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# GNU Fortran Command Options

The `g77` command supports all the options supported by the `gcc` command. See section `GNU CC Command Options' in Using and Porting GNU CC, for information on the non-Fortran-specific aspects of the `gcc` command (and, therefore, the `g77` command).

The `g77` command supports one option not supported by the `gcc` command:

`--driver=command`
  Specifies that command, rather than `gcc`, is to be invoked by `g77` to do its job. For example, within the `gcc` build directory after building GNU Fortran (but without having to install it), `./g77 --driver=./xgcc foo.f -B./`.

All other options are supported both by `g77` and by `gcc` as modified (and reinstalled) by the `g77` distribution. In some cases, options have positive and negative forms; the negative form of ``-ffoo'` would be ``-fno-foo'`. This manual documents only one of these two forms, whichever one is not the default.

## Option Summary

Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections.

Overall Options
  See section Options Controlling the Kind of Output.

      --driver  -fversion  -fset-g77-defaults  -fno-silent

Shorthand Options
  See section Shorthand Options.

      -ff66  -fno-f66  -ff77  -fno-f77  -fugly  -fno-ugly

Fortran Language Options
  See section Options Controlling Fortran Dialect.

      -ffree-form  -fno-fixed-form  -ff90
      -fvxt  -fdollar-ok  -fno-backslash
      -fno-ugly-args  -fno-ugly-assign  -fno-ugly-assumed
      -fugly-comma  -fugly-complex  -fugly-init  -fugly-logint
      -fonetrip  -ftypeless-boz
      -fintrin-case-initcap  -fintrin-case-upper
      -fintrin-case-lower  -fintrin-case-any
      -fmatch-case-initcap  -fmatch-case-upper
      -fmatch-case-lower  -fmatch-case-any
      -fsource-case-upper -fsource-case-lower  -fsource-case-preserve
      -fsymbol-case-initcap  -fsymbol-case-upper
      -fsymbol-case-lower  -fsymbol-case-any
      -fcase-strict-upper  -fcase-strict-lower
      -fcase-initcap  -fcase-upper  -fcase-lower  -fcase-preserve
      -ff2c-intrinsics-delete  -ff2c-intrinsics-hide
      -ff2c-intrinsics-disable  -ff2c-intrinsics-enable
      -ff90-intrinsics-delete  -ff90-intrinsics-hide
      -ff90-intrinsics-disable  -ff90-intrinsics-enable
      -fgnu-intrinsics-delete  -fgnu-intrinsics-hide
      -fgnu-intrinsics-disable  -fgnu-intrinsics-enable
      -fmil-intrinsics-delete  -fmil-intrinsics-hide
      -fmil-intrinsics-disable  -fmil-intrinsics-enable
      -funix-intrinsics-delete  -funix-intrinsics-hide
      -funix-intrinsics-disable  -funix-intrinsics-enable
      -fvxt-intrinsics-delete  -fvxt-intrinsics-hide
      -fvxt-intrinsics-disable  -fvxt-intrinsics-enable
      -ffixed-line-length-n  -ffixed-line-length-none

Warning Options
  See section Options to Request or Suppress Warnings.

      -fsyntax-only  -pedantic  -pedantic-errors  -fpedantic
      -w  -Wimplicit -Wunused  -Wuninitialized  -Wall  -Wsurprising
      -Werror  -W

Debugging Options
  See section Options for Debugging Your Program or GNU Fortran.

      -g

Optimization Options
  See section Options That Control Optimization.

      -malign-double
      -ffloat-store  -fforce-mem  -fforce-addr  -fno-inline
      -ffast-math  -fstrength-reduce  -frerun-cse-after-loop
      -fexpensive-optimizations  -fdelayed-branch
      -fschedule-insns  -fschedule-insn2  -fcaller-saves
      -funroll-loops  -funroll-all-loops
      -fno-move-all-movables  -fno-reduce-all-givs
      -fno-rerun-loop-opt

Directory Options
  See section Options for Directory Search.

      -Idir  -I-

Code Generation Options
  See section Options for Code Generation Conventions.

      -fno-automatic  -finit-local-zero  -fno-f2c
      -ff2c-library  -fno-underscoring  -fno-ident
      -fpcc-struct-return  -freg-struct-return
      -fshort-double  -fno-common  -fpack-struct
      -fzeros  -fno-second-underscore
      -fdebug-kludge  -fno-emulate-complex
      -falias-check  -fargument-alias
      -fargument-noalias  -fno-argument-noalias-global

## Options Controlling the Kind of Output

Compilation can involve as many as four stages: preprocessing, code generation (often what is really meant by the term "compilation"), 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 program is contained in the file--that is, the language in which the program is written is generally indicated by the suffix. Suffixes specific to GNU Fortran are listed below. See section Options Controlling the Kind of Output, 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 constructs such as Hollerith and character constants can nevertheless present problems, especially when these are continued across multiple source lines. These problems result, primarily, from differences between the way such constants are interpreted by the C preprocessor and by a Fortran compiler.

Note: The ``-traditional'` and ``-undef'` flags are supplied to `cpp` by default, to avoid unpleasant surprises. See section `Options Controlling the Preprocessor' in Using and Porting GNU CC. This means that ANSI C preprocessor features (such as the ``#'` operator) aren't available, and only variables in the C reserved namespace (generally, names with a leading underscore) are liable to substitution by C predefines. Thus, if you want to do system-specific tests, use, for example, ``#ifdef __linux__'` rather than ``#ifdef linux'`. Use the ``-v'` option to see exactly how the preprocessor is invoked.

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

`--driver=command`
  This works when invoking only the `g77` command, not when invoking the `gcc` command. See section GNU Fortran Command Options, 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 ``--verbose'` 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, and avoid running internal consistency checks that might take some time. As of version 0.5.20, this is equivalent to ``-fmove-all-movables -freduce-all-givs -frerun-loop-opt -fargument-noalias-global'`. This option is supplied automatically when compiling Fortran code via the `g77` or `gcc` command. The description of this option is provided 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`. However, such developers should, after linking a new `f771` executable, invoke it without this option once, e.g. via `./f771 -quiet < /dev/null`, to ensure that they have not introduced any internal inconsistencies (such as in the table of intrinsics) before proceeding---`g77` will crash with a diagnostic if it detects an inconsistency.
`-fno-silent`
  Print (to `stderr`) the names of the program units as they are compiled, in a form similar to that used by popular UNIX `f77` implementations and `f2c`.

See section `Options Controlling the Kind of Output' in Using and Porting GNU CC, for information on more options that control the overall operation of the `gcc` command (and, by extension, the `g77` command).

## Shorthand Options

The following options serve as "shorthand" for other options accepted by the compiler:

`-fugly`
  Specify that certain "ugly" constructs are to be quietly accepted. Same as:

      -fugly-args -fugly-assign -fugly-assumed
      -fugly-comma -fugly-complex -fugly-init
      -fugly-logint

  These constructs are considered inappropriate to use in new or well-maintained portable Fortran code, but widely used in old code. See section Distensions, for more information. Note: The ``-fugly'` option is likely to be removed in a future version. Implicitly enabling all the ``-fugly-*'` options is unlikely to be feasible, or sensible, in the future, so users should learn to specify only those ``-fugly-*'` options they really need for a particular source file.
`-fno-ugly`
  Specify that all "ugly" constructs are to be noisily rejected. Same as:

      -fno-ugly-args -fno-ugly-assign -fno-ugly-assumed
      -fno-ugly-comma -fno-ugly-complex -fno-ugly-init
      -fno-ugly-logint

  See section Distensions, for more information.
`-ff66`
  Specify that the program is written in idiomatic FORTRAN 66. Same as ``-fonetrip -fugly-assumed'`. The ``-fno-f66'` option is the inverse of ``-ff66'`. As such, it is the same as ``-fno-onetrip -fno-ugly-assumed'`. The meaning of this option is likely to be refined as future versions of `g77` provide more compatibility with other existing and obsolete Fortran implementations.
`-ff77`
  Specify that the program is written in idiomatic UNIX FORTRAN 77 and/or the dialect accepted by the `f2c` product. Same as ``-fbackslash -fno-typeless-boz'`. The meaning of this option is likely to be refined as future versions of `g77` provide more compatibility with other existing and obsolete Fortran implementations.
`-fno-f77`
  The ``-fno-f77'` option is not the inverse of ``-ff77'`. It specifies that the program is not written in idiomatic UNIX FORTRAN 77 or `f2c`, but in a more widely portable dialect. ``-fno-f77'` is the same as ``-fno-backslash'`. The meaning of this option is likely to be refined as future versions of `g77` provide more compatibility with other existing and obsolete Fortran implementations.

## 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'`, ``-ff90-intrinsics-enable'`, and the current level of support for Fortran 90.) See section Fortran 90, for more information.
`-fvxt`
  Specify the treatment of certain constructs that have different meanings depending on whether the code is written in GNU Fortran (based on FORTRAN 77 and akin to Fortran 90) or VXT Fortran (more like VAX FORTRAN). The default is ``-fno-vxt'`. ``-fvxt'` specifies that the VXT Fortran interpretations for those constructs are to be chosen. See section VXT Fortran, 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. See section Backslash in Constants, for information on why ``-fbackslash'` is the default instead of ``-fno-backslash'`.
`-fno-ugly-args`
  Disallow passing Hollerith and typeless constants as actual arguments (for example, ``CALL FOO(4HABCD)'`). See section Implicit Argument Conversion, for more information.
`-fugly-assign`
  Use the same storage for a given variable regardless of whether it is used to hold an assigned-statement label (as in ``ASSIGN 10 TO I'`) or used to hold numeric data (as in ``I = 3'`). See section Ugly Assigned Labels, for more information.
`-fugly-assumed`
  Assume any dummy array with a final dimension specified as ``1'` is really an assumed-size array, as if ``*'` had been specified for the final dimension instead of ``1'`. For example, ``DIMENSION X(1)'` is treated as if it had read ``DIMENSION X(*)'`. See section Ugly Assumed-Size Arrays, for more information.
`-fugly-comma`
  Treat a trailing comma in an argument list as specification of a trailing null argument, and treat an empty argument list as specification of a single null argument. For example, ``CALL FOO(,)'` is treated as ``CALL FOO(%VAL(0), %VAL(0))'`. That is, two null arguments are specified by the procedure call when ``-fugly-comma'` is in force. And ``F = FUNC()'` is treated as ``F = FUNC(%VAL(0))'`. The default behavior, ``-fno-ugly-comma'`, is to ignore a single trailing comma in an argument list. See section Ugly Null Arguments, for more information.
`-fugly-complex`
  Do not complain about ``REAL(expr)'` or ``AIMAG(expr)'` when expr is a `COMPLEX` type other than `COMPLEX(KIND=1)`---usually this is used to permit `COMPLEX(KIND=2)` (`DOUBLE COMPLEX`) operands. The ``-ff90'` option controls the interpretation of this construct. See section Ugly Complex Part Extraction, for more information.
`-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'`. See section Ugly Conversion of Initializers, for more information.
`-fugly-logint`
  Treat `INTEGER` and `LOGICAL` variables and expressions as potential stand-ins for each other. For example, automatic conversion between `INTEGER` and `LOGICAL` is enabled, for many contexts, via this option. See section Ugly Integer Conversions, for more information.
`-fonetrip`
  Imperative executable `DO` loops are to be executed at least once each time they are reached. ANSI FORTRAN 77 and more recent versions of the Fortran standard specify that the body of an imperative `DO` loop is not executed if the number of iterations calculated from the parameters of the loop is less than 1. (For example, ``DO 10 I = 1, 0'`.) Such a loop is called a zero-trip loop. Prior to ANSI FORTRAN 77, many compilers implemented `DO` loops such that the body of a loop would be executed at least once, even if the iteration count was zero. Fortran code written assuming this behavior is said to require one-trip loops. For example, some code written to the FORTRAN 66 standard expects this behavior from its `DO` loops, although that standard did not specify this behavior. The ``-fonetrip'` option specifies that the source file(s) being compiled require one-trip loops. This option affects only those loops specified by the (imperative) `DO` statement and by implied-`DO` lists in I/O statements. Loops specified by implied-`DO` lists in `DATA` and specification (non-executable) statements are not affected.
`-ftypeless-boz`
  Specifies that prefix-radix non-decimal constants, such as ``Z'ABCD''`, are typeless instead of `INTEGER(KIND=1)`. You can test for yourself whether a particular compiler treats the prefix form as `INTEGER(KIND=1)` or typeless by running the following program:

      EQUIVALENCE (I, R)
      R = Z'ABCD1234'
      J = Z'ABCD1234'
      IF (J .EQ. I) PRINT *, 'Prefix form is TYPELESS'
      IF (J .NE. I) PRINT *, 'Prefix form is INTEGER'
      END

  Reports indicate that many compilers process this form as `INTEGER(KIND=1)`, though a few as typeless, and at least one based on a command-line option specifying some kind of compatibility.
`-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'`.)
`-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-enable'` is the default.
`-fgnu-intrinsics-delete`
`-fgnu-intrinsics-hide`
`-fgnu-intrinsics-disable`
`-fgnu-intrinsics-enable`
  Specify status of Digital's COMPLEX-related intrinsics. ``-fgnu-intrinsics-enable'` 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.
`-fvxt-intrinsics-delete`
`-fvxt-intrinsics-hide`
`-fvxt-intrinsics-disable`
`-fvxt-intrinsics-enable`
  Specify status of VXT intrinsics. ``-fvxt-intrinsics-enable'` 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'`.

## 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.
`-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 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 might 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 = 9.4
      ...
      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 resulting from 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. An example of an expression producing different results in a surprising way is ``-I*S'`, where I holds the value ``-2147483648'` and S holds ``0.5'`. On many systems, negating I results in the same value, not a positive number, because it is already the lower bound of what an `INTEGER(KIND=1)` variable can hold. So, the expression evaluates to a positive number, while the "expected" interpretation, ``(-I)*S'`, would evaluate to a negative number. 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` 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.) See section Loops, for more information.
`-Werror`
  Make all warnings into errors.
`-W`
  Turns on "extra warnings" and, if optimization is specified via ``-O'`, the ``-Wuninitialized'` 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).

See section `Options to Request or Suppress Warnings' in Using and Porting GNU CC, 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.

## 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. However, version 0.5.19 of `g77` does provide this information in a rudimentary way, as controlled by the ``-fdebug-kludge'` option. See section Options for Code Generation Conventions, for more information.

See section `Options for Debugging Your Program or GNU CC' in Using and Porting GNU CC, for more information on debugging options.

## 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:

`-malign-double`
  (Intel 386 architecture only.) Noticeably improves performance of `g77` programs making heavy use of `REAL(KIND=2)` (`DOUBLE PRECISION`) data on some systems. In particular, systems using Pentium, Pentium Pro, 585, and 686 implementations of the i386 architecture execute programs faster when `REAL(KIND=2)` (`DOUBLE PRECISION`) data are aligned on 64-bit boundaries in memory. This option can, at least, make benchmark results more consistent across various system configurations, versions of the program, and data sets. Note: The warning in the `gcc` documentation about this option does not apply, generally speaking, to Fortran code compiled by `g77`. Also note: Apparently due to a `gcc` backend bug, ``-malign-double'` does not align stack-allocated data (such as local variables neither `SAVE`d nor reckoned to take up too much space to put on the stack). Also also note: The negative form of ``-malign-double'` is ``-mno-align-double'`, not ``-benign-double'`.
`-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`). These three options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving the performance of Fortran code. 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.

See section `Options That Control Optimization' in Using and Porting GNU CC, for more information on options to optimize the generated machine code.

## Options Controlling the Preprocessor

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

See section `Options Controlling the Preprocessor' in Using and Porting GNU CC, for information on C preprocessor options.

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

## 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` directive. These options are:

`-I-`
`-Idir`
  These affect interpretation of the `INCLUDE` directive (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. See section `Options for Directory Search' in Using and Porting GNU CC, for information on the ``-I'` option.

## 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. The ``-fno-second-underscore'` option disables appending of the second underscore in all cases.) 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 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.
`-fdebug-kludge`
  Emit information on `COMMON` and `EQUIVALENCE` members that might help users of debuggers work around lack of proper debugging information on such members. As of version 0.5.19, `g77` offers this option to emit information on members of aggregate areas to help users while debugging. This information consists of establishing the type and contents of each such member so that, when a debugger is asked to print the contents, the printed information provides rudimentary debugging information. This information identifies the name of the aggregate area (either the `COMMON` block name, or the `g77`-assigned name for the `EQUIVALENCE` name) and the offset, in bytes, of the member from the beginning of the area. Using `gdb`, this information is not coherently displayed in the Fortran language mode, so temporarily switching to the C language mode to display the information is suggested. Use ``set language c'` and ``set language fortran'` to accomplish this. For example:

            COMMON /X/A,B
            EQUIVALENCE (C,D)
            CHARACTER XX*50
            EQUIVALENCE (I,XX(20:20))
            END

      GDB is free software and you are welcome to distribute copies of it
       under certain conditions; type "show copying" to see the conditions.
      There is absolutely no warranty for GDB; type "show warranty" for details.
      GDB 4.16 (lm-gnits-dwim), Copyright 1996 Free Software Foundation, Inc...
      (gdb) b MAIN__
      Breakpoint 1 at 0t1200000201120112: file cd.f, line 5.
      (gdb) r
      Starting program: /home/user/a.out

      Breakpoint 1, MAIN__ () at cd.f:5
      Current language:  auto; currently fortran
      (gdb) set language c
      Warning: the current language does not match this frame.
      (gdb) p a
      $2 = "At (COMMON) `x_' plus 0 bytes"
      (gdb) p b
      $3 = "At (COMMON) `x_' plus 4 bytes"
      (gdb) p c
      $4 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes"
      (gdb) p d
      $5 = "At (EQUIVALENCE) `__g77_equiv_c' plus 0 bytes"
      (gdb) p i
      $6 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 20 bytes"
      (gdb) p xx
      $7 = "At (EQUIVALENCE) `__g77_equiv_xx' plus 1 bytes"
      (gdb) set language fortran
      (gdb)

  Use ``-fdebug-kludge'` to generate this information, which might make some programs noticeably larger. Caution: Future versions of `g77` might disregard this option (and its negative form). Current plans call for this to happen when published versions of `g77` and `gdb` exist that provide proper access to debugging information on `COMMON` and `EQUIVALENCE` members.
`-fno-emulate-complex`
  Implement `COMPLEX` arithmetic using the facilities in the `gcc` back end that provide direct support of `complex` arithmetic, instead of emulating the arithmetic. `gcc` has some known problems in its back-end support for `complex` arithmetic, due primarily to the support not being completed as of version 2.7.2.2. Other front ends for the `gcc` back end avoid this problem by emulating `complex` arithmetic at a higher level, so the back end sees arithmetic on the real and imaginary components. To make `g77` more portable to systems where `complex` support in the `gcc` back end is particularly troublesome, `g77` now defaults to performing the same kinds of emulations done by these other front ends. Use ``-fno-emulate-complex'` to try the `complex` support in the `gcc` back end, in case it works and produces faster programs. So far, all the known bugs seem to involve compile-time crashes, rather than the generation of incorrect code. Use of this option should not affect how Fortran code compiled by `g77` works in terms of its interfaces to other code, e.g. that compiled by `f2c`. Caution: Future versions of `g77` are likely to change the default for this option to ``-fno-emulate-complex'`, and perhaps someday ignore both forms of this option. Also, it is possible that use of the ``-fno-emulate-complex'` option could result in incorrect code being silently produced by `g77`. But, this is generally true of compilers anyway, so, as usual, test the programs you compile before assuming they are working.
`-falias-check`
`-fargument-alias`
`-fargument-noalias`
`-fno-argument-noalias-global`
  These options specify to what degree aliasing (overlap) is permitted between arguments (passed as pointers) and `COMMON` (external, or public) storage. The default for Fortran code, as mandated by the FORTRAN 77 and Fortran 90 standards, is ``-fargument-noalias-global'`. The default for code written in the C language family is ``-fargument-alias'`. Note that, on some systems, compiling with ``-fforce-addr'` in effect can produce more optimal code when the default aliasing options are in effect (and when optimization is enabled). See section Aliasing Assumed To Work, for detailed information on the implications of compiling Fortran code that depends on the ability to alias dummy arguments.

See section `Options for Code Generation Conventions' in Using and Porting GNU CC, 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.

## 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`.

See section `Environment Variables Affecting GNU CC' in Using and Porting GNU CC, for information on environment variables.

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