path: root/gnu/usr.bin/gcc/f/g77.info-2

This is Info file g77.info, produced by Makeinfo-1.64 from the input
file g77.texi.

   This file explains how to use the GNU Fortran system.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

   Copyright (C) 1995, 1996 Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.

   Contributed by James Craig Burley (`burley@gnu.ai.mit.edu').
Inspired by a first pass at translating `g77-0.5.16/f/DOC' that was
contributed to Craig by David Ronis (`ronis@onsager.chem.mcgill.ca').


File: g77.info,  Node: Overall Options,  Next: Fortran Dialect Options,  Prev: Option Summary,  Up: Invoking G77

Options Controlling the Kind of Output
======================================

   Compilation can involve as many as four stages: preprocessing,
compilation proper, assembly, and linking, always in that order.  The
first three stages apply to an individual source file, and end by
producing an object file; linking combines all the object files (those
newly compiled, and those specified as input) into an executable file.

   For any given input file, the file name suffix determines what kind
of compilation is done.  Suffixes specific to GNU Fortran are listed
below.  *Note gcc: (Using and Porting GNU CC)Overall Options, for
information on suffixes recognized by GNU CC.

`FILE.f'
`FILE.for'
     Fortran source code that should not be preprocessed.

`FILE.F'
`FILE.fpp'
     Fortran source code that must be preprocessed (by the C
     preprocessor `cpp', which is part of GNU CC).

   UNIX users typically use the `FILE.f' and `FILE.F' nomenclature.
Users of other operating systems, especially those that cannot
distinguish upper-case letters from lower-case letters in their file
names, typically use the `FILE.for' and `FILE.fpp' nomenclature.

   Use of the preprocessor `cpp' allows use of C-like constructs such
as `#define' and `#include', but can lead to unexpected, even mistaken,
results due to Fortran's source file format.  It is recommended that
use of the C preprocessor be limited to `#include' and, in conjunction
with `#define', only `#if' and related directives, thus avoiding
in-line macro expansion entirely.  This recommendation applies
especially when using the traditional fixed source form.  With free
source form, fewer unexpected transformations are likely to happen, but
use of Hollerith and things like continued character constants can
nevertheless present problems.

   The following options that affect overall processing are recognized
by the `g77' and `gcc' commands in a GNU Fortran installation:

`--driver=COMMAND'
     This works only when invoking the `g77' command, not when invoking
     the `gcc' command.  *Note GNU Fortran Command Options: Invoking
     G77, for information on this option.

`-fversion'
     Ensure that the `g77'-specific version of the compiler phase is
     reported, if run.  (This is supplied automatically when `-v' or
     `--version' is specified as a command-line option for `g77' or
     `gcc' and when the resulting commands compile Fortran source
     files.)

`-fset-g77-defaults'
     Set up whatever `gcc' options are to apply to Fortran compilations.
     For version 0.5.18, this is equivalent to `-fmove-all-movables
     -freduce-all-givs -frerun-loop-opt'.  (This is supplied
     automatically when compiling Fortran code.  The description of
     this option is here so that users seeing it in the output of, say,
     `g77 -v' understand why it is there.  Also, developers who run
     `f771' directly might want to specify it by hand to get the same
     defaults as they would running `f771' via `g77' or `gcc'.)

   *Note Options Controlling the Kind of Output: (gcc)Overall Options,
for information on more options that control the overall operation of
the `gcc' command (and, by extension, the `g77' command).


File: g77.info,  Node: Fortran Dialect Options,  Next: Warning Options,  Prev: Overall Options,  Up: Invoking G77

Options Controlling Fortran Dialect
===================================

   The following options control the dialect of Fortran that the
compiler accepts:

`-ffree-form'
`-fno-fixed-form'
     Specify that the source file is written in free form (introduced
     in Fortran 90) instead of the more-traditional fixed form.

`-ff90'
     Allow certain Fortran-90 constructs.

     This option controls whether certain Fortran 90 constructs are
     recognized.  (Other Fortran 90 constructs might or might not be
     recognized depending on other options such as `-fvxt-not-f90',
     `-ff90-intrinsics-enable', and the current level of support for
     Fortran 90.)

     *Note GNU Fortran Extensions: Extensions, for more information.

`-fvxt-not-f90'
`-ff90-not-vxt'
     Specify whether Fortran 90 or other popular extensions are to be
     assumed for ambiguous constructs.  The default is -fvxt-not-f90.

     For example, with `-ff90-not-vxt', `PRINT *,"double-quoted
     string"' is valid, while with `-fvxt-not-f90', `PRINT *,"2000' is
     valid.

     (There is no way to allow both constructs in the general case,
     since statements like `PRINT *,"2000 !comment?"' would be
     ambiguous.)

     *Note GNU Fortran Dialects: Dialects, for more information.

`-fdollar-ok'
     Allow `$' as a valid character in a symbol name.

`-fno-backslash'
     Specify that `\' is not to be specially interpreted in character
     and Hollerith constants a la C and many UNIX Fortran compilers.

     For example, with `-fbackslash' in effect, `A\nB' specifies three
     characters, with the second one being newline.  With
     `-fno-backslash', it specifies four characters, `A', `\', `n', and
     `B'.

     Note that `g77' implements a fairly general form of backslash
     processing that is incompatible with the narrower forms supported
     by some other compilers.  For example, `'A\003B'' is a
     three-character string in `g77', whereas other compilers that
     support backslash might not support the three-octal-digit form,
     and thus treat that string as longer than three characters.

     *Note Certain Changes We Don't Want to Make: Non-bugs, for
     information on why `-fbackslash' is the default instead of
     `-fno-backslash'.

`-fintrin-case-initcap'
`-fintrin-case-upper'
`-fintrin-case-lower'
`-fintrin-case-any'
     Specify expected case for intrinsic names.  `-fintrin-case-lower'
     is the default.

`-fmatch-case-initcap'
`-fmatch-case-upper'
`-fmatch-case-lower'
`-fmatch-case-any'
     Specify expected case for keywords.  `-fmatch-case-lower' is the
     default.

`-fsource-case-upper'
`-fsource-case-lower'
`-fsource-case-preserve'
     Specify whether source text other than character and Hollerith
     constants is to be translated to uppercase, to lowercase, or
     preserved as is.  `-fsource-case-lower' is the default.

`-fsymbol-case-initcap'
`-fsymbol-case-upper'
`-fsymbol-case-lower'
`-fsymbol-case-any'
     Specify valid cases for user-defined symbol names.
     `-fsymbol-case-any' is the default.

`-fcase-strict-upper'
     Same as `-fintrin-case-upper -fmatch-case-upper
     -fsource-case-preserve -fsymbol-case-upper'.  (Requires all
     pertinent source to be in uppercase.)

`-fcase-strict-lower'
     Same as `-fintrin-case-lower -fmatch-case-lower
     -fsource-case-preserve -fsymbol-case-lower'.  (Requires all
     pertinent source to be in lowercase.)

`-fcase-initcap'
     Same as `-fintrin-case-initcap -fmatch-case-initcap
     -fsource-case-preserve -fsymbol-case-initcap'.  (Requires all
     pertinent source to be in initial capitals, as in `Print
     *,SqRt(Value)'.)

`-fcase-upper'
     Same as `-fintrin-case-any -fmatch-case-any -fsource-case-upper
     -fsymbol-case-any'.  (Maps all pertinent source to uppercase.)

`-fcase-lower'
     Same as `-fintrin-case-any -fmatch-case-any -fsource-case-lower
     -fsymbol-case-any'.  (Maps all pertinent source to lowercase.)

`-fcase-preserve'
     Same as `-fintrin-case-any -fmatch-case-any -fsource-case-preserve
     -fsymbol-case-any'.  (Preserves all case in user-defined symbols,
     while allowing any-case matching of intrinsics and keywords.  For
     example, `call Foo(i,I)' would pass two *different* variables
     named `i' and `I' to a procedure named `Foo'.)

`-fdcp-intrinsics-delete'
`-fdcp-intrinsics-hide'
`-fdcp-intrinsics-disable'
`-fdcp-intrinsics-enable'
     Specify status of Digital's COMPLEX-related intrinsics.
     `-fdcp-intrinsics-enable' is the default.

`-ff2c-intrinsics-delete'
`-ff2c-intrinsics-hide'
`-ff2c-intrinsics-disable'
`-ff2c-intrinsics-enable'
     Specify status of f2c-specific intrinsics.
     `-ff2c-intrinsics-enable' is the default.

`-ff90-intrinsics-delete'
`-ff90-intrinsics-hide'
`-ff90-intrinsics-disable'
`-ff90-intrinsics-enable'
     Specify status of F90-specific intrinsics.
     `-ff90-intrinsics-delete' is the default.

`-fmil-intrinsics-delete'
`-fmil-intrinsics-hide'
`-fmil-intrinsics-disable'
`-fmil-intrinsics-enable'
     Specify status of MIL-STD-1753-specific intrinsics.
     `-fmil-intrinsics-enable' is the default.

`-funix-intrinsics-delete'
`-funix-intrinsics-hide'
`-funix-intrinsics-disable'
`-funix-intrinsics-enable'
     Specify status of UNIX intrinsics.  `-funix-intrinsics-enable' is
     the default.

     For example, if your code invokes `FLUSH' as a library function
     and thus works with other UNIX Fortran compilers or earlier
     version of `g77', either add the `EXTERNAL FLUSH' statement or,
     perhaps more convenient for you, compile with the
     -funix-intrinsics-hide or -funix-intrinsics-delete option.

     Note that `ABORT', `EXIT', `FLUSH', `SIGNAL', and `SYSTEM' are
     intrinsic subroutines, not functions (since they have side
     effects), so to get the return values from `SIGNAL' and `SYSTEM',
     append a final argument specifying an `INTEGER' variable or array
     element to receive the returned status.  (For example, `CALL
     SYSTEM('rm foo',ISTAT)'.)

     `FLUSH()' accepts an optional single `INTEGER' argument, since
     many Fortran implementations allow or require a unit number.
     Currently, since `libf2c' does not flush a given unit number, this
     argument is not used--all units are flushed by `libf2c''s
     implementation of `FLUSH()'.  Do not depend on this behavior--if
     you want to flush all units, use `CALL FLUSH' (that is, specify no
     arguments to `FLUSH').

     `EXIT()' accepts an optional single `INTEGER' argument.  If
     omitted, zero is the default (as in `CALL EXIT(0)').  The default
     might change on configurations where the "normal return status" is
     not zero, however.  If you want to return a "success" status, it
     is best to call `EXIT' with no arguments in your code, and let
     `g77' choose the appropriate default.

`-fvxt-intrinsics-delete'
`-fvxt-intrinsics-hide'
`-fvxt-intrinsics-disable'
`-fvxt-intrinsics-enable'
     Specify status of VXT intrinsics.  `-fvxt-intrinsics-delete' is
     the default.

`-ffixed-line-length-N'
     Set column after which characters are ignored in typical fixed-form
     lines in the source file.

     Popular values for N include 72 (the standard and the default), 80
     (card image), and 132 (corresponds to "extended-source" options in
     some popular compilers).  N may be `none', meaning that the entire
     line is meaningful and that continued character constants never
     have implicit spaces appended to them to fill out the line.
     `-ffixed-line-length-0' means the same thing as
     `-ffixed-line-length-none'.


File: g77.info,  Node: Warning Options,  Next: Debugging Options,  Prev: Fortran Dialect Options,  Up: Invoking G77

Options to Request or Suppress Warnings
=======================================

   Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there might
have been an error.

   You can request many specific warnings with options beginning `-W',
for example `-Wimplicit' to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'.
This manual lists only one of the two forms, whichever is not the
default.

   These options control the amount and kinds of warnings produced by
GNU Fortran:

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

`-pedantic'
     Issue warnings for uses of extensions to ANSI FORTRAN 77.
     `-pedantic' also applies to C-language constructs where they occur
     in GNU Fortran source files, such as use of `\e' in a character
     constant within a directive like `#include'.

     Valid ANSI FORTRAN 77 programs should compile properly with or
     without this option.  However, without this option, certain GNU
     extensions and traditional Fortran features are supported as well.
     With this option, many of them are rejected.

     Some users try to use `-pedantic' to check programs for strict ANSI
     conformance.  They soon find that it does not do quite what they
     want: it finds some non-ANSI practices, but not all--however,
     improvements to `g77' in this area are welcome.

`-pedantic-errors'
     Like `-pedantic', except that errors are produced rather than
     warnings.

`-fpedantic'
     Like `-pedantic', but applies only to Fortran constructs.

`-fugly'
     Specify that certain "ugly" constructs are to be quietly accepted.
     Implies `-fugly-args' and `-fugly-init' as well.

     For example, with `-fno-ugly', `CALL FOO(,)' means to pass one
     null argument, whereas with `-fugly', it means to pass two null
     arguments.

`-fno-ugly-args'
     Disallow passing Hollerith and typeless constants as actual
     arguments (for example, `CALL FOO(4HABCD)').

`-fno-ugly-init'
     Disallow use of Hollerith and typeless constants as initial values
     (in `PARAMETER' and `DATA' statements), and use of character
     constants to initialize numeric types and vice versa.

     For example, `DATA I/'F'/, CHRVAR/65/, J/4HABCD/' is disallowed by
     `-fno-ugly-init'.

`-w'
     Inhibit all warning messages.

`-Wimplicit'
     Warn whenever a variable, array, or function is implicitly
     declared.  Has an effect similar to using the `IMPLICIT NONE'
     statement in every program unit.  (Some Fortran compilers provide
     this feature by an option named `-u' or `/WARNINGS=DECLARATIONS'.)

`-Wunused'
     Warn whenever a variable is unused aside from its declaration.

`-Wuninitialized'
     Warn whenever an automatic variable is used without first being
     initialized.

     These warnings are possible only in optimizing compilation,
     because they require data flow information that is computed only
     when optimizing.  If you don't specify `-O', you simply won't get
     these warnings.

     These warnings occur only for variables that are candidates for
     register allocation.  Therefore, they do not occur for a variable
     that is declared `volatile', or whose address is taken, or whose
     size is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for
     arrays, even when they are in registers.

     Note that there may be no warning about a variable that is used
     only to compute a value that itself is never used, because such
     computations may be deleted by data flow analysis before the
     warnings are printed.

     These warnings are made optional because GNU Fortran is not smart
     enough to see all the reasons why the code might be correct
     despite appearing to have an error.  Here is one example of how
     this can happen:

          SUBROUTINE DISPAT(J)
          IF (J.EQ.1) I=1
          IF (J.EQ.2) I=4
          IF (J.EQ.3) I=5
          CALL FOO(I)
          END

     If the value of `J' is always 1, 2 or 3, then `I' is always
     initialized, but GNU Fortran doesn't know this.  Here is another
     common case:

          SUBROUTINE MAYBE(FLAG)
          LOGICAL FLAG
          IF (FLAG) VALUE = 3.14
          ...
          IF (FLAG) PRINT *, VALUE
          END

     This has no bug because `VALUE' is used only if it is set.

`-Wall'
     The `-Wunused' and `-Wuninitialized' options combined.  These are
     all the options which pertain to usage that we recommend avoiding
     and that we believe is easy to avoid.  (As more warnings are added
     to `g77', some might be added to the list enabled by `-Wall'.)

   The remaining `-W...' options are not implied by `-Wall' because
they warn about constructions that we consider reasonable to use, on
occasion, in clean programs.

`-Wsurprising'
     Warn about "suspicious" constructs that are interpreted by the
     compiler in a way that might well be surprising to someone reading
     the code.  These differences can result in subtle,
     compiler-dependent (even machine-dependent) behavioral differences.
     The constructs warned about include:

        * Expressions having two arithmetic operators in a row, such as
          `X*-Y'.  Such a construct is nonstandard, and can produce
          unexpected results in more complicated situations such as
          `X**-Y*Z'.  `g77', along with many other compilers, interprets
          this example differently than many programmers, and a few
          other compilers.  Specifically, `g77' interprets `X**-Y*Z' as
          `(X**(-Y))*Z', while others might think it should be
          interpreted as `X**(-(Y*Z))'.

          A revealing example is the constant expression `2**-2*1.',
          which `g77' evaluates to .25, while others might evaluate it
          to 0., the difference being the way precedence affects type
          promotion.

          (The `-fpedantic' option also warns about expressions having
          two arithmetic operators in a row.)

        * Expressions with a unary minus followed by an operand and then
          a binary operator other than plus or minus.  For example,
          `-2**2' produces a warning, because the precedence is
          `-(2**2)', yielding -4, not `(-2)**2', which yields 4, and
          which might represent what a programmer expects.  Even cases
          such as `-I*J' produce warnings, even though, in most
          configurations and situations, there is no computational
          difference between the results of the two
          interpretations--the purpose of this warning is to warn about
          differing interpretations and encourage a better style of
          coding, not to identify only those places where bugs might
          exist in the user's code.

        * `DO' loops with `DO' variables that are not of integral
          type--that is, using `REAL' or `DOUBLE PRECISION' variables
          as loop control variables.  Although such loops can be
          written to work in the "obvious" way, the way `g77' is
          required by the Fortran standard to interpret such code is
          likely to be quite different from the way many programmers
          expect.  (This is true of all `DO' loops, but the differences
          are pronounced for non-integral loop control variables.)

          *Note Loops::, for more information.

`-Werror'
     Make all warnings into errors.

`-W'
     Turns on "extra warnings" and the `uninitialized' option.  (This
     might change in future versions of `g77'.)

     "Extra warnings" are issued for:

        * Unused parameters to a procedure (when `-Wunused' also is
          specified).

        * Overflows involving floating-point constants (not available
          for certain configurations?).

   *Note Options to Request or Suppress Warnings: (gcc)Warning Options,
for information on more options offered by the GBE shared by `g77',
`gcc', and other GNU compilers.

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

`-Wcomment'
`-Wformat'
`-Wparentheses'
`-Wswitch'
`-Wtraditional'
`-Wshadow'
`-Wid-clash-LEN'
`-Wlarger-than-LEN'
`-Wconversion'
`-Waggregate-return'
`-Wredundant-decls'
     These options all could have some relevant meaning for GNU Fortran
     programs, but are not yet supported.


File: g77.info,  Node: Debugging Options,  Next: Optimize Options,  Prev: Warning Options,  Up: Invoking G77

Options for Debugging Your Program or GNU Fortran
=================================================

   GNU Fortran has various special options that are used for debugging
either your program or `g77'.

`-g'
     Produce debugging information in the operating system's native
     format (stabs, COFF, XCOFF, or DWARF).  GDB can work with this
     debugging information.

     Support for this option in Fortran programs is incomplete.  In
     particular, names of variables and arrays in common blocks or that
     are storage-associated via `EQUIVALENCE' are unavailable to the
     debugger.

   *Note Options for Debugging Your Program or GNU CC: (gcc)Debugging
Options, for more information on debugging options.


File: g77.info,  Node: Optimize Options,  Next: Preprocessor Options,  Prev: Debugging Options,  Up: Invoking G77

Options That Control Optimization
=================================

   Most Fortran users will want to use no optimization when developing
and testing programs, and use `-O' or `-O2' when compiling programs for
late-cycle testing and for production use.

   The following flags have particular applicability when compiling
Fortran programs:

`-ffloat-store'
     Might help a Fortran program that depends on exact IEEE conformance
     on some machines, but might slow down a program that doesn't.

`-fforce-mem'
`-fforce-addr'
     Might improve optimization of loops.

`-fno-inline'
     Don't compile statement functions inline.  Might reduce the size
     of a program unit--which might be at expense of some speed (though
     it should compile faster).  Note that if you are not optimizing,
     no functions can be expanded inline.

`-ffast-math'
     Might allow some programs designed to not be too dependent on IEEE
     behavior for floating-point to run faster, or die trying.

`-fstrength-reduce'
     Might make some loops run faster.

`-frerun-cse-after-loop'
`-fexpensive-optimizations'
`-fdelayed-branch'
`-fschedule-insns'
`-fschedule-insns2'
`-fcaller-saves'
     Might improve performance on some code.

`-funroll-loops'
     Definitely improves performance on some code.

`-funroll-all-loops'
     Definitely improves performance on some code.

`-fno-move-all-movables'
`-fno-reduce-all-givs'
`-fno-rerun-loop-opt'
     Each of these might improve performance on some code.

     Analysis of Fortran code optimization and the resulting
     optimizations triggered by the above options were contributed by
     Toon Moene (`toon@moene.indiv.nluug.nl').

     Please let us know how use of these options affects the
     performance of your production code.  We're particularly
     interested in code that runs faster when these options are
     *disabled*, and in non-Fortran code that benefits when they are
     *enabled* via the above `gcc' command-line options.

   *Note Options That Control Optimization: (gcc)Optimize Options, for
more information on options to optimize the generated machine code.


File: g77.info,  Node: Preprocessor Options,  Next: Directory Options,  Prev: Optimize Options,  Up: Invoking G77

Options Controlling the Preprocessor
====================================

   These options control the C preprocessor, which is run on each C
source file before actual compilation.

   *Note Options Controlling the Preprocessor: (gcc)Preprocessor
Options, for information on C preprocessor options.

   Some of these options also affect how `g77' processes the `INCLUDE'
statement.  Since this statement is processed even when preprocessing
is not requested, it is not described in this section.  *Note Options
for Directory Search: Directory Options, for information on how `g77'
processes the `INCLUDE' statement.


File: g77.info,  Node: Directory Options,  Next: Code Gen Options,  Prev: Preprocessor Options,  Up: Invoking G77

Options for Directory Search
============================

   These options affect how the `cpp' preprocessor searches for files
specified via the `#include' directive.  Therefore, when compiling
Fortran programs, they are meaningful when the preproecssor is used.

   Some of these options also affect how `g77' searches for files
specified via the `INCLUDE' statement.  These options are:

`-I-'
`-IDIR'
     These affect interpretation of the `INCLUDE' statement (as well as
     of the `#include' directive of the `cpp' preprocessor).

     Note that `-IDIR' must be specified *without* any spaces between
     `-I' and the directory name--that is, `-Ifoo/bar' is valid, but
     `-I foo/bar' is rejected by the `g77' compiler (though the
     preprocessor supports the latter form).  Also note that the
     general behavior of `-I' and `INCLUDE' is pretty much the same as
     of `-I' with `#include' in the `cpp' preprocessor, with regard to
     looking for `header.gcc' files and other such things.

     *Note Options for Directory Search: (gcc)Directory Optoins, for
     information on the `-I' option.


File: g77.info,  Node: Code Gen Options,  Next: Environment Variables,  Prev: Directory Options,  Up: Invoking G77

Options for Code Generation Conventions
=======================================

   These machine-independent options control the interface conventions
used in code generation.

   Most of them have both positive and negative forms; the negative form
of `-ffoo' would be `-fno-foo'.  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 `no-' or adding it.

`-fno-automatic'
     Treat each program unit as if the `SAVE' 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 `-static'.)

`-finit-local-zero'
     Specify that variables and arrays that are local to a program unit
     (not in a common block and not passed as an argument) are to be
     initialized to binary zeros.

     Since there is a run-time penalty for initialization of variables
     that are not given the `SAVE' attribute, it might be a good idea
     to also use `-fno-automatic' with `-finit-local-zero'.

`-fno-f2c'
     Do not generate code designed to be compatible with code generated
     by `f2c'.

     This does not affect the generation of code that interfaces with
     the `libf2c' library.

     *Caution:* If `-fno-f2c' is used when compiling any source file
     used in a program, it must be used when compiling *all* Fortran
     source files used in that program.

`-ff2c-library'
     Specify that use of `libf2c' is required.  This is the default for
     the current version of `g77'.

     Currently it is not valid to specify `-fno-f2c-library'.  This
     option is provided so users can specify it in shell scripts that
     build programs and libraries that require the `libf2c' library,
     even when being compiled by future versions of `g77' that might
     otherwise default to generating code for an incompatible library.

`-fno-underscoring'
     Do not transform names of entities specified in the Fortran source
     file by appending underscores to them.

     With `-funderscoring' in effect, `g77' appends two underscores to
     names with underscores and one underscore to external names with
     no underscores.  (`g77' also appends two underscores to internal
     names with underscores to avoid naming collisions with external
     names.)

     This is done to ensure compatibility with code produced by many
     UNIX Fortran compilers, including `f2c', which perform the same
     transformations.

     Use of `-fno-underscoring' is not recommended unless you are
     experimenting with issues such as integration of (GNU) Fortran into
     existing system environments (vis-a-vis existing libraries, tools,
     and so on).

     For example, with `-funderscoring', and assuming other defaults
     like `-fcase-lower' and that `j()' and `max_count()' are external
     functions while `my_var' and `lvar' are local variables, a
     statement like

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

     is implemented as something akin to:

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

     With `-fno-underscoring', the same statement is implemented as:

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

     Use of `-fno-underscoring' allows direct specification of
     user-defined names while debugging and when interfacing
     `g77'-compiled code with other languages.

     Note that just because the names match does *not* mean that the
     interface implemented by `g77' for an external name matches the
     interface implemented by some other language for that same name.
     That is, getting code produced by `g77' 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.

     Also, note that with `-fno-underscoring', 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.

     In future versions of `g77', 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.

`-fno-second-underscore'
     Do not append a second underscore to names of entities specified
     in the Fortran source file.

     This option has no effect if `-fno-underscoring' is not in effect.

     Otherwise, with this option, an external name such as `MAX_COUNT'
     is implemented as a reference to the link-time external symbol
     `max_count_', instead of `max_count__'.

`-fno-ident'
     Ignore the `#ident' directive.

`-fzeros'
     Treat initial values of zero as if they were any other value.

     As of version 0.5.18, `g77' normally treats `DATA' and other
     statements that are used specify initial values of zero for
     variables and arrays as if no values were actually specified, in
     the sense that no diagnostics regarding multiple initializations
     are produced.

     This is done to speed up compiling of programs that initialize
     large arrays to zeros.

     Use `-fzeros' to revert to the simpler, slower behavior that can
     catch multiple initializations by keeping track of all
     initializations, zero or otherwise.

     *Caution:* Future versions of `g77' might disregard this option
     (and its negative form, the default) or interpret it somewhat
     differently.  The interpretation changes will affect only
     non-standard programs; standard-conforming programs should not be
     affected.

   *Note Options for Code Generation Conventions: (gcc)Code Gen
Options, for information on more options offered by the GBE shared by
`g77', `gcc', and other GNU compilers.

   Some of these do *not* work when compiling programs written in
Fortran:

`-fpcc-struct-return'
`-freg-struct-return'
     You should not use these except strictly the same way as you used
     them to build the version of `libf2c' with which you will be
     linking all code compiled by `g77' with the same option.

`-fshort-double'
     This probably either has no effect on Fortran programs, or makes
     them act loopy.

`-fno-common'
     Do not use this when compiling Fortran programs, or there will be
     Trouble.

`-fpack-struct'
     This probably will break any calls to the `libf2c' library, at the
     very least, even if it is built with the same option.


File: g77.info,  Node: Environment Variables,  Prev: Code Gen Options,  Up: Invoking G77

Environment Variables Affecting GNU Fortran
===========================================

   GNU Fortran currently does not make use of any environment variables
to control its operation above and beyond those that affect the
operation of `gcc'.

   *Note Environment Variables Affecting GNU CC: (gcc)Environment
Variables, for information on environment variables.