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2.4 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 messages produced.
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 #include.

Valid Fortran 95 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 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, -std=f2003 or -std=f2008.

Like -pedantic, except that errors are produced rather than warnings.
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, -Wconversion, -Wsurprising, -Wintrinsics-std, -Wno-tabs, -Wintrinsic-shadow, -Wline-truncation, -Wreal-q-constant and -Wunused.
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.

              subroutine bar(a,b)
                integer, intent(in) :: a
                integer, intent(out) :: b
              end subroutine
            end interface
            integer :: a
            call bar(a,a)

Warn about missing ampersand in continued character constants. The warning is given with -Wampersand, -pedantic, -std=f95, -std=f2003 and -std=f2008. 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 about array temporaries generated by the compiler. The information generated by this warning is sometimes useful in optimization, in order to avoid such temporaries.
Warn when a character assignment will truncate the assigned string.
Warn when a source code line will be truncated.
Warn about implicit conversions that are likely to change the value of the expression after conversion. Implied by -Wall.
Warn about implicit conversions between different types and kinds.
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 a procedure is called that has neither an explicit interface nor has been declared as EXTERNAL.
Warn if gfortran finds a procedure named like an intrinsic not available in the currently selected standard (with -std) and treats it as EXTERNAL procedure because of this. -fall-intrinsics can be used to never trigger this behavior and always link to the intrinsic regardless of the selected standard.
Produce a warning if a real-literal-constant contains a q exponent-letter.
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:

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, -std=f2008 and -Wall.
Produce a warning when numerical constant expressions are encountered, which yield an UNDERFLOW during compilation.
Warn if a user-defined procedure or module procedure has the same name as an intrinsic; in this case, an explicit interface or EXTERNAL or INTRINSIC declaration might be needed to get calls later resolved to the desired intrinsic/procedure.
Warn about unused dummy arguments. This option is implied by -Wall.
Contrary to gcc's meaning of -Wunused-parameter, gfortran's implementation of this option does not warn about unused dummy arguments (see -Wunused-dummy-argument), but about unused PARAMETER values. -Wunused-parameter is not included in -Wall but is implied by -Wall -Wextra.
By default, gfortran warns about any occasion of variables being padded for proper alignment inside a COMMON block. This warning can be turned off via -Wno-align-commons. See also -falign-commons.
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.