2.3 Options to request or suppress errors and warnings
Errors are diagnostic messages that report that the GNU Fortran compiler
cannot compile the relevant piece of source code. The compiler will
continue to process the program in an attempt to report further errors
to aid in debugging, but will not produce any compiled output.
Warnings are diagnostic messages that report constructions which
are not inherently erroneous but which are risky or suggest there is
likely to be a bug in the program. Unless -Werror is specified,
they do not prevent compilation of the program.
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 errors and warnings produced
by GNU Fortran:
- Limits the maximum number of error messages to n, at which point
GNU Fortran bails out rather than attempting to continue processing the
source code. If n is 0, there is no limit on the number of error
- Check the code for syntax errors, but don't actually compile it. This
will generate module files for each module present in the code, but no
other output file.
- Issue warnings for uses of extensions to Fortran 95.
-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
Valid Fortran 95 programs should compile properly with or without
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 conformance.
They soon find that it does not do quite what they want—it finds some
nonstandard practices, but not all.
However, improvements to GNU Fortran in this area are welcome.
This should be used in conjunction with -std=f95 or
- Like -pedantic, except that errors are produced rather than
- Enables commonly used warning options pertaining to usage that
we recommend avoiding and that we believe are easy to avoid.
This currently includes -Waliasing,
-Wampersand, -Wsurprising, -Wnonstd-intrinsics,
-Wno-tabs, and -Wline-truncation.
- Warn about possible aliasing of dummy arguments. Specifically, it warns
if the same actual argument is associated with a dummy argument with
INTENT(IN) and a dummy argument with
INTENT(OUT) in a call
with an explicit interface.
The following example will trigger the warning.
integer, intent(in) :: a
integer, intent(out) :: b
integer :: a
- Warn about missing ampersand in continued character constants. The warning is
given with -Wampersand, -pedantic, -std=f95, and
-std=f2003. Note: With no ampersand given in a continued character
constant, GNU Fortran assumes continuation at the first non-comment,
non-whitespace character after the ampersand that initiated the continuation.
- Warn when a character assignment will truncate the assigned string.
- Warn about implicit conversions between different types.
- Warn if a procedure is called without an explicit interface.
Note this only checks that an explicit interface is present. It does not
check that the declared interfaces are consistent across program units.
- Warn if the user tries to use an intrinsic that does not belong to the
standard the user has chosen via the -std option.
- Produce a warning when “suspicious” code constructs are encountered.
While technically legal these usually indicate that an error has been made.
This currently produces a warning under the following circumstances:
- An INTEGER SELECT construct has a CASE that can never be matched as its
lower value is greater than its upper value.
- A LOGICAL SELECT construct has three CASE statements.
- A TRANSFER specifies a source that is shorter than the destination.
- By default, tabs are accepted as whitespace, but tabs are not members
of the Fortran Character Set. For continuation lines, a tab followed
by a digit between 1 and 9 is supported. -Wno-tabs will cause
a warning to be issued if a tab is encountered. Note, -Wno-tabs
is active for -pedantic, -std=f95, -std=f2003,
- Produce a warning when numerical constant expressions are
encountered, which yield an UNDERFLOW during compilation.
- Contrary to gcc's meaning of -Wunused-parameter,
gfortran's implementation of this option does not warn
about unused dummy arguments, but about unused
-Wunused-parameter is not included in -Wall but is
implied by -Wall -Wextra.
- Turns all warnings into errors.
See Options to Request or Suppress Errors and Warnings, for information on
more options offered by the GBE shared by gfortran, gcc
and other GNU compilers.
Some of these have no effect when compiling programs written in Fortran.