These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form
-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
SAVEstatement 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
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
f2cuse the GNU calling conventions instead.
f2c calling conventions require functions that return
REAL(KIND=1) to actually return the C type
and functions that return type
COMPLEX to return the
values via an extra 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--
return the C type
return the GNU C type
complex (or its
This does not affect the generation of code that interfaces with the
However, because the
libg2c library uses
g77 rejects attempts to pass
intrinsics implemented by routines in this library as actual
-fno-f2c is used, to avoid bugs when
they are actually called by code expecting the GNU calling
conventions to work.
INTRINSIC ABS;CALL FOO(ABS) is
-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
that may be passed as actual arguments, so that
valid programs need not be rejected when
-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.
libg2c(or the original
libf2c) is required. This is the default for the current version of
Currently it is not
valid to specify
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
g77 that might otherwise default to
generating code for an incompatible library.
-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.
-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
-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
For example, with
-funderscoring, and assuming other defaults like
-fcase-lower and that
external functions while
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);
-fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
-fno-underscoring allows direct specification of
user-defined names while debugging and when interfacing
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
This option has no effect if
Otherwise, with this option, an external name such as
is implemented as a reference to the link-time external symbol
max_count_, instead of
As of version 0.5.18,
g77 normally treats
other statements 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
This is done to speed up compiling of programs that initialize large arrays to zeros.
-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
The interpretation changes will affect only non-standard
programs; standard-conforming programs should not be affected.
COMPLEXarithmetic via emulation, instead of using the facilities of the
gccback end that provide direct support of
gcc had some bugs in its back-end support
complex arithmetic, due primarily to the support not being
completed as of version 2.8.1 and
-femulate-complex if you suspect code-generation bugs,
or experience compiler crashes,
that might result from
g77 using the
gcc back end.
If using that option fixes the bugs or crashes you are seeing,
that indicates a likely
(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
g77 works in terms of its interfaces to other code,
e.g. that compiled by
As of GCC version 3.0, this option is not necessary anymore.
Caution: Future versions of
g77 might ignore both forms
of this option.
These options specify to what degree aliasing
is permitted between
arguments (passed as pointers) and
COMMON (external, or
The default for Fortran code, as mandated by the FORTRAN 77 and
Fortran 90 standards, is
The default for code written in the C language family is
Note that, on some systems, compiling with
effect can produce more optimal code when the default aliasing
options are in effect (and when optimization is enabled).
See Aliasing Assumed To Work, for detailed information on the implications
of compiling Fortran code that depends on the ability to alias dummy
-Wno-globalsis 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 diagnosed.
As such, this option might be quite useful when
compiling existing, "working" code that happens
to have a few bugs that do not generally show themselves,
Use of this option therefore has the effect of
g77 to behave more like it did
up through version 0.5.19.1, when it paid little or
no attention to disagreements between program units
about a procedure's type and argument information,
and when it performed 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 incorrect code.
ARRAY_REFconstruct to handle all array references.
Note: This option is not supported.
It is intended for use only by
to evaluate code-generation issues.
It might be removed at any time.
The current implementation uses the
s_rnge to print the diagnostic.
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,
f2c currently does not do.
Use the new
to specify bounds-checking 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 subscript,
g77 provides the
libg2c run-time library routine
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
within the program unit (or statement function) named
The offended array is named
The offended array dimension is the second for a two-dimensional array,
and the offending, computed subscript expression was
CHARACTER substring reference, the second line has
Attempt to access the 11-th element of variable a[start-substring].
This indicates that the offended
CHARACTER variable or array
the offended substring position is the starting (leftmost) position,
and the offending substring expression is
(Though the verbage of
s_rnge is not ideal
for the purpose of the
the above information should provide adequate diagnostic abilities
to it users.)
See Options for Code Generation Conventions, for information on more options
offered by the GBE
gcc and other GNU compilers.
Some of these do not work when compiling programs written in Fortran:
libg2cwith which you will be linking all code compiled by
g77with the same option.
libg2clibrary, at the very least, even if it is built with the same option.