g77 (1)





NAME

       g77 - GNU project Fortran 77 compiler


SYNOPSIS

       g77 [-c|-S|-E]
           [-g] [-pg] [-Olevel]
           [-Wwarn...] [-pedantic]
           [-Idir...] [-Ldir...]
           [-Dmacro[=defn]...] [-Umacro]
           [-foption...] [-mmachine-option...]
           [-o outfile] infile...

       Only the most useful options are listed here; see below for the remain-
       der.


DESCRIPTION

       The g77 command supports all the options supported by the gcc command.

       All gcc and g77 options are accepted both by g77 and by gcc (as well as
       any other drivers built at the same time, such as g++), since adding
       g77 to the gcc distribution enables acceptance of g77 options by all of
       the relevant drivers.

       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.


OPTIONS

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

       Overall Options
           -fversion  -fset-g77-defaults  -fno-silent

       Shorthand Options
           -ff66  -fno-f66  -ff77  -fno-f77  -fno-ugly

       Fortran Language Options
           -ffree-form  -fno-fixed-form  -ff90 -fvxt  -fdollar-ok  -fno-back-
           slash -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 -fin-
           trin-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-intrin-
           sics-disable  -ff2c-intrinsics-enable -fbadu77-intrinsics-delete
           -fbadu77-intrinsics-hide -fbadu77-intrinsics-disable
           -fbadu77-intrinsics-enable -ff90-intrinsics-delete  -ff90-intrin-
           sics-hide -ff90-intrinsics-disable  -ff90-intrinsics-enable
           -fgnu-intrinsics-delete  -fgnu-intrinsics-hide -fgnu-intrin-
           sics-disable  -fgnu-intrinsics-enable -fmil-intrinsics-delete
           -Wno-globals  -Wimplicit  -Wunused  -Wuninitialized -Wall  -Wsur-
           prising -Werror  -W

       Debugging Options
           -g

       Optimization Options
           -malign-double -ffloat-store  -fforce-mem  -fforce-addr
           -fno-inline -ffast-math  -fstrength-reduce  -frerun-cse-after-loop
           -funsafe-math-optimizations -fno-trapping-math -fexpensive-opti-
           mizations  -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
           -Idir  -I-

       Code Generation Options
           -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 -femulate-complex -falias-check
           -fargument-alias -fargument-noalias  -fno-argument-noalias-global
           -fno-globals  -fflatten-arrays -fbounds-check  -ffor-
           tran-bounds-check

       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 spe-
       cific to GNU Fortran are listed below.

       file.f
       file.for
       file.FOR
           Fortran source code that should not be preprocessed.

           Such source code cannot contain any preprocessor directives, such
           as "#include", "#define", "#if", and so on.

           You can force .f files to be preprocessed by cpp by using -x
           f77-cpp-input.

       file.F
       file.fpp
       file.FPP
           Fortran source code that must be preprocessed (by the C preproces-
           sor cpp, which is part of GNU CC).

           Note that preprocessing is not extended to the contents of files
           included by the "INCLUDE" directive---the "#include" preprocessor

       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, typi-
       cally 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 con-
       structs 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.

       Another example of a problem that results from using the C preprocessor
       is that a Fortran comment line that happens to contain any characters
       ``interesting'' to the C preprocessor, such as a backslash at the end
       of the line, is not recognized by the preprocessor as a comment line,
       so instead of being passed through ``raw'', the line is edited accord-
       ing to the rules for the preprocessor.  For example, the backslash at
       the end of the line is removed, along with the subsequent newline,
       resulting in the next line being effectively commented out---unfortu-
       nate if that line is a non-comment line of important code!

       Note: The -traditional and -undef flags are supplied to cpp by default,
       to help avoid unpleasant surprises.

       This means that ANSI C preprocessor features (such as the # operator)
       aren't available, and only variables in the C reserved namespace (gen-
       erally, 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.

       Unfortunately, the -traditional flag will not avoid an error from any-
       thing that cpp sees as an unterminated C comment, such as:

               C Some Fortran compilers accept /* as starting
               C an inline comment.

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

       -fversion
           Ensure that the g77 version of the compiler phase is reported, if
           run, and, starting in "egcs" version 1.1, that internal consistency
           checks in the f771 program are run.

           This option is supplied automatically when -v or --verbose is spec-
           ified as a command-line option for g77 or gcc and when the result-
           ing commands compile Fortran source files.
           Set up whatever gcc options are to apply to Fortran compilations,
           and avoid running internal consistency checks that might take some
           time.

           This option is supplied automatically when compiling Fortran code
           via the g77 or gcc command.  The description of this option is pro-
           vided so that users seeing it in the output of, say, g77 -v under-
           stand 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 intro-
           duced any internal inconsistencies (such as in the table of intrin-
           sics) 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 com-
           piled, in a form similar to that used by popular UNIX f77 implemen-
           tations and f2c

       Shorthand Options

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

       -fugly
           Note: This option is no longer supported.  The information, below,
           is provided to aid in the conversion of old scripts.

           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.

       -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

       -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 ver-
           sions 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 ver-
           sions 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 rec-
           ognized.  (Other Fortran 90 constructs might or might not be recog-
           nized depending on other options such as -fvxt, -ff90-intrin-
           sics-enable, and the current level of support for Fortran 90.)

       -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.

       -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 char-
           acters, 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 pro-
           cessing 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
           or used to hold numeric data (as in I = 3).

       -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(*).

       -fugly-comma
           In an external-procedure invocation, treat a trailing comma in the
           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(%VVAALL(0), %VVAALL(0)).
           That is, two null arguments are specified by the procedure call
           when -fugly-comma is in force.  And F = FFUUNNCC(()) is treated as F =
           FUNC(%VVAALL(0)).

           The default behavior, -fno-ugly-comma, is to ignore a single trail-
           ing comma in an argument list.  So, by default, CALL FOO(X,) is
           treated exactly the same as CALL FOO(X).

       -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.

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

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

       -fugly-logint
           Treat "INTEGER" and "LOGICAL" variables and expressions as poten-
           tial stand-ins for each other.

           For example, automatic conversion between "INTEGER" and "LOGICAL"
           is enabled, for many contexts, via this option.

       -fonetrip
           Executable iterative "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 iterative "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.

           piled require one-trip loops.

           This option affects only those loops specified by the (iterative)
           "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 follow-
           ing 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 "INTE-
           GER(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 con-
           stants 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.  -fsym-
           bol-case-any is the default.

       -fcase-strict-upper
           Same as -fintrin-case-upper -fmatch-case-upper -fsource-case-pre-
           serve -fsymbol-case-upper.  (Requires all pertinent source to be in
           -fsource-case-preserve -fsymbol-case-initcap.  (Requires all perti-
           nent 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.)

       -fbadu77-intrinsics-delete
       -fbadu77-intrinsics-hide
       -fbadu77-intrinsics-disable
       -fbadu77-intrinsics-enable
           Specify status of UNIX intrinsics having inappropriate forms.
           -fbadu77-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-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-intrin-
           sics-enable is the default.

       -funix-intrinsics-delete
       -funix-intrinsics-hide
       -funix-intrinsics-disable
       -funix-intrinsics-enable

       -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.

           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.  -pedan-
           tic 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 warn-
           ings.

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

       -w  Inhibit all warning messages.
           types of arguments.

       -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 opti-
           mizing.  If you don't specify -O, you simply won't get these warn-
           ings.

           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 warn-
           ings 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 ini-
           tialized, but GNU Fortran doesn't know this.  Here is another com-
           mon 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
       -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-depen-
           dent (even machine-dependent) behavioral differences.  The con-
           structs warned about include:

              Expressions having two arithmetic operators in a row, such as
               X*-Y.  Such a construct is nonstandard, and can produce unex-
               pected 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 oth-
               ers 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 nega-
               tive number.

               Even cases such as -I*J produce warnings, even though, in most
               configurations and situations, there is no computational dif-
               ference between the results of the two interpretations---the
               purpose of this warning is to warn about differing interpreta-
               tions 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 vari-
               ables.  Although such loops can be written to work in the
               ``obvious'' way, the way g77 is required by the Fortran stan-
               dard 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.)

       -Werror
           Make all warnings into errors.

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

       Some of these have no effect when compiling programs written in For-
       tran:

       -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 for-
           mat (stabs, COFF, XCOFF, or DWARF).  GDB can work with this debug-
           ging information.

           A sample debugging session looks like this (note the use of the
           breakpoint):

                   $ cat gdb.f
                         PROGRAM PROG
                         DIMENSION A(10)
                         DATA A /1.,2.,3.,4.,5.,6.,7.,8.,9.,10./
                         A(5) = 4.
                         PRINT*,A
                         END
                   $ g77 -g -O gdb.f
                   $ gdb a.out
                   ...
                   (gdb) break MAIN__
                   Breakpoint 1 at 0x8048e96: file gdb.f, line 4.
                   (gdb) run
                   Starting program: /home/toon/g77-bugs/./a.out
                   Breakpoint 1, MAIN__ () at gdb.f:4
                   4             A(5) = 4.
                   Current language:  auto; currently fortran
                   (gdb) print a(5)
                   $1 = 5
                   (gdb) step
                   5             PRINT*,A
                   (gdb) print a(5)
                   $2 = 4
                   ...

       nostics---such as for uninitialized variables---depend on the flow
       analysis done by -O, i.e. you must use -O or -O2 to get such diagnos-
       tics.

       The following flags have particular applicability when compiling For-
       tran programs:

       -malign-double
           (Intel x86 architecture only.)

           Noticeably improves performance of g77 programs making heavy use of
           "REAL(KIND=2)" ("DOUBLE PRECISION") data on some systems.  In par-
           ticular, systems using Pentium, Pentium Pro, 586, and 686 implemen-
           tations 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 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.

           This option is effective when the floating-point unit is set to
           work in IEEE 854 `extended precision'---as it typically is on x86
           and m68k GNU systems---rather than IEEE 754 double precision.
           -ffloat-store tries to remove the extra precision by spilling data
           from floating-point registers into memory and this typically
           involves a big performance hit.  However, it doesn't affect inter-
           mediate results, so that it is only partially effective.  `Excess
           precision' is avoided in code like:

                   a = b + c
                   d = a * e

           but not in code like:

                         d = (b + c) * e

           For another, potentially better, way of controlling the precision,
           see @ref{Floating-point precision}.

       -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
           Allow optimizations that may be give incorrect results for certain
           IEEE inputs.

       -fno-trapping-math
           Allow the compiler to assume that floating-point arithmetic will
           not generate traps on any inputs.  This is useful, for example,
           when running a program using IEEE "non-stop" floating-point arith-
           metic.

       -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
           Typically improves performance on code using iterative "DO" loops
           by unrolling them and is probably generally appropriate for For-
           tran, though it is not turned on at any optimization level.  Note
           that outer loop unrolling isn't done specifically; decisions about
           whether to unroll a loop are made on the basis of its instruction
           count.

           Also, no `loop discovery'[1] is done, so only loops written with
           "DO" benefit from loop optimizations, including---but not limited
           to---unrolling.  Loops written with "IF" and "GOTO" are not cur-
           rently recognized as such.  This option unrolls only iterative "DO"
           loops, not "DO WHILE" loops.

       -funroll-all-loops
           Probably improves performance on code using "DO WHILE" loops by
           unrolling them in addition to iterative "DO" loops.  In the absence
           of "DO WHILE", this option is equivalent to -funroll-loops but pos-
           sibly slower.

       -fno-move-all-movables
       -fno-reduce-all-givs
       -fno-rerun-loop-opt
           In general, the optimizations enabled with these options will lead
           to faster code being generated by GNU Fortran; hence they are
           enabled by default when issuing the g77 command.

           -fmove-all-movables and -freduce-all-givs will enable loop opti-
           mization to move all loop-invariant index computations in nested
           loops over multi-rank array dummy arguments out of these loops.

           -frerun-loop-opt will move offset calculations resulting from the
           fact that Fortran arrays by default have a lower bound of 1 out of
           the loops.

           These three options are intended to be removed someday, once loop
           optimization is sufficiently advanced to perform all those trans-

       is not requested, it is not described in this section.

       However, the "INCLUDE" directive does not apply preprocessing to the
       contents of the included file itself.

       Therefore, any file that contains preprocessor directives (such as
       "#include", "#define", and "#if") must be included via the "#include"
       directive, not via the "INCLUDE" directive.  Therefore, any file con-
       taining preprocessor directives, if included, is necessarily included
       by a file that itself contains preprocessor directives.

       Options for Directory Search

       These options affect how the cpp preprocessor searches for files speci-
       fied via the "#include" directive.  Therefore, when compiling Fortran
       programs, they are meaningful when the preprocessor is used.

       Some of these options also affect how g77 searches for files specified
       via the "INCLUDE" directive, although files included by that directive
       are not, themselves, preprocessed.  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.

       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
           argument in the calling sequence that points to where to store the
           return value.  Under the GNU calling conventions, such functions
           simply return their results as they would in GNU C---"REAL(KIND=1)"
           functions return the C type "float", and "COMPLEX" functions return
           the GNU C type "complex" (or its "struct" equivalent).

           This does not affect the generation of code that interfaces with
           the "libg2c" library.

           However, because the "libg2c" library uses f2c calling conventions,
           g77 rejects attempts to pass intrinsics implemented by routines in
           this library as actual arguments when -fno-f2c is used, to avoid
           bugs when they are actually called by code expecting the GNU call-
           ing conventions to work.

           For example, INTRINSIC ABS;CALL FOO(ABS) is rejected when -fno-f2c
           is in force.  (Future versions of the g77 run-time library might
           offer routines that provide GNU-callable versions of the routines
           that implement the f2c intrinsics that may be passed as actual
           arguments, so that valid programs need not be rejected when
           -fno-f2c is used.)

           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 "libg2c" (or the original "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 trans-
           formations.

           Use of -fno-underscoring is not recommended unless you are experi-
           menting 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
           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 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 nor-
           mally cannot detect disagreements in these other areas.

           Also, note that with -fno-underscoring, the lack of appended under-
           scores 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 state-
           ments that are used to 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 pro-
           duced.

           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
           Implement "COMPLEX" arithmetic via emulation, instead of using the
           facilities of the gcc back end that provide direct support of "com-
           plex" arithmetic.

           (gcc had some bugs in its back-end support for "complex" arith-
           metic, due primarily to the support not being completed as of ver-
           sion 2.8.1 and "egcs" 1.1.2.)

           Use -femulate-complex if you suspect code-generation bugs, or expe-
           rience compiler crashes, that might result from g77 using the "COM-
           PLEX" support in the gcc back end.  If using that option fixes the
           bugs or crashes you are seeing, that indicates a likely g77 bugs
           (though, all compiler crashes are considered bugs), so, please
           report it.  (Note that the known bugs, now believed fixed, produced
           compiler crashes rather than causing 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 com-
           piled by f2c

           As of GCC version 3.0, this option is not necessary anymore.

           Caution: Future versions of g77 might ignore both forms of this
           option.

       -falias-check
       -fargument-alias
       -fargument-noalias
       -fno-argument-noalias-global
           Version info: These options are not supported by versions of g77
           based on gcc version 2.8.

           These options specify to what degree aliasing (overlap) is permit-
           ted 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).

       -fno-globals
           Disable diagnostics about inter-procedural analysis problems, such
           as disagreements about the type of a function or a procedure's
           argument, that might cause a compiler crash when attempting to
           inline a reference to a procedure within a program unit.  (The
           diagnostics themselves are still produced, but as warnings, unless
           -Wno-globals is specified, in which case no relevant diagnostics
           are produced.)

           Further, this option disables such inlining, to avoid compiler
           crashes resulting from incorrect code that would otherwise be diag-
           nosed.
           no inlining of procedures (except statement functions).

           Without this option, g77 defaults to performing the potentially
           inlining procedures as it started doing in version 0.5.20, but as
           of version 0.5.21, it also diagnoses disagreements that might cause
           such inlining to crash the compiler as (fatal) errors, and warns
           about similar disagreements that are currently believed to not
           likely to result in the compiler later crashing or producing incor-
           rect code.

       -fflatten-arrays
           Use back end's C-like constructs (pointer plus offset) instead of
           its "ARRAY_REF" construct to handle all array references.

           Note: This option is not supported.  It is intended for use only by
           g77 developers, to evaluate code-generation issues.  It might be
           removed at any time.

       -fbounds-check
       -ffortran-bounds-check
           Enable generation of run-time checks for array subscripts and sub-
           string start and end points against the (locally) declared minimum
           and maximum values.

           The current implementation uses the "libf2c" library routine
           "s_rnge" to print the diagnostic.

           However, whereas f2c generates a single check per reference for a
           multi-dimensional array, of the computed offset against the valid
           offset range (0 through the size of the array), g77 generates a
           single check per subscript expression.  This catches some cases of
           potential bugs that f2c does not, such as references to below the
           beginning of an assumed-size array.

           g77 also generates checks for "CHARACTER" substring references,
           something f2c currently does not do.

           Use the new -ffortran-bounds-check option to specify bounds-check-
           ing for only the Fortran code you are compiling, not necessarily
           for code written in other languages.

           Note: To provide more detailed information on the offending sub-
           script, g77 provides the "libg2c" run-time library routine "s_rnge"
           with somewhat differently-formatted information.  Here's a sample
           diagnostic:

                   Subscript out of range on file line 4, procedure rnge.f/bf.
                   Attempt to access the -6-th element of variable b[subscript-2-of-2].
                   Aborted

           The above message indicates that the offending source line is line
           4 of the file rnge.f, within the program unit (or statement func-
           tion) named bf.  The offended array is named b.  The offended array
           dimension is the second for a two-dimensional array, and the
           offending, computed subscript expression was -6.

           For a "CHARACTER" substring reference, the second line has this
           g77 compiler, the above information should provide adequate diag-
           nostic abilities to it users.)

       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 "libg2c" with which you will be link-
           ing 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 "libg2c" library, at the
           very least, even if it is built with the same option.


ENVIRONMENT

       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.


BUGS

       For instructions on reporting bugs, see <http://gcc.gnu.org/bugs.html>.
       Use of the gccbug script to report bugs is recommended.


FOOTNOTES

       1.  loop discovery refers to the process by which a compiler, or indeed
           any reader of a program, determines which portions of the program
           are more likely to be executed repeatedly as it is being run.  Such
           discovery typically is done early when compiling using optimization
           techniques, so the ``discovered'' loops get more attention---and
           more run-time resources, such as registers---from the compiler.  It
           is easy to ``discover'' loops that are constructed out of looping
           constructs in the language (such as Fortran's "DO").  For some pro-
           grams, ``discovering'' loops constructed out of lower-level con-
           structs (such as "IF" and "GOTO") can lead to generation of more
           optimal code than otherwise.


SEE ALSO

       gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1), as(1), ld(1),
       gdb(1), adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, g77,
       as, ld, binutils and gdb.


AUTHOR

       See the Info entry for g77 for contributors to GCC and G77.


COPYRIGHT

       Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software
       Foundation, Inc.


            A GNU Manual

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

            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.

gcc-3.2.2                         2003-02-25                            g77(1)