\fR is specified,

they do not prevent compilation of the program.

.PP

You can request many specific warnings with options beginning \fB\-W\fR,

for example \fB\-Wimplicit\fR to request warnings on implicit

declarations.  Each of these specific warning options also has a

negative form beginning \fB\-Wno\-\fR to turn off warnings;

for example, \fB\-Wno\-implicit\fR.  This manual lists only one of the

two forms, whichever is not the default.

.PP

These options control the amount and kinds of errors and warnings produced

by \s-1GNU\s0 Fortran:

.IP "\fB\-fmax\-errors\-\fR\fIn\fR" 4

.IX Item "-fmax-errors-n"

Limits the maximum number of error messages to \fIn\fR, at which point

\&\s-1GNU\s0 Fortran bails out rather than attempting to continue processing the

source code.  If \fIn\fR is 0, there is no limit on the number of error

messages produced.

.IP "\fB\-fsyntax\-only\fR" 4

.IX Item "-fsyntax-only"

Check the code for syntax errors, but don't do anything beyond that.

.IP "\fB\-pedantic\fR" 4

.IX Item "-pedantic"

Issue warnings for uses of extensions to Fortran 95.

\&\fB\-pedantic\fR also applies to C\-language constructs where they

occur in \s-1GNU\s0 Fortran source files, such as use of \fB\ee\fR in a

character constant within a directive like \f(CW\*(C`#include\*(C'\fR.

.Sp

Valid Fortran 95 programs should compile properly with or without

this option.

However, without this option, certain \s-1GNU\s0 extensions and traditional

Fortran features are supported as well.

With this option, many of them are rejected.

.Sp

Some users try to use \fB\-pedantic\fR to check programs for conformance.

They soon find that it does not do quite what they want\-\-\-it finds some

nonstandard practices, but not all.

However, improvements to \s-1GNU\s0 Fortran in this area are welcome.

.Sp

This should be used in conjunction with \fB\-std=f95\fR or

\&\fB\-std=f2003\fR.

.IP "\fB\-pedantic\-errors\fR" 4

.IX Item "-pedantic-errors"

Like \fB\-pedantic\fR, except that errors are produced rather than

warnings.

.IP "\fB\-Wall\fR" 4

.IX Item "-Wall"

Enables commonly used warning options pertaining to usage that

we recommend avoiding and that we believe are easy to avoid.

This currently includes \fB\-Waliasing\fR,

\&\fB\-Wampersand\fR, \fB\-Wsurprising\fR, \fB\-Wnonstd\-intrinsics\fR,

\&\fB\-Wno\-tabs\fR, and \fB\-Wline\-truncation\fR.

.IP "\fB\-Waliasing\fR" 4

.IX Item "-Waliasing"

Warn about possible aliasing of dummy arguments. Specifically, it warns

if the same actual argument is associated with a dummy argument with

\&\f(CW\*(C`INTENT(IN)\*(C'\fR and a dummy argument with \f(CW\*(C`INTENT(OUT)\*(C'\fR in a call

with an explicit interface.

.Sp

The following example will trigger the warning.

.Sp

.Vb 7

\&          interface

\&            subroutine bar(a,b)

\&              integer, intent(in) :: a

\&              integer, intent(out) :: b

\&            end subroutine

\&          end interface

\&          integer :: a

.Ve

.Sp

.Vb 1

\&          call bar(a,a)

.Ve

.IP "\fB\-Wampersand\fR" 4

.IX Item "-Wampersand"

Warn about missing ampersand in continued character constants. The warning is

given with \fB\-Wampersand\fR, \fB\-pedantic\fR, \fB\-std=f95\fR, and

\&\fB\-std=f2003\fR. Note: With no ampersand given in a continued character

constant, \s-1GNU\s0 Fortran assumes continuation at the first non\-comment,

non-whitespace character after the ampersand that initiated the continuation.

.IP "\fB\-Wcharacter\-truncation\fR" 4

.IX Item "-Wcharacter-truncation"

Warn when a character assignment will truncate the assigned string.

.IP "\fB\-Wconversion\fR" 4

.IX Item "-Wconversion"

Warn about implicit conversions between different types.

.IP "\fB\-Wimplicit\-interface\fR" 4

.IX Item "-Wimplicit-interface"

Warn if a procedure is called without an explicit interface.

Note this only checks that an explicit interface is present.  It does not

check that the declared interfaces are consistent across program units.

.IP "\fB\-Wnonstd\-intrinsics\fR" 4

.IX Item "-Wnonstd-intrinsics"

Warn if the user tries to use an intrinsic that does not belong to the

standard the user has chosen via the \fB\-std\fR option.

.IP "\fB\-Wsurprising\fR" 4

.IX Item "-Wsurprising"

Produce a warning when \*(L"suspicious\*(R" code constructs are encountered.

While technically legal these usually indicate that an error has been made.

.Sp

This currently produces a warning under the following circumstances:

.RS 4

.IP "*" 4

An \s-1INTEGER\s0 \s-1SELECT\s0 construct has a \s-1CASE\s0 that can never be matched as its

lower value is greater than its upper value.

.IP "*" 4

A \s-1LOGICAL\s0 \s-1SELECT\s0 construct has three \s-1CASE\s0 statements.

.RE

.RS 4

.RE

.IP "\fB\-Wtabs\fR" 4

.IX Item "-Wtabs"

By default, tabs are accepted as whitespace, but tabs are not members

of the Fortran Character Set.  \fB\-Wno\-tabs\fR will cause a warning

to be issued if a tab is encountered. Note, \fB\-Wno\-tabs\fR is active

for \fB\-pedantic\fR, \fB\-std=f95\fR, \fB\-std=f2003\fR, and

\&\fB\-Wall\fR.

.IP "\fB\-Wunderflow\fR" 4

.IX Item "-Wunderflow"

Produce a warning when numerical constant expressions are

encountered, which yield an \s-1UNDERFLOW\s0 during compilation.

.IP "\fB\-Werror\fR" 4

.IX Item "-Werror"

Turns all warnings into errors.

.PP

Some of these have no effect when compiling programs written in Fortran.

.Sh "Options for debugging your program or \s-1GNU\s0 Fortran"

.IX Subsection "Options for debugging your program or GNU Fortran"

\&\s-1GNU\s0 Fortran has various special options that are used for debugging

either your program or the \s-1GNU\s0 Fortran compiler.

.IP "\fB\-fdump\-parse\-tree\fR" 4

.IX Item "-fdump-parse-tree"

Output the internal parse tree before starting code generation.  Only

really useful for debugging the \s-1GNU\s0 Fortran compiler itself.

.IP "\fB\-ffpe\-trap=\fR\fIlist\fR" 4

.IX Item "-ffpe-trap=list"

Specify a list of \s-1IEEE\s0 exceptions when a Floating Point Exception

(\s-1FPE\s0) should be raised.  On most systems, this will result in a \s-1SIGFPE\s0

signal being sent and the program being interrupted, producing a core

file useful for debugging.  \fIlist\fR is a (possibly empty) comma-separated

list of the following \s-1IEEE\s0 exceptions: \fBinvalid\fR (invalid floating

point operation, such as \f(CW\*(C`SQRT(\-1.0)\*(C'\fR), \fBzero\fR (division by

zero), \fBoverflow\fR (overflow in a floating point operation),

\&\fBunderflow\fR (underflow in a floating point operation),

\&\fBprecision\fR (loss of precision during operation) and \fBdenormal\fR

(operation produced a denormal value).

.Sh "Options for directory search"

.IX Subsection "Options for directory search"

These options affect how \s-1GNU\s0 Fortran searches

for files specified by the \f(CW\*(C`INCLUDE\*(C'\fR directive and where it searches

for previously compiled modules.

.PP

It also affects the search paths used by \fBcpp\fR when used to preprocess

Fortran source.

.IP "\fB\-I\fR\fIdir\fR" 4

.IX Item "-Idir"

These affect interpretation of the \f(CW\*(C`INCLUDE\*(C'\fR directive

(as well as of the \f(CW\*(C`#include\*(C'\fR directive of the \fBcpp\fR

preprocessor).

.Sp

Also note that the general behavior of \fB\-I\fR and

\&\f(CW\*(C`INCLUDE\*(C'\fR is pretty much the same as of \fB\-I\fR with

\&\f(CW\*(C`#include\*(C'\fR in the \fBcpp\fR preprocessor, with regard to

looking for \fIheader.gcc\fR files and other such things.

.Sp

This path is also used to search for \fI.mod\fR files when previously

compiled modules are required by a \f(CW\*(C`USE\*(C'\fR statement.

.IP "\fB\-M\fR\fIdir\fR" 4

.IX Item "-Mdir"

.PD 0

.IP "\fB\-J\fR\fIdir\fR" 4

.IX Item "-Jdir"

.PD

This option specifies where to put \fI.mod\fR files for compiled modules.

It is also added to the list of directories to searched by an \f(CW\*(C`USE\*(C'\fR

statement.

.Sp

The default is the current directory.

.Sp

\&\fB\-J\fR is an alias for \fB\-M\fR to avoid conflicts with existing

\&\s-1GCC\s0 options.

.Sh "Influencing runtime behavior"

.IX Subsection "Influencing runtime behavior"

These options affect the runtime behavior of programs compiled with \s-1GNU\s0 Fortran.

.IP "\fB\-fconvert=\fR\fIconversion\fR" 4

.IX Item "-fconvert=conversion"

Specify the representation of data for unformatted files.  Valid

values for conversion are: \fBnative\fR, the default; \fBswap\fR,

swap between big\- and little\-endian; \fBbig-endian\fR, use big-endian

representation for unformatted files; \fBlittle-endian\fR, use little-endian

representation for unformatted files.

.Sp

\&\fIThis option has an effect only when used in the main program.

The \f(CI\*(C`CONVERT\*(C'\fI specifier and the \s-1GFORTRAN_CONVERT_UNIT\s0 environment

variable override the default specified by \f(BI\-fconvert\fI.\fR

.IP "\fB\-frecord\-marker=\fR\fIlength\fR" 4

.IX Item "-frecord-marker=length"

Specify the length of record markers for unformatted files.

Valid values for \fIlength\fR are 4 and 8.  Default is 4.

\&\fIThis is different from previous versions of\fR \fBgfortran\fR,

which specified a default record marker length of 8 on most

systems.  If you want to read or write files compatible

with earlier versions of \fBgfortran\fR, use \fB\-frecord\-marker=8\fR.

.IP "\fB\-fmax\-subrecord\-length=\fR\fIlength\fR" 4

.IX Item "-fmax-subrecord-length=length"

Specify the maximum length for a subrecord.  The maximum permitted

value for length is 2147483639, which is also the default.  Only

really useful for use by the gfortran testsuite.

.Sh "Options for code generation conventions"

.IX Subsection "Options for code generation conventions"

These machine-independent options control the interface conventions

used in code generation.

.PP

Most of them have both positive and negative forms; the negative form

of \fB\-ffoo\fR would be \fB\-fno\-foo\fR.  In the table below, only

one of the forms is listed\-\-\-the one which is not the default.  You

can figure out the other form by either removing \fBno\-\fR or adding

it.

.IP "\fB\-fno\-automatic\fR" 4

.IX Item "-fno-automatic"

Treat each program unit as if the \f(CW\*(C`SAVE\*(C'\fR statement was specified for

every local variable and array referenced in it. Does not affect common

blocks. (Some Fortran compilers provide this option under the name

\&\fB\-static\fR.)

.IP "\fB\-ff2c\fR" 4

.IX Item "-ff2c"

Generate code designed to be compatible with code generated

by \fBg77\fR and \fBf2c\fR.

.Sp

The calling conventions used by \fBg77\fR (originally implemented

in \fBf2c\fR) require functions that return type

default \f(CW\*(C`REAL\*(C'\fR to actually return the C type \f(CW\*(C`double\*(C'\fR, and

functions that return type \f(CW\*(C`COMPLEX\*(C'\fR to return the values via an

extra argument in the calling sequence that points to where to

store the return value.  Under the default \s-1GNU\s0 calling conventions, such

functions simply return their results as they would in \s-1GNU\s0

C\-\-\-default \f(CW\*(C`REAL\*(C'\fR functions return the C type \f(CW\*(C`float\*(C'\fR, and

\&\f(CW\*(C`COMPLEX\*(C'\fR functions return the \s-1GNU\s0 C type \f(CW\*(C`complex\*(C'\fR.

Additionally, this option implies the \fB\-fsecond\-underscore\fR

option, unless \fB\-fno\-second\-underscore\fR is explicitly requested.

.Sp

This does not affect the generation of code that interfaces with

the \fBlibgfortran\fR library.

.Sp

\&\fICaution:\fR It is not a good idea to mix Fortran code compiled with

\&\fB\-ff2c\fR with code compiled with the default \fB\-fno\-f2c\fR

calling conventions as, calling \f(CW\*(C`COMPLEX\*(C'\fR or default \f(CW\*(C`REAL\*(C'\fR

functions between program parts which were compiled with different

calling conventions will break at execution time.

.Sp

\&\fICaution:\fR This will break code which passes intrinsic functions

of type default \f(CW\*(C`REAL\*(C'\fR or \f(CW\*(C`COMPLEX\*(C'\fR as actual arguments, as

the library implementations use the \fB\-fno\-f2c\fR calling conventions.

.IP "\fB\-fno\-underscoring\fR" 4

.IX Item "-fno-underscoring"

Do not transform names of entities specified in the Fortran

source file by appending underscores to them.

.Sp

With \fB\-funderscoring\fR in effect, \s-1GNU\s0 Fortran appends one

underscore to external names with no underscores.  This is done to ensure

compatibility with code produced by many \s-1UNIX\s0 Fortran compilers.

.Sp

\&\fICaution\fR: The default behavior of \s-1GNU\s0 Fortran is

incompatible with \fBf2c\fR and \fBg77\fR, please use the

\&\fB\-ff2c\fR option if you want object files compiled with

\&\s-1GNU\s0 Fortran to be compatible with object code created with these

tools.

.Sp

Use of \fB\-fno\-underscoring\fR is not recommended unless you are

experimenting with issues such as integration of \s-1GNU\s0 Fortran into

existing system environments (vis\-a\-vis existing libraries, tools, and

so on).

.Sp

For example, with \fB\-funderscoring\fR, and assuming other defaults like

\&\fB\-fcase\-lower\fR and that \f(CW\*(C`j()\*(C'\fR and \f(CW\*(C`max_count()\*(C'\fR are

external functions while \f(CW\*(C`my_var\*(C'\fR and \f(CW\*(C`lvar\*(C'\fR are local variables,

a statement like

.Sp

.Vb 1

\&        I = J() + MAX_COUNT (MY_VAR, LVAR)

.Ve

.Sp

is implemented as something akin to:

.Sp

.Vb 1

\&        i = j_() + max_count__(&my_var__, &lvar);

.Ve

.Sp

With \fB\-fno\-underscoring\fR, the same statement is implemented as:

.Sp

.Vb 1

\&        i = j() + max_count(&my_var, &lvar);

.Ve

.Sp

Use of \fB\-fno\-underscoring\fR allows direct specification of

user-defined names while debugging and when interfacing \s-1GNU\s0 Fortran

code with other languages.

.Sp

Note that just because the names match does \fInot\fR mean that the

interface implemented by \s-1GNU\s0 Fortran for an external name matches the

interface implemented by some other language for that same name.

That is, getting code produced by \s-1GNU\s0 Fortran to link to code produced

by some other compiler using this or any other method can be only a

small part of the overall solution\-\-\-getting the code generated by

both compilers to agree on issues other than naming can require

significant effort, and, unlike naming disagreements, linkers normally

cannot detect disagreements in these other areas.

.Sp

Also, note that with \fB\-fno\-underscoring\fR, the lack of appended

underscores introduces the very real possibility that a user-defined

external name will conflict with a name in a system library, which

could make finding unresolved-reference bugs quite difficult in some

cases\-\-\-they might occur at program run time, and show up only as

buggy behavior at run time.

.Sp

In future versions of \s-1GNU\s0 Fortran we hope to improve naming and linking

issues so that debugging always involves using the names as they appear

in the source, even if the names as seen by the linker are mangled to

prevent accidental linking between procedures with incompatible

interfaces.

.IP "\fB\-fsecond\-underscore\fR" 4

.IX Item "-fsecond-underscore"

By default, \s-1GNU\s0 Fortran appends an underscore to external

names.  If this option is used \s-1GNU\s0 Fortran appends two

underscores to names with underscores and one underscore to external names

with no underscores.  \s-1GNU\s0 Fortran also appends two underscores to

internal names with underscores to avoid naming collisions with external

names.

.Sp

This option has no effect if \fB\-fno\-underscoring\fR is

in effect.  It is implied by the \fB\-ff2c\fR option.

.Sp

Otherwise, with this option, an external name such as \f(CW\*(C`MAX_COUNT\*(C'\fR

is implemented as a reference to the link-time external symbol

\&\f(CW\*(C`max_count_\|_\*(C'\fR, instead of \f(CW\*(C`max_count_\*(C'\fR.  This is required

for compatibility with \fBg77\fR and \fBf2c\fR, and is implied

by use of the \fB\-ff2c\fR option.

.IP "\fB\-fbounds\-check\fR" 4

.IX Item "-fbounds-check"

Enable generation of run-time checks for array subscripts

and against the declared minimum and maximum values.  It also

checks array indices for assumed and deferred

shape arrays against the actual allocated bounds.

.Sp

In the future this may also include other forms of checking, e.g., checking

substring references.

.IP "\fB\-fmax\-stack\-var\-size=\fR\fIn\fR" 4

.IX Item "-fmax-stack-var-size=n"

This option specifies the size in bytes of the largest array that will be put

on the stack.

.Sp

This option currently only affects local arrays declared with constant

bounds, and may not apply to all character variables.

Future versions of \s-1GNU\s0 Fortran may improve this behavior.

.Sp

The default value for \fIn\fR is 32768.

.IP "\fB\-fpack\-derived\fR" 4

.IX Item "-fpack-derived"

This option tells \s-1GNU\s0 Fortran to pack derived type members as closely as

possible.  Code compiled with this option is likely to be incompatible

with code compiled without this option, and may execute slower.

.IP "\fB\-frepack\-arrays\fR" 4

.IX Item "-frepack-arrays"

In some circumstances \s-1GNU\s0 Fortran may pass assumed shape array

sections via a descriptor describing a noncontiguous area of memory.

This option adds code to the function prologue to repack the data into

a contiguous block at runtime.

.Sp

This should result in faster accesses to the array.  However it can introduce

significant overhead to the function call, especially  when the passed data

is noncontiguous.

.IP "\fB\-fshort\-enums\fR" 4

.IX Item "-fshort-enums"

This option is provided for interoperability with C code that was

compiled with the \fB\-fshort\-enums\fR option.  It will make

\&\s-1GNU\s0 Fortran choose the smallest \f(CW\*(C`INTEGER\*(C'\fR kind a given

enumerator set will fit in, and give all its enumerators this kind.

.SH "ENVIRONMENT"

.IX Header "ENVIRONMENT"

The \fBgfortran\fR compiler currently does not make use of any environment

variables to control its operation above and beyond those

that affect the operation of \fBgcc\fR.

.SH "BUGS"

.IX Header "BUGS"

For instructions on reporting bugs, see

<\fBhttp://gcc.gnu.org/bugs.html\fR>.

.SH "SEE ALSO"

.IX Header "SEE ALSO"

\&\fIgpl\fR\|(7), \fIgfdl\fR\|(7), \fIfsf\-funding\fR\|(7),

\&\fIcpp\fR\|(1), \fIgcov\fR\|(1), \fIgcc\fR\|(1), \fIas\fR\|(1), \fIld\fR\|(1), \fIgdb\fR\|(1), \fIadb\fR\|(1), \fIdbx\fR\|(1), \fIsdb\fR\|(1)

and the Info entries for \fIgcc\fR, \fIcpp\fR, \fIgfortran\fR, \fIas\fR,

\&\fIld\fR, \fIbinutils\fR and \fIgdb\fR.

.SH "AUTHOR"

.IX Header "AUTHOR"

See the Info entry for \fBgfortran\fR for contributors to \s-1GCC\s0 and

\&\s-1GNU\s0 Fortran.

.SH "COPYRIGHT"

.IX Header "COPYRIGHT"

Copyright (c) 2004, 2005, 2006, 2007

Free Software Foundation, Inc.

.PP

Permission is granted to copy, distribute and/or modify this document

under the terms of the \s-1GNU\s0 Free Documentation License, Version 1.2 or

any later version published by the Free Software Foundation; with the

Invariant Sections being \*(L"\s-1GNU\s0 General Public License\*(R" and \*(L"Funding

Free Software\*(R", the Front-Cover texts being (a) (see below), and with

the Back-Cover Texts being (b) (see below).  A copy of the license is

included in the \fIgfdl\fR\|(7) man page.

.PP

(a) The \s-1FSF\s0's Front-Cover Text is:

.PP

.Vb 1

\&     A GNU Manual

.Ve

.PP

(b) The \s-1FSF\s0's Back-Cover Text is:

.PP

.Vb 3

\&     You have freedom to copy and modify this GNU Manual, like GNU

\&     software.  Copies published by the Free Software Foundation raise

\&     funds for GNU development.

.Ve