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:

-fmax-errors=n

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.

-fsyntax-only

Check the code for syntax errors, but do not actually compile it. This will generate module files for each module present in the code, but no other output file.

-Wpedantic

-pedantic

Issue warnings for uses of extensions to Fortran. -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 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, -std=f2008 or -std=f2018.

-pedantic-errors

Like -pedantic, except that errors are produced rather than warnings.

-Wall

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, -Wc-binding-type, -Wintrinsics-std, -Wtabs, -Wintrinsic-shadow, -Wline-truncation, -Wtarget-lifetime, -Winteger-division, -Wreal-q-constant, -Wunused and -Wundefined-do-loop.

-Waliasing

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.

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

  call bar(a,a)

-Wampersand

Warn about missing ampersand in continued character constants. The warning is given with -Wampersand, -pedantic, -std=f95, -std=f2003, -std=f2008 and -std=f2018. 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.

-Warray-temporaries

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.

-Wc-binding-type

Warn if the a variable might not be C interoperable. In particular, warn if the variable has been declared using an intrinsic type with default kind instead of using a kind parameter defined for C interoperability in the intrinsic ISO_C_Binding module. This option is implied by -Wall.

-Wcharacter-truncation

Warn when a character assignment will truncate the assigned string.

-Wline-truncation

Warn when a source code line will be truncated. This option is implied by -Wall. For free-form source code, the default is -Werror=line-truncation such that truncations are reported as error.

-Wconversion

Warn about implicit conversions that are likely to change the value of the expression after conversion. Implied by -Wall.

-Wconversion-extra

Warn about implicit conversions between different types and kinds. This option does not imply -Wconversion.

-Wextra

Enables some warning options for usages of language features which may be problematic. This currently includes -Wcompare-reals, -Wunused-parameter and -Wdo-subscript.

-Wfrontend-loop-interchange

Warn when using -ffrontend-loop-interchange for performing loop interchanges.

-Wimplicit-interface

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.

-Wimplicit-procedure

Warn if a procedure is called that has neither an explicit interface nor has been declared as EXTERNAL.

-Winteger-division

Warn if a constant integer division truncates its result. As an example, 3/5 evaluates to 0.

-Wintrinsics-std

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.

-Wno-overwrite-recursive

Do not warn when -fno-automatic is used with -frecursive. Recursion will be broken if the relevant local variables do not have the attribute AUTOMATIC explicitly declared. This option can be used to suppress the warning when it is known that recursion is not broken. Useful for build environments that use -Werror.

-Wreal-q-constant

Produce a warning if a real-literal-constant contains a q exponent-letter.

-Wsurprising

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.
• The type of a function result is declared more than once with the same type. If -pedantic or standard-conforming mode is enabled, this is an error.
• A CHARACTER variable is declared with negative length.

-Wtabs

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. -Wtabs will cause a warning to be issued if a tab is encountered. Note, -Wtabs is active for -pedantic, -std=f95, -std=f2003, -std=f2008, -std=f2018 and -Wall.

-Wundefined-do-loop

Warn if a DO loop with step either 1 or -1 yields an underflow or an overflow during iteration of an induction variable of the loop. This option is implied by -Wall.

-Wunderflow

Produce a warning when numerical constant expressions are encountered, which yield an UNDERFLOW during compilation. Enabled by default.

-Wintrinsic-shadow

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. This option is implied by -Wall.

-Wuse-without-only

Warn if a USE statement has no ONLY qualifier and thus implicitly imports all public entities of the used module.

-Wunused-dummy-argument

Warn about unused dummy arguments. This option is implied by -Wall.

-Wunused-parameter

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 implied by -Wextra if also -Wunused or -Wall is used.

-Walign-commons

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.

-Wfunction-elimination

Warn if any calls to impure functions are eliminated by the optimizations enabled by the -ffrontend-optimize option. This option is implied by -Wextra.

-Wrealloc-lhs

Warn when the compiler might insert code to for allocation or reallocation of an allocatable array variable of intrinsic type in intrinsic assignments. In hot loops, the Fortran 2003 reallocation feature may reduce the performance. If the array is already allocated with the correct shape, consider using a whole-array array-spec (e.g. (:,:,:)) for the variable on the left-hand side to prevent the reallocation check. Note that in some cases the warning is shown, even if the compiler will optimize reallocation checks away. For instance, when the right-hand side contains the same variable multiplied by a scalar. See also -frealloc-lhs.

-Wrealloc-lhs-all

Warn when the compiler inserts code to for allocation or reallocation of an allocatable variable; this includes scalars and derived types.

-Wcompare-reals

Warn when comparing real or complex types for equality or inequality. This option is implied by -Wextra.

-Wtarget-lifetime

Warn if the pointer in a pointer assignment might be longer than the its target. This option is implied by -Wall.

-Wzerotrip

Warn if a DO loop is known to execute zero times at compile time. This option is implied by -Wall.

-Wdo-subscript

Warn if an array subscript inside a DO loop could lead to an out-of-bounds access even if the compiler cannot prove that the statement is actually executed, in cases like

  real a(3)
  do i=1,4
    if (condition(i)) then
      a(i) = 1.2
    end if
  end do

This option is implied by -Wextra.

-Werror

Turns all warnings into errors.

See Options to Request or Suppress Errors and Warnings in Using the GNU Compiler Collection (GCC), 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.