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[thirdparty/gcc.git] / gcc / doc / gcc / gcc-command-options / options-to-request-or-suppress-warnings.rst

..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: options to control warnings, warning messages, messages, warning, suppressing warnings

.. _warning-options:

Options to Request or Suppress Warnings
***************************************

Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.

The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.

.. index:: syntax checking

.. option:: -fsyntax-only

  Check the code for syntax errors, but don't do anything beyond that.

.. option:: -fmax-errors={n}

  Limits the maximum number of error messages to :samp:`{n}`, at which point
  GCC bails out rather than attempting to continue processing the source
  code.  If :samp:`{n}` is 0 (the default), there is no limit on the number
  of error messages produced.  If :option:`-Wfatal-errors` is also
  specified, then :option:`-Wfatal-errors` takes precedence over this
  option.

.. option:: -w

  Inhibit all warning messages.

.. option:: -Werror

  Make all warnings into errors.

.. option:: -Wno-error

  Default setting; overrides :option:`-Werror`.

.. option:: -Werror=

  Make the specified warning into an error.  The specifier for a warning
  is appended; for example :option:`-Werror=switch` turns the warnings
  controlled by :option:`-Wswitch` into errors.  This switch takes a
  negative form, to be used to negate :option:`-Werror` for specific
  warnings; for example :option:`-Wno-error=switch` makes
  :option:`-Wswitch` warnings not be errors, even when :option:`-Werror`
  is in effect.

  The warning message for each controllable warning includes the
  option that controls the warning.  That option can then be used with
  :option:`-Werror=` and :option:`-Wno-error=` as described above.
  (Printing of the option in the warning message can be disabled using the
  :option:`-fno-diagnostics-show-option` flag.)

  Note that specifying :option:`-Werror=foo` automatically implies
  :option:`-Wfoo`.  However, :option:`-Wno-error=foo` does not
  imply anything.

.. option:: -Wno-error=

  Default setting; overrides :option:`-Werror=`.

.. option:: -Wfatal-errors

  This option causes the compiler to abort compilation on the first error
  occurred rather than trying to keep going and printing further error
  messages.

.. option:: -Wno-fatal-errors

  Default setting; overrides :option:`-Wfatal-errors`.

You can request many specific warnings with options beginning with
:samp:`-W`, for example :option:`-Wimplicit` to request warnings on
implicit declarations.  Each of these specific warning options also
has a negative form beginning :samp:`-Wno-` to turn off warnings; for
example, :option:`-Wno-implicit`.  This manual lists only one of the
two forms, whichever is not the default.  For further
language-specific options also refer to :ref:`c++-dialect-options` and
:ref:`objective-c-and-objective-c++-dialect-options`.
Additional warnings can be produced by enabling the static analyzer;
See :ref:`static-analyzer-options`.

Some options, such as :option:`-Wall` and :option:`-Wextra`, turn on other
options, such as :option:`-Wunused`, which may turn on further options,
such as :option:`-Wunused-value`. The combined effect of positive and
negative forms is that more specific options have priority over less
specific ones, independently of their position in the command-line. For
options of the same specificity, the last one takes effect. Options
enabled or disabled via pragmas (see :ref:`diagnostic-pragmas`) take effect
as if they appeared at the end of the command-line.

When an unrecognized warning option is requested (e.g.,
:option:`-Wunknown-warning`), GCC emits a diagnostic stating
that the option is not recognized.  However, if the :option:`-Wno-` form
is used, the behavior is slightly different: no diagnostic is
produced for :option:`-Wno-unknown-warning` unless other diagnostics
are being produced.  This allows the use of new :option:`-Wno-` options
with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.

The effectiveness of some warnings depends on optimizations also being
enabled. For example :option:`-Wsuggest-final-types` is more effective
with link-time optimization and some instances of other warnings may
not be issued at all unless optimization is enabled.  While optimization
in general improves the efficacy of control and data flow sensitive
warnings, in some cases it may also cause false positives.

.. option:: -Wpedantic, -pedantic

  Issue all the warnings demanded by strict ISO C and ISO C++;
  reject all programs that use forbidden extensions, and some other
  programs that do not follow ISO C and ISO C++.  For ISO C, follows the
  version of the ISO C standard specified by any :option:`-std` option used.

  Valid ISO C and ISO C++ programs should compile properly with or without
  this option (though a rare few require :option:`-ansi` or a
  :option:`-std` option specifying the required version of ISO C).  However,
  without this option, certain GNU extensions and traditional C and C++
  features are supported as well.  With this option, they are rejected.

  :option:`-Wpedantic` does not cause warning messages for use of the
  alternate keywords whose names begin and end with :samp:`__`.  This alternate
  format can also be used to disable warnings for non-ISO :samp:`__intN` types,
  i.e. :samp:`__intN__`.
  Pedantic warnings are also disabled in the expression that follows
  ``__extension__``.  However, only system header files should use
  these escape routes; application programs should avoid them.
  See :ref:`alternate-keywords`.

  Some users try to use :option:`-Wpedantic` to check programs for strict ISO
  C conformance.  They soon find that it does not do quite what they want:
  it finds some non-ISO practices, but not all---only those for which
  ISO C *requires* a diagnostic, and some others for which
  diagnostics have been added.

  A feature to report any failure to conform to ISO C might be useful in
  some instances, but would require considerable additional work and would
  be quite different from :option:`-Wpedantic`.  We don't have plans to
  support such a feature in the near future.

  Where the standard specified with :option:`-std` represents a GNU
  extended dialect of C, such as :samp:`gnu90` or :samp:`gnu99`, there is a
  corresponding :dfn:`base standard`, the version of ISO C on which the GNU
  extended dialect is based.  Warnings from :option:`-Wpedantic` are given
  where they are required by the base standard.  (It does not make sense
  for such warnings to be given only for features not in the specified GNU
  C dialect, since by definition the GNU dialects of C include all
  features the compiler supports with the given option, and there would be
  nothing to warn about.)

.. option:: -Wno-pedantic

  Default setting; overrides :option:`-Wpedantic`.

.. option:: -pedantic-errors

  Give an error whenever the :dfn:`base standard` (see :option:`-Wpedantic`)
  requires a diagnostic, in some cases where there is undefined behavior
  at compile-time and in some other cases that do not prevent compilation
  of programs that are valid according to the standard. This is not
  equivalent to :option:`-Werror=pedantic`, since there are errors enabled
  by this option and not enabled by the latter and vice versa.

.. option:: -Wall

  This enables all the warnings about constructions that some users
  consider questionable, and that are easy to avoid (or modify to
  prevent the warning), even in conjunction with macros.  This also
  enables some language-specific warnings described in :ref:`c++-dialect-options` and :ref:`objective-c-and-objective-c++-dialect-options`.

  :option:`-Wall` turns on the following warning flags:

  :option:`-Waddress`   |gol|
  :option:`-Warray-bounds=1` (only with :option:`-O2` )  |gol|
  :option:`-Warray-compare` |gol|
  :option:`-Warray-parameter=2` (C and Objective :option:`-C` only) |gol|
  :option:`-Wbool-compare`  |gol|
  :option:`-Wbool-operation`  |gol|
  :option:`-Wc++11-compat`  :option:`-Wc++14-compat`  |gol|
  :option:`-Wcatch-value` (C++ and Objective :option:`-C++` only)  |gol|
  :option:`-Wchar-subscripts`  |gol|
  :option:`-Wcomment`  |gol|
  :option:`-Wdangling-pointer=2`  |gol|
  :option:`-Wduplicate-decl-specifier` (C and Objective :option:`-C` only) |gol|
  :option:`-Wenum-compare` (in C/ObjC; this is on by default in C++) |gol|
  :option:`-Wenum-int-mismatch` (C and Objective :option:`-C` only) |gol|
  :option:`-Wformat`   |gol|
  :option:`-Wformat-overflow`  |gol|
  :option:`-Wformat-truncation`  |gol|
  :option:`-Wint-in-bool-context`  |gol|
  :option:`-Wimplicit` (C and Objective :option:`-C` only) |gol|
  :option:`-Wimplicit-int` (C and Objective :option:`-C` only) |gol|
  :option:`-Wimplicit-function-declaration` (C and Objective :option:`-C` only) |gol|
  :option:`-Winit-self` (only for C++) |gol|
  :option:`-Wlogical-not-parentheses` |gol|
  :option:`-Wmain` (only for C/ObjC and unless :option:`-ffreestanding` )  |gol|
  :option:`-Wmaybe-uninitialized` |gol|
  :option:`-Wmemset-elt-size` |gol|
  :option:`-Wmemset-transposed-args` |gol|
  :option:`-Wmisleading-indentation` (only for C/C++) |gol|
  :option:`-Wmismatched-dealloc` |gol|
  :option:`-Wmismatched-new-delete` (only for C/C++) |gol|
  :option:`-Wmissing-attributes` |gol|
  :option:`-Wmissing-braces` (only for C/ObjC) |gol|
  :option:`-Wmultistatement-macros`  |gol|
  :option:`-Wnarrowing` (only for C++)  |gol|
  :option:`-Wnonnull`  |gol|
  :option:`-Wnonnull-compare`  |gol|
  :option:`-Wopenmp-simd` |gol|
  :option:`-Wparentheses`  |gol|
  :option:`-Wpessimizing-move` (only for C++)  |gol|
  :option:`-Wpointer-sign`  |gol|
  :option:`-Wrange-loop-construct` (only for C++)  |gol|
  :option:`-Wreorder`   |gol|
  :option:`-Wrestrict`   |gol|
  :option:`-Wreturn-type`  |gol|
  :option:`-Wself-move` (only for C++)  |gol|
  :option:`-Wsequence-point`  |gol|
  :option:`-Wsign-compare` (only in C++)  |gol|
  :option:`-Wsizeof-array-div` |gol|
  :option:`-Wsizeof-pointer-div` |gol|
  :option:`-Wsizeof-pointer-memaccess` |gol|
  :option:`-Wstrict-aliasing`  |gol|
  :option:`-Wstrict-overflow=1`  |gol|
  :option:`-Wswitch`  |gol|
  :option:`-Wtautological-compare`  |gol|
  :option:`-Wtrigraphs`  |gol|
  :option:`-Wuninitialized`  |gol|
  :option:`-Wunknown-pragmas`  |gol|
  :option:`-Wunused-function`  |gol|
  :option:`-Wunused-label`     |gol|
  :option:`-Wunused-value`     |gol|
  :option:`-Wunused-variable`  |gol|
  :option:`-Wuse-after-free=3`  |gol|
  :option:`-Wvla-parameter` (C and Objective :option:`-C` only) |gol|
  :option:`-Wvolatile-register-var`  |gol|
  :option:`-Wzero-length-bounds`

  Note that some warning flags are not implied by :option:`-Wall`.  Some of
  them warn about constructions that users generally do not consider
  questionable, but which occasionally you might wish to check for;
  others warn about constructions that are necessary or hard to avoid in
  some cases, and there is no simple way to modify the code to suppress
  the warning. Some of them are enabled by :option:`-Wextra` but many of
  them must be enabled individually.

.. option:: -Wno-all

  Default setting; overrides :option:`-Wall`.

.. option:: -Wextra

  This enables some extra warning flags that are not enabled by
  :option:`-Wall`. (This option used to be called :option:`-W`.  The older
  name is still supported, but the newer name is more descriptive.)

  :option:`-Wclobbered`  |gol|
  :option:`-Wcast-function-type`  |gol|
  :option:`-Wdeprecated-copy` (C++ only) |gol|
  :option:`-Wempty-body`  |gol|
  :option:`-Wenum-conversion` (C only) |gol|
  :option:`-Wignored-qualifiers` |gol|
  :option:`-Wimplicit-fallthrough=3` |gol|
  :option:`-Wmissing-field-initializers`  |gol|
  :option:`-Wmissing-parameter-type` (C only)  |gol|
  :option:`-Wold-style-declaration` (C only)  |gol|
  :option:`-Woverride-init`  |gol|
  :option:`-Wsign-compare` (C only) |gol|
  :option:`-Wstring-compare` |gol|
  :option:`-Wredundant-move` (only for C++)  |gol|
  :option:`-Wtype-limits`  |gol|
  :option:`-Wuninitialized`  |gol|
  :option:`-Wshift-negative-value` (in C++11 to C++17 and in C99 and newer)  |gol|
  :option:`-Wunused-parameter` (only with :option:`-Wunused` or :option:`-Wall` ) |gol|
  :option:`-Wunused-but-set-parameter` (only with :option:`-Wunused` or :option:`-Wall` )
  The option :option:`-Wextra` also prints warning messages for the
  following cases:

  * A pointer is compared against integer zero with ``<``, ``<=``,
    ``>``, or ``>=``.

  * (C++ only) An enumerator and a non-enumerator both appear in a
    conditional expression.

  * (C++ only) Ambiguous virtual bases.

  * (C++ only) Subscripting an array that has been declared ``register``.

  * (C++ only) Taking the address of a variable that has been declared
    ``register``.

  * (C++ only) A base class is not initialized in the copy constructor
    of a derived class.

.. option:: -Wno-extra

  Default setting; overrides :option:`-Wextra`.

.. option:: -Wabi

  .. note::

    C, Objective-C, C++ and Objective-C++ only

  Warn about code affected by ABI changes.  This includes code that may
  not be compatible with the vendor-neutral C++ ABI as well as the psABI
  for the particular target.

  Since G++ now defaults to updating the ABI with each major release,
  normally :option:`-Wabi` warns only about C++ ABI compatibility
  problems if there is a check added later in a release series for an
  ABI issue discovered since the initial release.  :option:`-Wabi` warns
  about more things if an older ABI version is selected (with
  :option:`-fabi-version=n`).

  :option:`-Wabi` can also be used with an explicit version number to
  warn about C++ ABI compatibility with a particular :option:`-fabi-version`
  level, e.g. :option:`-Wabi=2` to warn about changes relative to
  :option:`-fabi-version=2`.

  If an explicit version number is provided and
  :option:`-fabi-compat-version` is not specified, the version number
  from this option is used for compatibility aliases.  If no explicit
  version number is provided with this option, but
  :option:`-fabi-compat-version` is specified, that version number is
  used for C++ ABI warnings.

  Although an effort has been made to warn about
  all such cases, there are probably some cases that are not warned about,
  even though G++ is generating incompatible code.  There may also be
  cases where warnings are emitted even though the code that is generated
  is compatible.

  You should rewrite your code to avoid these warnings if you are
  concerned about the fact that code generated by G++ may not be binary
  compatible with code generated by other compilers.

  Known incompatibilities in :option:`-fabi-version=2` (which was the
  default from GCC 3.4 to 4.9) include:

  * A template with a non-type template parameter of reference type was
    mangled incorrectly:

    .. code-block:: c++

      extern int N;
      template <int &> struct S {};
      void n (S<N>) {2}

    This was fixed in :option:`-fabi-version=3`.

  * SIMD vector types declared using ``__attribute ((vector_size))`` were
    mangled in a non-standard way that does not allow for overloading of
    functions taking vectors of different sizes.

    The mangling was changed in :option:`-fabi-version=4`.

  * ``__attribute ((const))`` and :fn-attr:`noreturn` were mangled as type
    qualifiers, and ``decltype`` of a plain declaration was folded away.

    These mangling issues were fixed in :option:`-fabi-version=5`.

  * Scoped enumerators passed as arguments to a variadic function are
    promoted like unscoped enumerators, causing ``va_arg`` to complain.
    On most targets this does not actually affect the parameter passing
    ABI, as there is no way to pass an argument smaller than ``int``.

    Also, the ABI changed the mangling of template argument packs,
    ``const_cast``, ``static_cast``, prefix increment/decrement, and
    a class scope function used as a template argument.

    These issues were corrected in :option:`-fabi-version=6`.

  * Lambdas in default argument scope were mangled incorrectly, and the
    ABI changed the mangling of ``nullptr_t``.

    These issues were corrected in :option:`-fabi-version=7`.

  * When mangling a function type with function-cv-qualifiers, the
    un-qualified function type was incorrectly treated as a substitution
    candidate.

    This was fixed in :option:`-fabi-version=8`, the default for GCC 5.1.

  * ``decltype(nullptr)`` incorrectly had an alignment of 1, leading to
    unaligned accesses.  Note that this did not affect the ABI of a
    function with a ``nullptr_t`` parameter, as parameters have a
    minimum alignment.

    This was fixed in :option:`-fabi-version=9`, the default for GCC 5.2.

  * Target-specific attributes that affect the identity of a type, such as
    ia32 calling conventions on a function type (stdcall, regparm, etc.),
    did not affect the mangled name, leading to name collisions when
    function pointers were used as template arguments.

    This was fixed in :option:`-fabi-version=10`, the default for GCC 6.1.

  This option also enables warnings about psABI-related changes.
  The known psABI changes at this point include:

  * For SysV/x86-64, unions with ``long double`` members are
    passed in memory as specified in psABI.  Prior to GCC 4.4, this was not
    the case.  For example:

    .. code-block:: c++

      union U {
        long double ld;
        int i;
      };

    ``union U`` is now always passed in memory.

.. option:: -Wno-abi

  Default setting; overrides :option:`-Wabi`.

.. option:: -Wchar-subscripts

  Warn if an array subscript has type ``char``.  This is a common cause
  of error, as programmers often forget that this type is signed on some
  machines.
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-char-subscripts

  Default setting; overrides :option:`-Wchar-subscripts`.

.. option:: -Wno-coverage-mismatch

  Warn if feedback profiles do not match when using the
  :option:`-fprofile-use` option.
  If a source file is changed between compiling with :option:`-fprofile-generate`
  and with :option:`-fprofile-use`, the files with the profile feedback can fail
  to match the source file and GCC cannot use the profile feedback
  information.  By default, this warning is enabled and is treated as an
  error.  :option:`-Wno-coverage-mismatch` can be used to disable the
  warning or :option:`-Wno-error=coverage-mismatch` can be used to
  disable the error.  Disabling the error for this warning can result in
  poorly optimized code and is useful only in the
  case of very minor changes such as bug fixes to an existing code-base.
  Completely disabling the warning is not recommended.

.. option:: -Wcoverage-mismatch

  Default setting; overrides :option:`-Wno-coverage-mismatch`.

.. option:: -Wno-coverage-invalid-line-number

  Warn in case a function ends earlier than it begins due
  to an invalid linenum macros.  The warning is emitted only
  with :option:`--coverage` enabled.

  By default, this warning is enabled and is treated as an
  error.  :option:`-Wno-coverage-invalid-line-number` can be used to disable the
  warning or :option:`-Wno-error=coverage-invalid-line-number` can be used to
  disable the error.

.. option:: -Wcoverage-invalid-line-number

  Default setting; overrides :option:`-Wno-coverage-invalid-line-number`.

.. option:: -Wno-cpp

  .. note::

    C, Objective-C, C++, Objective-C++ and Fortran only

  Suppress warning messages emitted by ``#warning`` directives.

.. option:: -Wcpp

  Default setting; overrides :option:`-Wno-cpp`.

.. option:: -Wdouble-promotion

  .. note::

    C, C++, Objective-C and Objective-C++ only

  Give a warning when a value of type ``float`` is implicitly
  promoted to ``double``.  CPUs with a 32-bit 'single-precision'
  floating-point unit implement ``float`` in hardware, but emulate
  ``double`` in software.  On such a machine, doing computations
  using ``double`` values is much more expensive because of the
  overhead required for software emulation.

  It is easy to accidentally do computations with ``double`` because
  floating-point literals are implicitly of type ``double``.  For
  example, in:

  .. code-block:: c++

    float area(float radius)
    {
       return 3.14159 * radius * radius;
    }

  the compiler performs the entire computation with ``double``
  because the floating-point literal is a ``double``.

.. option:: -Wno-double-promotion

  Default setting; overrides :option:`-Wdouble-promotion`.

.. option:: -Wduplicate-decl-specifier

  .. note::

    C and Objective-C only

  Warn if a declaration has duplicate ``const``, ``volatile``,
  ``restrict`` or ``_Atomic`` specifier.  This warning is enabled by
  :option:`-Wall`.

.. option:: -Wno-duplicate-decl-specifier

  Default setting; overrides :option:`-Wduplicate-decl-specifier`.

.. option:: -Wformat, -Wformat={n}

  Check calls to ``printf`` and ``scanf``, etc., to make sure that
  the arguments supplied have types appropriate to the format string
  specified, and that the conversions specified in the format string make
  sense.  This includes standard functions, and others specified by format
  attributes (see :ref:`function-attributes`), in the ``printf``,
  ``scanf``, ``strftime`` and ``strfmon`` (an X/Open extension,
  not in the C standard) families (or other target-specific families).
  Which functions are checked without format attributes having been
  specified depends on the standard version selected, and such checks of
  functions without the attribute specified are disabled by
  :option:`-ffreestanding` or :option:`-fno-builtin`.

  The formats are checked against the format features supported by GNU
  libc version 2.2.  These include all ISO C90 and C99 features, as well
  as features from the Single Unix Specification and some BSD and GNU
  extensions.  Other library implementations may not support all these
  features; GCC does not support warning about features that go beyond a
  particular library's limitations.  However, if :option:`-Wpedantic` is used
  with :option:`-Wformat`, warnings are given about format features not
  in the selected standard version (but not for ``strfmon`` formats,
  since those are not in any version of the C standard).  See :ref:`c-dialect-options`.

  .. option:: -Wformat=1

    Option :option:`-Wformat` is equivalent to :option:`-Wformat=1`, and
    :option:`-Wno-format` is equivalent to :option:`-Wformat=0`.  Since
    :option:`-Wformat` also checks for null format arguments for several
    functions, :option:`-Wformat` also implies :option:`-Wnonnull`.  Some
    aspects of this level of format checking can be disabled by the
    options: :option:`-Wno-format-contains-nul`,
    :option:`-Wno-format-extra-args`, and :option:`-Wno-format-zero-length`.
    :option:`-Wformat` is enabled by :option:`-Wall`.

  .. option:: -Wformat=2

    Enable :option:`-Wformat` plus additional format checks.  Currently
    equivalent to :option:`-Wformat` :option:`-Wformat-nonliteral` :option:`-Wformat-security`
    :option:`-Wformat-y2k`.

.. option:: -Wno-format

  Default setting; overrides :option:`-Wformat`.

.. option:: -Wno-format-contains-nul

  If :option:`-Wformat` is specified, do not warn about format strings that
  contain NUL bytes.

.. option:: -Wformat-contains-nul

  Default setting; overrides :option:`-Wno-format-contains-nul`.

.. option:: -Wno-format-extra-args

  If :option:`-Wformat` is specified, do not warn about excess arguments to a
  ``printf`` or ``scanf`` format function.  The C standard specifies
  that such arguments are ignored.

  Where the unused arguments lie between used arguments that are
  specified with :samp:`$` operand number specifications, normally
  warnings are still given, since the implementation could not know what
  type to pass to ``va_arg`` to skip the unused arguments.  However,
  in the case of ``scanf`` formats, this option suppresses the
  warning if the unused arguments are all pointers, since the Single
  Unix Specification says that such unused arguments are allowed.

.. option:: -Wformat-extra-args

  Default setting; overrides :option:`-Wno-format-extra-args`.

.. option:: -Wformat-overflow, -Wformat-overflow={level}

  Warn about calls to formatted input/output functions such as ``sprintf``
  and ``vsprintf`` that might overflow the destination buffer.  When the
  exact number of bytes written by a format directive cannot be determined
  at compile-time it is estimated based on heuristics that depend on the
  :samp:`{level}` argument and on optimization.  While enabling optimization
  will in most cases improve the accuracy of the warning, it may also
  result in false positives.

  .. option:: -Wformat-overflow, -Wformat-overflow=1

    Level :samp:`{1}` of :option:`-Wformat-overflow` enabled by :option:`-Wformat`
    employs a conservative approach that warns only about calls that most
    likely overflow the buffer.  At this level, numeric arguments to format
    directives with unknown values are assumed to have the value of one, and
    strings of unknown length to be empty.  Numeric arguments that are known
    to be bounded to a subrange of their type, or string arguments whose output
    is bounded either by their directive's precision or by a finite set of
    string literals, are assumed to take on the value within the range that
    results in the most bytes on output.  For example, the call to ``sprintf``
    below is diagnosed because even with both :samp:`{a}` and :samp:`{b}` equal to zero,
    the terminating NUL character (``'\0'``) appended by the function
    to the destination buffer will be written past its end.  Increasing
    the size of the buffer by a single byte is sufficient to avoid the
    warning, though it may not be sufficient to avoid the overflow.

    .. code-block:: c++

      void f (int a, int b)
      {
        char buf [13];
        sprintf (buf, "a = %i, b = %i\n", a, b);
      }

  .. option:: -Wno-format-overflow

    Default setting; overrides :option:`-Wformat-overflow`.

  ``-Wformat-overflow=2``
    Level :samp:`{2}` warns also about calls that might overflow the destination
    buffer given an argument of sufficient length or magnitude.  At level
    :samp:`{2}`, unknown numeric arguments are assumed to have the minimum
    representable value for signed types with a precision greater than 1, and
    the maximum representable value otherwise.  Unknown string arguments whose
    length cannot be assumed to be bounded either by the directive's precision,
    or by a finite set of string literals they may evaluate to, or the character
    array they may point to, are assumed to be 1 character long.

    At level :samp:`{2}`, the call in the example above is again diagnosed, but
    this time because with :samp:`{a}` equal to a 32-bit ``INT_MIN`` the first
    ``%i`` directive will write some of its digits beyond the end of
    the destination buffer.  To make the call safe regardless of the values
    of the two variables, the size of the destination buffer must be increased
    to at least 34 bytes.  GCC includes the minimum size of the buffer in
    an informational note following the warning.

    An alternative to increasing the size of the destination buffer is to
    constrain the range of formatted values.  The maximum length of string
    arguments can be bounded by specifying the precision in the format
    directive.  When numeric arguments of format directives can be assumed
    to be bounded by less than the precision of their type, choosing
    an appropriate length modifier to the format specifier will reduce
    the required buffer size.  For example, if :samp:`{a}` and :samp:`{b}` in the
    example above can be assumed to be within the precision of
    the ``short int`` type then using either the ``%hi`` format
    directive or casting the argument to ``short`` reduces the maximum
    required size of the buffer to 24 bytes.

    .. code-block:: c++

      void f (int a, int b)
      {
        char buf [23];
        sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
      }

.. option:: -Wno-format-overflow

  Default setting; overrides :option:`-Wformat-overflow`.

.. option:: -Wno-format-zero-length

  If :option:`-Wformat` is specified, do not warn about zero-length formats.
  The C standard specifies that zero-length formats are allowed.

.. option:: -Wformat-zero-length

  Default setting; overrides :option:`-Wno-format-zero-length`.

.. option:: -Wformat-nonliteral

  If :option:`-Wformat` is specified, also warn if the format string is not a
  string literal and so cannot be checked, unless the format function
  takes its format arguments as a ``va_list``.

.. option:: -Wno-format-nonliteral

  Default setting; overrides :option:`-Wformat-nonliteral`.

.. option:: -Wformat-security

  If :option:`-Wformat` is specified, also warn about uses of format
  functions that represent possible security problems.  At present, this
  warns about calls to ``printf`` and ``scanf`` functions where the
  format string is not a string literal and there are no format arguments,
  as in ``printf (foo);``.  This may be a security hole if the format
  string came from untrusted input and contains :samp:`%n`.  (This is
  currently a subset of what :option:`-Wformat-nonliteral` warns about, but
  in future warnings may be added to :option:`-Wformat-security` that are not
  included in :option:`-Wformat-nonliteral`.)

.. option:: -Wno-format-security

  Default setting; overrides :option:`-Wformat-security`.

.. option:: -Wformat-signedness

  If :option:`-Wformat` is specified, also warn if the format string
  requires an unsigned argument and the argument is signed and vice versa.

.. option:: -Wno-format-signedness

  Default setting; overrides :option:`-Wformat-signedness`.

.. option:: -Wformat-truncation, -Wformat-truncation={level}

  Warn about calls to formatted input/output functions such as ``snprintf``
  and ``vsnprintf`` that might result in output truncation.  When the exact
  number of bytes written by a format directive cannot be determined at
  compile-time it is estimated based on heuristics that depend on
  the :samp:`{level}` argument and on optimization.  While enabling optimization
  will in most cases improve the accuracy of the warning, it may also result
  in false positives.  Except as noted otherwise, the option uses the same
  logic :option:`-Wformat-overflow`.

  .. option:: -Wformat-truncation, -Wformat-truncation=1

    Level :samp:`{1}` of :option:`-Wformat-truncation` enabled by :option:`-Wformat`
    employs a conservative approach that warns only about calls to bounded
    functions whose return value is unused and that will most likely result
    in output truncation.

  .. option:: -Wformat-truncation=2

    Level :samp:`{2}` warns also about calls to bounded functions whose return
    value is used and that might result in truncation given an argument of
    sufficient length or magnitude.

.. option:: -Wno-format-truncation

  Default setting; overrides :option:`-Wformat-truncation`.

.. option:: -Wformat-y2k

  If :option:`-Wformat` is specified, also warn about ``strftime``
  formats that may yield only a two-digit year.

.. option:: -Wno-format-y2k

  Default setting; overrides :option:`-Wformat-y2k`.

.. option:: -Wnonnull

  Warn about passing a null pointer for arguments marked as
  requiring a non-null value by the :fn-attr:`nonnull` function attribute.

  :option:`-Wnonnull` is included in :option:`-Wall` and :option:`-Wformat`.  It
  can be disabled with the :option:`-Wno-nonnull` option.

.. option:: -Wno-nonnull

  Default setting; overrides :option:`-Wnonnull`.

.. option:: -Wnonnull-compare

  Warn when comparing an argument marked with the :fn-attr:`nonnull`
  function attribute against null inside the function.

  :option:`-Wnonnull-compare` is included in :option:`-Wall`.  It
  can be disabled with the :option:`-Wno-nonnull-compare` option.

.. option:: -Wno-nonnull-compare

  Default setting; overrides :option:`-Wnonnull-compare`.

.. option:: -Wnull-dereference

  Warn if the compiler detects paths that trigger erroneous or
  undefined behavior due to dereferencing a null pointer.  This option
  is only active when :option:`-fdelete-null-pointer-checks` is active,
  which is enabled by optimizations in most targets.  The precision of
  the warnings depends on the optimization options used.

.. option:: -Wno-null-dereference

  Default setting; overrides :option:`-Wnull-dereference`.

.. option:: -Winfinite-recursion

  Warn about infinitely recursive calls.  The warning is effective at all
  optimization levels but requires optimization in order to detect infinite
  recursion in calls between two or more functions.
  :option:`-Winfinite-recursion` is included in :option:`-Wall`.

.. option:: -Wno-infinite-recursion

  Default setting; overrides :option:`-Winfinite-recursion`.

.. option:: -Winit-self

  .. note::

    C, C++, Objective-C and Objective-C++ only

  Warn about uninitialized variables that are initialized with themselves.
  Note this option can only be used with the :option:`-Wuninitialized` option.

  For example, GCC warns about ``i`` being uninitialized in the
  following snippet only when :option:`-Winit-self` has been specified:

  .. code-block:: c++

    int f()
    {
      int i = i;
      return i;
    }

  This warning is enabled by :option:`-Wall` in C++.

.. option:: -Wno-init-self

  Default setting; overrides :option:`-Winit-self`.

.. option:: -Wno-implicit-int

  .. note::

    C and Objective-C only

  This option controls warnings when a declaration does not specify a type.
  This warning is enabled by default in C99 and later dialects of C,
  and also by :option:`-Wall`.

.. option:: -Wimplicit-int

  Default setting; overrides :option:`-Wno-implicit-int`.

.. option:: -Wno-implicit-function-declaration

  .. note::

    C and Objective-C only

  This option controls warnings when a function is used before being declared.
  This warning is enabled by default in C99 and later dialects of C,
  and also by :option:`-Wall`.
  The warning is made into an error by :option:`-pedantic-errors`.

.. option:: -Wimplicit-function-declaration

  Default setting; overrides :option:`-Wno-implicit-function-declaration`.

.. option:: -Wimplicit

  .. note::

    C and Objective-C only

  Same as :option:`-Wimplicit-int` and :option:`-Wimplicit-function-declaration`.
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-implicit

  Default setting; overrides :option:`-Wimplicit`.

.. option:: -Wimplicit-fallthrough

  :option:`-Wimplicit-fallthrough` is the same as :option:`-Wimplicit-fallthrough=3`
  and :option:`-Wno-implicit-fallthrough` is the same as
  :option:`-Wimplicit-fallthrough=0`.

.. option:: -Wno-implicit-fallthrough

  Default setting; overrides :option:`-Wimplicit-fallthrough`.

.. option:: -Wimplicit-fallthrough={n}

  Warn when a switch case falls through.  For example:

  .. code-block:: c++

    switch (cond)
      {
      case 1:
        a = 1;
        break;
      case 2:
        a = 2;
      case 3:
        a = 3;
        break;
      }

  This warning does not warn when the last statement of a case cannot
  fall through, e.g. when there is a return statement or a call to function
  declared with the noreturn attribute.  :option:`-Wimplicit-fallthrough=`
  also takes into account control flow statements, such as ifs, and only
  warns when appropriate.  E.g.

  .. code-block:: c++

    switch (cond)
      {
      case 1:
        if (i > 3) {
          bar (5);
          break;
        } else if (i < 1) {
          bar (0);
        } else
          return;
      default:
        ...
      }

  Since there are occasions where a switch case fall through is desirable,
  GCC provides an attribute, ``__attribute__ ((fallthrough))``, that is
  to be used along with a null statement to suppress this warning that
  would normally occur:

  .. code-block:: c++

    switch (cond)
      {
      case 1:
        bar (0);
        __attribute__ ((fallthrough));
      default:
        ...
      }

  C++17 provides a standard way to suppress the :option:`-Wimplicit-fallthrough`
  warning using ``[[fallthrough]];`` instead of the GNU attribute.  In C++11
  or C++14 users can use ``[[gnu::fallthrough]];``, which is a GNU extension.
  Instead of these attributes, it is also possible to add a fallthrough comment
  to silence the warning.  The whole body of the C or C++ style comment should
  match the given regular expressions listed below.  The option argument :samp:`{n}`
  specifies what kind of comments are accepted:

  * :option:`-Wimplicit-fallthrough=0` disables the warning altogether.

  * :option:`-Wimplicit-fallthrough=1` matches ``.*`` regular
    expression, any comment is used as fallthrough comment.

  * :option:`-Wimplicit-fallthrough=2` case insensitively matches
    ``.*falls?[ \t-]*thr(ough|u).*`` regular expression.

  * :option:`-Wimplicit-fallthrough=3` case sensitively matches one of the
    following regular expressions:

    * ``-fallthrough``

    * ``@fallthrough@``

    * ``lint -fallthrough[ \t]*``

    * ``[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?
      FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?``

    * ``[ \t.!]*(Else,? |Intentional(ly)? )?
      Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?``

    * ``[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?
      fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?``

  * :option:`-Wimplicit-fallthrough=4` case sensitively matches one of the
    following regular expressions:

    * ``-fallthrough``

    * ``@fallthrough@``

    * ``lint -fallthrough[ \t]*``

    * ``[ \t]*FALLTHR(OUGH|U)[ \t]*``

  * :option:`-Wimplicit-fallthrough=5` doesn't recognize any comments as
    fallthrough comments, only attributes disable the warning.

  The comment needs to be followed after optional whitespace and other comments
  by ``case`` or ``default`` keywords or by a user label that precedes some
  ``case`` or ``default`` label.

  .. code-block:: c++

    switch (cond)
      {
      case 1:
        bar (0);
        /* FALLTHRU */
      default:
        ...
      }

  The :option:`-Wimplicit-fallthrough=3` warning is enabled by :option:`-Wextra`.

.. option:: -Wno-if-not-aligned

  .. note::

    C, C++, Objective-C and Objective-C++ only

  Control if warnings triggered by the ``warn_if_not_aligned`` attribute
  should be issued.  These warnings are enabled by default.

.. option:: -Wif-not-aligned

  Default setting; overrides :option:`-Wno-if-not-aligned`.

.. option:: -Wignored-qualifiers

  .. note::

    C and C++ only

  Warn if the return type of a function has a type qualifier
  such as ``const``.  For ISO C such a type qualifier has no effect,
  since the value returned by a function is not an lvalue.
  For C++, the warning is only emitted for scalar types or ``void``.
  ISO C prohibits qualified ``void`` return types on function
  definitions, so such return types always receive a warning
  even without this option.

  This warning is also enabled by :option:`-Wextra`.

.. option:: -Wno-ignored-qualifiers

  Default setting; overrides :option:`-Wignored-qualifiers`.

.. option:: -Wno-ignored-attributes

  .. note::

    C and C++ only

  This option controls warnings when an attribute is ignored.
  This is different from the
  :option:`-Wattributes` option in that it warns whenever the compiler decides
  to drop an attribute, not that the attribute is either unknown, used in a
  wrong place, etc.  This warning is enabled by default.

.. option:: -Wignored-attributes

  Default setting; overrides :option:`-Wno-ignored-attributes`.

.. option:: -Wmain

  Warn if the type of ``main`` is suspicious.  ``main`` should be
  a function with external linkage, returning int, taking either zero
  arguments, two, or three arguments of appropriate types.  This warning
  is enabled by default in C++ and is enabled by either :option:`-Wall`
  or :option:`-Wpedantic`.

.. option:: -Wno-main

  Default setting; overrides :option:`-Wmain`.

.. option:: -Wmisleading-indentation

  .. note::

    C and C++ only

  Warn when the indentation of the code does not reflect the block structure.
  Specifically, a warning is issued for ``if``, ``else``, ``while``, and
  ``for`` clauses with a guarded statement that does not use braces,
  followed by an unguarded statement with the same indentation.

  In the following example, the call to 'bar' is misleadingly indented as
  if it were guarded by the 'if' conditional.

  .. code-block:: c++

      if (some_condition ())
        foo ();
        bar ();  /* Gotcha: this is not guarded by the "if".  */

  In the case of mixed tabs and spaces, the warning uses the
  :option:`-ftabstop=` option to determine if the statements line up
  (defaulting to 8).

  The warning is not issued for code involving multiline preprocessor logic
  such as the following example.

  .. code-block:: c++

      if (flagA)
        foo (0);
    #if SOME_CONDITION_THAT_DOES_NOT_HOLD
      if (flagB)
    #endif
        foo (1);

  The warning is not issued after a ``#line`` directive, since this
  typically indicates autogenerated code, and no assumptions can be made
  about the layout of the file that the directive references.

  This warning is enabled by :option:`-Wall` in C and C++.

.. option:: -Wno-misleading-indentation

  Default setting; overrides :option:`-Wmisleading-indentation`.

.. option:: -Wmissing-attributes

  Warn when a declaration of a function is missing one or more attributes
  that a related function is declared with and whose absence may adversely
  affect the correctness or efficiency of generated code.  For example,
  the warning is issued for declarations of aliases that use attributes
  to specify less restrictive requirements than those of their targets.
  This typically represents a potential optimization opportunity.
  By contrast, the :option:`-Wattribute-alias=2` option controls warnings
  issued when the alias is more restrictive than the target, which could
  lead to incorrect code generation.
  Attributes considered include ``alloc_align``, ``alloc_size``,
  :fn-attr:`cold`, :fn-attr:`const`, :fn-attr:`hot`, :fn-attr:`leaf`, :fn-attr:`malloc`,
  :fn-attr:`nonnull`, :fn-attr:`noreturn`, :fn-attr:`nothrow`, :fn-attr:`pure`,
  :fn-attr:`returns_nonnull`, and :fn-attr:`returns_twice`.

  In C++, the warning is issued when an explicit specialization of a primary
  template declared with attribute ``alloc_align``, ``alloc_size``,
  ``assume_aligned``, ``format``, ``format_arg``, ``malloc``,
  or :fn-attr:`nonnull` is declared without it.  Attributes :fn-attr:`deprecated`,
  ``error``, and ``warning`` suppress the warning.
  (see :ref:`function-attributes`).

  You can use the ``copy`` attribute to apply the same
  set of attributes to a declaration as that on another declaration without
  explicitly enumerating the attributes. This attribute can be applied
  to declarations of functions (see :ref:`common-function-attributes`),
  variables (see :ref:`common-variable-attributes`), or types
  (see :ref:`common-type-attributes`).

  :option:`-Wmissing-attributes` is enabled by :option:`-Wall`.

  For example, since the declaration of the primary function template
  below makes use of both attribute ``malloc`` and ``alloc_size``
  the declaration of the explicit specialization of the template is
  diagnosed because it is missing one of the attributes.

  .. code-block:: c++

    template <class T>
    T* __attribute__ ((malloc, alloc_size (1)))
    allocate (size_t);

    template <>
    void* __attribute__ ((malloc))   // missing alloc_size
    allocate<void> (size_t);

.. option:: -Wno-missing-attributes

  Default setting; overrides :option:`-Wmissing-attributes`.

.. option:: -Wmissing-braces

  Warn if an aggregate or union initializer is not fully bracketed.  In
  the following example, the initializer for ``a`` is not fully
  bracketed, but that for ``b`` is fully bracketed.

  .. code-block:: c++

    int a[2][2] = { 0, 1, 2, 3 };
    int b[2][2] = { { 0, 1 }, { 2, 3 } };

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-missing-braces

  Default setting; overrides :option:`-Wmissing-braces`.

.. option:: -Wmissing-include-dirs

  .. note::

    C, C++, Objective-C, Objective-C++ and Fortran only

  Warn if a user-supplied include directory does not exist. This opions is disabled
  by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially
  enabled by default by warning for -I and -J, only.

.. option:: -Wno-missing-include-dirs

  Default setting; overrides :option:`-Wmissing-include-dirs`.

.. option:: -Wno-missing-profile

  This option controls warnings if feedback profiles are missing when using the
  :option:`-fprofile-use` option.
  This option diagnoses those cases where a new function or a new file is added
  between compiling with :option:`-fprofile-generate` and with
  :option:`-fprofile-use`, without regenerating the profiles.
  In these cases, the profile feedback data files do not contain any
  profile feedback information for
  the newly added function or file respectively.  Also, in the case when profile
  count data (.gcda) files are removed, GCC cannot use any profile feedback
  information.  In all these cases, warnings are issued to inform you that a
  profile generation step is due.
  Ignoring the warning can result in poorly optimized code.
  :option:`-Wno-missing-profile` can be used to
  disable the warning, but this is not recommended and should be done only
  when non-existent profile data is justified.

.. option:: -Wmissing-profile

  Default setting; overrides :option:`-Wno-missing-profile`.

.. option:: -Wmismatched-dealloc

  Warn for calls to deallocation functions with pointer arguments returned
  from from allocations functions for which the former isn't a suitable
  deallocator.  A pair of functions can be associated as matching allocators
  and deallocators by use of attribute ``malloc``.  Unless disabled by
  the :option:`-fno-builtin` option the standard functions ``calloc``,
  ``malloc``, ``realloc``, and ``free``, as well as the corresponding
  forms of C++ ``operator new`` and ``operator delete`` are implicitly
  associated as matching allocators and deallocators.  In the following
  example ``mydealloc`` is the deallocator for pointers returned from
  ``myalloc``.

  .. code-block:: c++

    void mydealloc (void*);

    __attribute__ ((malloc (mydealloc, 1))) void*
    myalloc (size_t);

    void f (void)
    {
      void *p = myalloc (32);
      // ...use p...
      free (p);   // warning: not a matching deallocator for myalloc
      mydealloc (p);   // ok
    }

  In C++, the related option :option:`-Wmismatched-new-delete` diagnoses
  mismatches involving either ``operator new`` or ``operator delete``.

  Option :option:`-Wmismatched-dealloc` is included in :option:`-Wall`.

.. option:: -Wno-mismatched-dealloc

  Default setting; overrides :option:`-Wmismatched-dealloc`.

.. option:: -Wmultistatement-macros

  Warn about unsafe multiple statement macros that appear to be guarded
  by a clause such as ``if``, ``else``, ``for``, ``switch``, or
  ``while``, in which only the first statement is actually guarded after
  the macro is expanded.

  For example:

  .. code-block:: c++

    #define DOIT x++; y++
    if (c)
      DOIT;

  will increment ``y`` unconditionally, not just when ``c`` holds.
  The can usually be fixed by wrapping the macro in a do-while loop:

  .. code-block:: c++

    #define DOIT do { x++; y++; } while (0)
    if (c)
      DOIT;

  This warning is enabled by :option:`-Wall` in C and C++.

.. option:: -Wno-multistatement-macros

  Default setting; overrides :option:`-Wmultistatement-macros`.

.. option:: -Wparentheses

  Warn if parentheses are omitted in certain contexts, such
  as when there is an assignment in a context where a truth value
  is expected, or when operators are nested whose precedence people
  often get confused about.

  Also warn if a comparison like ``x<=y<=z`` appears; this is
  equivalent to ``(x<=y ? 1 : 0) <= z``, which is a different
  interpretation from that of ordinary mathematical notation.

  Also warn for dangerous uses of the GNU extension to
  ``?:`` with omitted middle operand. When the condition
  in the ``?`` : operator is a boolean expression, the omitted value is
  always 1.  Often programmers expect it to be a value computed
  inside the conditional expression instead.

  For C++ this also warns for some cases of unnecessary parentheses in
  declarations, which can indicate an attempt at a function call instead
  of a declaration:

  .. code-block:: c++

    {
      // Declares a local variable called mymutex.
      std::unique_lock<std::mutex> (mymutex);
      // User meant std::unique_lock<std::mutex> lock (mymutex);
    }

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-parentheses

  Default setting; overrides :option:`-Wparentheses`.

.. option:: -Wno-self-move

  .. note::

    C++ and Objective-C++ only

  This warning warns when a value is moved to itself with ``std::move``.
  Such a ``std::move`` typically has no effect.

  .. code-block:: c++

    struct T {
    ...
    };
    void fn()
    {
      T t;
      ...
      t = std::move (t);
    }

  This warning is enabled by :option:`-Wall`.

.. option:: -Wself-move

  Default setting; overrides :option:`-Wno-self-move`.

.. option:: -Wsequence-point

  Warn about code that may have undefined semantics because of violations
  of sequence point rules in the C and C++ standards.

  The C and C++ standards define the order in which expressions in a C/C++
  program are evaluated in terms of :dfn:`sequence points`, which represent
  a partial ordering between the execution of parts of the program: those
  executed before the sequence point, and those executed after it.  These
  occur after the evaluation of a full expression (one which is not part
  of a larger expression), after the evaluation of the first operand of a
  ``&&``, ``||``, ``? :`` or ``,`` (comma) operator, before a
  function is called (but after the evaluation of its arguments and the
  expression denoting the called function), and in certain other places.
  Other than as expressed by the sequence point rules, the order of
  evaluation of subexpressions of an expression is not specified.  All
  these rules describe only a partial order rather than a total order,
  since, for example, if two functions are called within one expression
  with no sequence point between them, the order in which the functions
  are called is not specified.  However, the standards committee have
  ruled that function calls do not overlap.

  It is not specified when between sequence points modifications to the
  values of objects take effect.  Programs whose behavior depends on this
  have undefined behavior; the C and C++ standards specify that 'Between
  the previous and next sequence point an object shall have its stored
  value modified at most once by the evaluation of an expression.
  Furthermore, the prior value shall be read only to determine the value
  to be stored.'.  If a program breaks these rules, the results on any
  particular implementation are entirely unpredictable.

  Examples of code with undefined behavior are ``a = a++;``, ``a[n]
  = b[n++]`` and ``a[i++] = i;``.  Some more complicated cases are not
  diagnosed by this option, and it may give an occasional false positive
  result, but in general it has been found fairly effective at detecting
  this sort of problem in programs.

  The C++17 standard will define the order of evaluation of operands in
  more cases: in particular it requires that the right-hand side of an
  assignment be evaluated before the left-hand side, so the above
  examples are no longer undefined.  But this option will still warn
  about them, to help people avoid writing code that is undefined in C
  and earlier revisions of C++.

  The standard is worded confusingly, therefore there is some debate
  over the precise meaning of the sequence point rules in subtle cases.
  Links to discussions of the problem, including proposed formal
  definitions, may be found on the GCC readings page, at
  https://gcc.gnu.org/readings.html.

  This warning is enabled by :option:`-Wall` for C and C++.

.. option:: -Wno-sequence-point

  Default setting; overrides :option:`-Wsequence-point`.

.. option:: -Wno-return-local-addr

  Do not warn about returning a pointer (or in C++, a reference) to a
  variable that goes out of scope after the function returns.

.. option:: -Wreturn-local-addr

  Default setting; overrides :option:`-Wno-return-local-addr`.

.. option:: -Wreturn-type

  Warn whenever a function is defined with a return type that defaults
  to ``int``.  Also warn about any ``return`` statement with no
  return value in a function whose return type is not ``void``
  (falling off the end of the function body is considered returning
  without a value).

  For C only, warn about a ``return`` statement with an expression in a
  function whose return type is ``void``, unless the expression type is
  also ``void``.  As a GNU extension, the latter case is accepted
  without a warning unless :option:`-Wpedantic` is used.  Attempting
  to use the return value of a non- ``void`` function other than ``main``
  that flows off the end by reaching the closing curly brace that terminates
  the function is undefined.

  Unlike in C, in C++, flowing off the end of a non- ``void`` function other
  than ``main`` results in undefined behavior even when the value of
  the function is not used.

  This warning is enabled by default in C++ and by :option:`-Wall` otherwise.

.. option:: -Wno-return-type

  Default setting; overrides :option:`-Wreturn-type`.

.. option:: -Wno-shift-count-negative

  Controls warnings if a shift count is negative.
  This warning is enabled by default.

.. option:: -Wshift-count-negative

  Default setting; overrides :option:`-Wno-shift-count-negative`.

.. option:: -Wno-shift-count-overflow

  Controls warnings if a shift count is greater than or equal to the bit width
  of the type.  This warning is enabled by default.

.. option:: -Wshift-count-overflow

  Default setting; overrides :option:`-Wno-shift-count-overflow`.

.. option:: -Wshift-negative-value

  Warn if left shifting a negative value.  This warning is enabled by
  :option:`-Wextra` in C99 (and newer) and C++11 to C++17 modes.

.. option:: -Wno-shift-negative-value

  Default setting; overrides :option:`-Wshift-negative-value`.

.. option:: -Wno-shift-overflow, -Wshift-overflow={n}

  These options control warnings about left shift overflows.

  ``-Wshift-overflow=1``
    This is the warning level of :option:`-Wshift-overflow` and is enabled
    by default in C99 and C++11 modes (and newer).  This warning level does
    not warn about left-shifting 1 into the sign bit.  (However, in C, such
    an overflow is still rejected in contexts where an integer constant expression
    is required.)  No warning is emitted in C++20 mode (and newer), as signed left
    shifts always wrap.

  ``-Wshift-overflow=2``
    This warning level also warns about left-shifting 1 into the sign bit,
    unless C++14 mode (or newer) is active.

.. option:: -Wshift-overflow

  Default setting; overrides :option:`-Wno-shift-overflow`.

.. option:: -Wswitch

  Warn whenever a ``switch`` statement has an index of enumerated type
  and lacks a ``case`` for one or more of the named codes of that
  enumeration.  (The presence of a ``default`` label prevents this
  warning.)  ``case`` labels outside the enumeration range also
  provoke warnings when this option is used (even if there is a
  ``default`` label).
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-switch

  Default setting; overrides :option:`-Wswitch`.

.. option:: -Wswitch-default

  Warn whenever a ``switch`` statement does not have a ``default``
  case.

.. option:: -Wno-switch-default

  Default setting; overrides :option:`-Wswitch-default`.

.. option:: -Wswitch-enum

  Warn whenever a ``switch`` statement has an index of enumerated type
  and lacks a ``case`` for one or more of the named codes of that
  enumeration.  ``case`` labels outside the enumeration range also
  provoke warnings when this option is used.  The only difference
  between :option:`-Wswitch` and this option is that this option gives a
  warning about an omitted enumeration code even if there is a
  ``default`` label.

.. option:: -Wno-switch-enum

  Default setting; overrides :option:`-Wswitch-enum`.

.. option:: -Wno-switch-bool

  Do not warn when a ``switch`` statement has an index of boolean type
  and the case values are outside the range of a boolean type.
  It is possible to suppress this warning by casting the controlling
  expression to a type other than ``bool``.  For example:

  .. code-block:: c++

    switch ((int) (a == 4))
      {
      ...
      }

  This warning is enabled by default for C and C++ programs.

.. option:: -Wswitch-bool

  Default setting; overrides :option:`-Wno-switch-bool`.

.. option:: -Wno-switch-outside-range

  This option controls warnings when a ``switch`` case has a value
  that is outside of its
  respective type range.  This warning is enabled by default for
  C and C++ programs.

.. option:: -Wswitch-outside-range

  Default setting; overrides :option:`-Wno-switch-outside-range`.

.. option:: -Wno-switch-unreachable

  Do not warn when a ``switch`` statement contains statements between the
  controlling expression and the first case label, which will never be
  executed.  For example:

  .. code-block:: c++

    switch (cond)
      {
       i = 15;
      ...
       case 5:
      ...
      }

  :option:`-Wswitch-unreachable` does not warn if the statement between the
  controlling expression and the first case label is just a declaration:

  .. code-block:: c++

    switch (cond)
      {
       int i;
      ...
       case 5:
       i = 5;
      ...
      }

  This warning is enabled by default for C and C++ programs.

.. option:: -Wswitch-unreachable

  Default setting; overrides :option:`-Wno-switch-unreachable`.

.. option:: -Wsync-nand

  .. note::

    C and C++ only

  Warn when ``__sync_fetch_and_nand`` and ``__sync_nand_and_fetch``
  built-in functions are used.  These functions changed semantics in GCC 4.4.

.. option:: -Wno-sync-nand

  Default setting; overrides :option:`-Wsync-nand`.

.. option:: -Wtrivial-auto-var-init

  Warn when ``-ftrivial-auto-var-init`` cannot initialize the automatic
  variable.  A common situation is an automatic variable that is declared
  between the controlling expression and the first case label of a ``switch``
  statement.

.. option:: -Wno-trivial-auto-var-init

  Default setting; overrides :option:`-Wtrivial-auto-var-init`.

.. option:: -Wunused-but-set-parameter

  Warn whenever a function parameter is assigned to, but otherwise unused
  (aside from its declaration).

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

  This warning is also enabled by :option:`-Wunused` together with
  :option:`-Wextra`.

.. option:: -Wno-unused-but-set-parameter

  Default setting; overrides :option:`-Wunused-but-set-parameter`.

.. option:: -Wunused-but-set-variable

  Warn whenever a local variable is assigned to, but otherwise unused
  (aside from its declaration).
  This warning is enabled by :option:`-Wall`.

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

  This warning is also enabled by :option:`-Wunused`, which is enabled
  by :option:`-Wall`.

.. option:: -Wno-unused-but-set-variable

  Default setting; overrides :option:`-Wunused-but-set-variable`.

.. option:: -Wunused-function

  Warn whenever a static function is declared but not defined or a
  non-inline static function is unused.
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-unused-function

  Default setting; overrides :option:`-Wunused-function`.

.. option:: -Wunused-label

  Warn whenever a label is declared but not used.
  This warning is enabled by :option:`-Wall`.

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

.. option:: -Wno-unused-label

  Default setting; overrides :option:`-Wunused-label`.

.. option:: -Wunused-local-typedefs

  .. note::

    C, Objective-C, C++ and Objective-C++ only

  Warn when a typedef locally defined in a function is not used.
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-unused-local-typedefs

  Default setting; overrides :option:`-Wunused-local-typedefs`.

.. option:: -Wunused-parameter

  Warn whenever a function parameter is unused aside from its declaration.

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

.. option:: -Wno-unused-parameter

  Default setting; overrides :option:`-Wunused-parameter`.

.. option:: -Wno-unused-result

  Do not warn if a caller of a function marked with attribute
  :fn-attr:`warn_unused_result` (see :ref:`function-attributes`) does not use
  its return value. The default is :option:`-Wunused-result`.

.. option:: -Wunused-result

  Default setting; overrides :option:`-Wno-unused-result`.

.. option:: -Wunused-variable

  Warn whenever a local or static variable is unused aside from its
  declaration. This option implies :option:`-Wunused-const-variable=1` for C,
  but not for C++. This warning is enabled by :option:`-Wall`.

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

.. option:: -Wno-unused-variable

  Default setting; overrides :option:`-Wunused-variable`.

.. option:: -Wunused-const-variable, -Wunused-const-variable={n}

  Warn whenever a constant static variable is unused aside from its declaration.
  :option:`-Wunused-const-variable=1` is enabled by :option:`-Wunused-variable`
  for C, but not for C++. In C this declares variable storage, but in C++ this
  is not an error since const variables take the place of ``#define`` s.

  To suppress this warning use the :var-attr:`unused` attribute
  (see :ref:`variable-attributes`).

  ``-Wunused-const-variable=1``
    This is the warning level that is enabled by :option:`-Wunused-variable` for
    C.  It warns only about unused static const variables defined in the main
    compilation unit, but not about static const variables declared in any
    header included.

  ``-Wunused-const-variable=2``
    This warning level also warns for unused constant static variables in
    headers (excluding system headers).  This is the warning level of
    :option:`-Wunused-const-variable` and must be explicitly requested since
    in C++ this isn't an error and in C it might be harder to clean up all
    headers included.

.. option:: -Wno-unused-const-variable

  Default setting; overrides :option:`-Wunused-const-variable`.

.. option:: -Wunused-value

  Warn whenever a statement computes a result that is explicitly not
  used. To suppress this warning cast the unused expression to
  ``void``. This includes an expression-statement or the left-hand
  side of a comma expression that contains no side effects. For example,
  an expression such as ``x[i,j]`` causes a warning, while
  ``x[(void)i,j]`` does not.

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-unused-value

  Default setting; overrides :option:`-Wunused-value`.

.. option:: -Wunused

  All the above :option:`-Wunused` options combined.

  In order to get a warning about an unused function parameter, you must
  either specify :option:`-Wextra -Wunused` (note that :option:`-Wall` implies
  :option:`-Wunused`), or separately specify :option:`-Wunused-parameter`.

.. option:: -Wno-unused

  Default setting; overrides :option:`-Wunused`.

.. option:: -Wuninitialized

  Warn if an object with automatic or allocated storage duration is used
  without having been initialized.  In C++, also warn if a non-static
  reference or non-static ``const`` member appears in a class without
  constructors.

  In addition, passing a pointer (or in C++, a reference) to an uninitialized
  object to a ``const`` -qualified argument of a built-in function known to
  read the object is also diagnosed by this warning.
  (:option:`-Wmaybe-uninitialized` is issued for ordinary functions.)

  If you want to warn about code that uses the uninitialized value of the
  variable in its own initializer, use the :option:`-Winit-self` option.

  These warnings occur for individual uninitialized elements of
  structure, union or array variables as well as for variables that are
  uninitialized as a whole.  They do not occur for variables or elements
  declared ``volatile``.  Because these warnings depend on
  optimization, the exact variables or elements for which there are
  warnings depend on the precise optimization options and version of GCC
  used.

  Note that there may 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 warnings
  are printed.

  In C++, this warning also warns about using uninitialized objects in
  member-initializer-lists.  For example, GCC warns about ``b`` being
  uninitialized in the following snippet:

  .. code-block:: c++

    struct A {
      int a;
      int b;
      A() : a(b) { }
    };

.. option:: -Wno-uninitialized

  Default setting; overrides :option:`-Wuninitialized`.

.. option:: -Wno-invalid-memory-model

  This option controls warnings
  for invocations of :ref:`atomic-builtins`, :ref:`sync-builtins`,
  and the C11 atomic generic functions with a memory consistency argument
  that is either invalid for the operation or outside the range of values
  of the ``memory_order`` enumeration.  For example, since the
  ``__atomic_store`` and ``__atomic_store_n`` built-ins are only
  defined for the relaxed, release, and sequentially consistent memory
  orders the following code is diagnosed:

  .. code-block:: c++

    void store (int *i)
    {
      __atomic_store_n (i, 0, memory_order_consume);
    }

  :option:`-Winvalid-memory-model` is enabled by default.

.. option:: -Winvalid-memory-model

  Default setting; overrides :option:`-Wno-invalid-memory-model`.

.. option:: -Wmaybe-uninitialized

  For an object with automatic or allocated storage duration, if there exists
  a path from the function entry to a use of the object that is initialized,
  but there exist some other paths for which the object is not initialized,
  the compiler emits a warning if it cannot prove the uninitialized paths
  are not executed at run time.

  In addition, passing a pointer (or in C++, a reference) to an uninitialized
  object to a ``const`` -qualified function argument is also diagnosed by
  this warning.  (:option:`-Wuninitialized` is issued for built-in functions
  known to read the object.)  Annotating the function with attribute
  ``access (none)`` indicates that the argument isn't used to access
  the object and avoids the warning (see :ref:`common-function-attributes`).

  These warnings are only possible in optimizing compilation, because otherwise
  GCC does not keep track of the state of variables.

  These warnings are made optional because GCC may not be able to determine when
  the code is correct in spite of appearing to have an error.  Here is one
  example of how this can happen:

  .. code-block:: c++

    {
      int x;
      switch (y)
        {
        case 1: x = 1;
          break;
        case 2: x = 4;
          break;
        case 3: x = 5;
        }
      foo (x);
    }

  If the value of ``y`` is always 1, 2 or 3, then ``x`` is
  always initialized, but GCC doesn't know this. To suppress the
  warning, you need to provide a default case with assert(0) or
  similar code.

  .. index:: longjmp warnings

  This option also warns when a non-volatile automatic variable might be
  changed by a call to ``longjmp``.
  The compiler sees only the calls to ``setjmp``.  It cannot know
  where ``longjmp`` will be called; in fact, a signal handler could
  call it at any point in the code.  As a result, you may get a warning
  even when there is in fact no problem because ``longjmp`` cannot
  in fact be called at the place that would cause a problem.

  Some spurious warnings can be avoided if you declare all the functions
  you use that never return as :fn-attr:`noreturn`.  See :ref:`function-attributes`.

  This warning is enabled by :option:`-Wall` or :option:`-Wextra`.

.. option:: -Wno-maybe-uninitialized

  Default setting; overrides :option:`-Wmaybe-uninitialized`.

.. index:: warning for unknown pragmas, unknown pragmas, warning, pragmas, warning of unknown

.. option:: -Wunknown-pragmas

  Warn when a ``#pragma`` directive is encountered that is not understood by
  GCC.  If this command-line option is used, warnings are even issued
  for unknown pragmas in system header files.  This is not the case if
  the warnings are only enabled by the :option:`-Wall` command-line option.

.. option:: -Wno-unknown-pragmas

  Default setting; overrides :option:`-Wunknown-pragmas`.

.. option:: -Wno-pragmas

  Do not warn about misuses of pragmas, such as incorrect parameters,
  invalid syntax, or conflicts between pragmas.  See also
  :option:`-Wunknown-pragmas`.

.. option:: -Wpragmas

  Default setting; overrides :option:`-Wno-pragmas`.

.. option:: -Wno-prio-ctor-dtor

  Do not warn if a priority from 0 to 100 is used for constructor or destructor.
  The use of constructor and destructor attributes allow you to assign a
  priority to the constructor/destructor to control its order of execution
  before ``main`` is called or after it returns.  The priority values must be
  greater than 100 as the compiler reserves priority values between 0--100 for
  the implementation.

.. option:: -Wprio-ctor-dtor

  Default setting; overrides :option:`-Wno-prio-ctor-dtor`.

.. option:: -Wstrict-aliasing

  This option is only active when :option:`-fstrict-aliasing` is active.
  It warns about code that might break the strict aliasing rules that the
  compiler is using for optimization.  The warning does not catch all
  cases, but does attempt to catch the more common pitfalls.  It is
  included in :option:`-Wall`.
  It is equivalent to :option:`-Wstrict-aliasing=3`

.. option:: -Wno-strict-aliasing

  Default setting; overrides :option:`-Wstrict-aliasing`.

.. option:: -Wstrict-aliasing=n

  This option is only active when :option:`-fstrict-aliasing` is active.
  It warns about code that might break the strict aliasing rules that the
  compiler is using for optimization.
  Higher levels correspond to higher accuracy (fewer false positives).
  Higher levels also correspond to more effort, similar to the way :option:`-O`
  works.
  :option:`-Wstrict-aliasing` is equivalent to :option:`-Wstrict-aliasing=3`.

  Level 1: Most aggressive, quick, least accurate.
  Possibly useful when higher levels
  do not warn but :option:`-fstrict-aliasing` still breaks the code, as it has very few
  false negatives.  However, it has many false positives.
  Warns for all pointer conversions between possibly incompatible types,
  even if never dereferenced.  Runs in the front end only.

  Level 2: Aggressive, quick, not too precise.
  May still have many false positives (not as many as level 1 though),
  and few false negatives (but possibly more than level 1).
  Unlike level 1, it only warns when an address is taken.  Warns about
  incomplete types.  Runs in the front end only.

  Level 3 (default for :option:`-Wstrict-aliasing`):
  Should have very few false positives and few false
  negatives.  Slightly slower than levels 1 or 2 when optimization is enabled.
  Takes care of the common pun+dereference pattern in the front end:
  ``*(int*)&some_float``.
  If optimization is enabled, it also runs in the back end, where it deals
  with multiple statement cases using flow-sensitive points-to information.
  Only warns when the converted pointer is dereferenced.
  Does not warn about incomplete types.

.. option:: -Wstrict-overflow, -Wstrict-overflow={n}

  This option is only active when signed overflow is undefined.
  It warns about cases where the compiler optimizes based on the
  assumption that signed overflow does not occur.  Note that it does not
  warn about all cases where the code might overflow: it only warns
  about cases where the compiler implements some optimization.  Thus
  this warning depends on the optimization level.

  An optimization that assumes that signed overflow does not occur is
  perfectly safe if the values of the variables involved are such that
  overflow never does, in fact, occur.  Therefore this warning can
  easily give a false positive: a warning about code that is not
  actually a problem.  To help focus on important issues, several
  warning levels are defined.  No warnings are issued for the use of
  undefined signed overflow when estimating how many iterations a loop
  requires, in particular when determining whether a loop will be
  executed at all.

  ``-Wstrict-overflow=1``
    Warn about cases that are both questionable and easy to avoid.  For
    example the compiler simplifies
    ``x + 1 > x`` to ``1``.  This level of
    :option:`-Wstrict-overflow` is enabled by :option:`-Wall` ; higher levels
    are not, and must be explicitly requested.

  ``-Wstrict-overflow=2``
    Also warn about other cases where a comparison is simplified to a
    constant.  For example: ``abs (x) >= 0``.  This can only be
    simplified when signed integer overflow is undefined, because
    ``abs (INT_MIN)`` overflows to ``INT_MIN``, which is less than
    zero.  :option:`-Wstrict-overflow` (with no level) is the same as
    :option:`-Wstrict-overflow=2`.

  ``-Wstrict-overflow=3``
    Also warn about other cases where a comparison is simplified.  For
    example: ``x + 1 > 1`` is simplified to ``x > 0``.

  ``-Wstrict-overflow=4``
    Also warn about other simplifications not covered by the above cases.
    For example: ``(x * 10) / 5`` is simplified to ``x * 2``.

  ``-Wstrict-overflow=5``
    Also warn about cases where the compiler reduces the magnitude of a
    constant involved in a comparison.  For example: ``x + 2 > y`` is
    simplified to ``x + 1 >= y``.  This is reported only at the
    highest warning level because this simplification applies to many
    comparisons, so this warning level gives a very large number of
    false positives.

.. option:: -Wno-strict-overflow

  Default setting; overrides :option:`-Wstrict-overflow`.

.. option:: -Wstring-compare

  Warn for calls to ``strcmp`` and ``strncmp`` whose result is
  determined to be either zero or non-zero in tests for such equality
  owing to the length of one argument being greater than the size of
  the array the other argument is stored in (or the bound in the case
  of ``strncmp``).  Such calls could be mistakes.  For example,
  the call to ``strcmp`` below is diagnosed because its result is
  necessarily non-zero irrespective of the contents of the array ``a``.

  .. code-block:: c++

    extern char a[4];
    void f (char *d)
    {
      strcpy (d, "string");
      ...
      if (0 == strcmp (a, d))   // cannot be true
        puts ("a and d are the same");
    }

  :option:`-Wstring-compare` is enabled by :option:`-Wextra`.

.. option:: -Wno-string-compare

  Default setting; overrides :option:`-Wstring-compare`.

.. option:: -Wstringop-overflow, -Wstringop-overflow={type}

  Warn for calls to string manipulation functions such as ``memcpy`` and
  ``strcpy`` that are determined to overflow the destination buffer.  The
  optional argument is one greater than the type of Object Size Checking to
  perform to determine the size of the destination.  See :ref:`object-size-checking`.
  The argument is meaningful only for functions that operate on character arrays
  but not for raw memory functions like ``memcpy`` which always make use
  of Object Size type-0.  The option also warns for calls that specify a size
  in excess of the largest possible object or at most ``SIZE_MAX / 2`` bytes.
  The option produces the best results with optimization enabled but can detect
  a small subset of simple buffer overflows even without optimization in
  calls to the GCC built-in functions like ``__builtin_memcpy`` that
  correspond to the standard functions.  In any case, the option warns about
  just a subset of buffer overflows detected by the corresponding overflow
  checking built-ins.  For example, the option issues a warning for
  the ``strcpy`` call below because it copies at least 5 characters
  (the string ``"blue"`` including the terminating NUL) into the buffer
  of size 4.

  .. code-block:: c++

    enum Color { blue, purple, yellow };
    const char* f (enum Color clr)
    {
      static char buf [4];
      const char *str;
      switch (clr)
        {
          case blue: str = "blue"; break;
          case purple: str = "purple"; break;
          case yellow: str = "yellow"; break;
        }

      return strcpy (buf, str);   // warning here
    }

  Option :option:`-Wstringop-overflow=2` is enabled by default.

  ``-Wstringop-overflow=1``
    The :option:`-Wstringop-overflow=1` option uses type-zero Object Size Checking
    to determine the sizes of destination objects.  At this setting the option
    does not warn for writes past the end of subobjects of larger objects accessed
    by pointers unless the size of the largest surrounding object is known.  When
    the destination may be one of several objects it is assumed to be the largest
    one of them.  On Linux systems, when optimization is enabled at this setting
    the option warns for the same code as when the ``_FORTIFY_SOURCE`` macro
    is defined to a non-zero value.

  ``-Wstringop-overflow=2``
    The :option:`-Wstringop-overflow=2` option uses type-one Object Size Checking
    to determine the sizes of destination objects.  At this setting the option
    warns about overflows when writing to members of the largest complete
    objects whose exact size is known.  However, it does not warn for excessive
    writes to the same members of unknown objects referenced by pointers since
    they may point to arrays containing unknown numbers of elements.  This is
    the default setting of the option.

  ``-Wstringop-overflow=3``
    The :option:`-Wstringop-overflow=3` option uses type-two Object Size Checking
    to determine the sizes of destination objects.  At this setting the option
    warns about overflowing the smallest object or data member.  This is the
    most restrictive setting of the option that may result in warnings for safe
    code.

  ``-Wstringop-overflow=4``
    The :option:`-Wstringop-overflow=4` option uses type-three Object Size Checking
    to determine the sizes of destination objects.  At this setting the option
    warns about overflowing any data members, and when the destination is
    one of several objects it uses the size of the largest of them to decide
    whether to issue a warning.  Similarly to :option:`-Wstringop-overflow=3` this
    setting of the option may result in warnings for benign code.

.. option:: -Wno-stringop-overflow

  Default setting; overrides :option:`-Wstringop-overflow`.

.. option:: -Wno-stringop-overread

  Warn for calls to string manipulation functions such as ``memchr``, or
  ``strcpy`` that are determined to read past the end of the source
  sequence.

  Option :option:`-Wstringop-overread` is enabled by default.

.. option:: -Wstringop-overread

  Default setting; overrides :option:`-Wno-stringop-overread`.

.. option:: -Wno-stringop-truncation

  Do not warn for calls to bounded string manipulation functions
  such as ``strncat``,
  ``strncpy``, and ``stpncpy`` that may either truncate the copied string
  or leave the destination unchanged.

  In the following example, the call to ``strncat`` specifies a bound that
  is less than the length of the source string.  As a result, the copy of
  the source will be truncated and so the call is diagnosed.  To avoid the
  warning use ``bufsize - strlen (buf) - 1)`` as the bound.

  .. code-block:: c++

    void append (char *buf, size_t bufsize)
    {
      strncat (buf, ".txt", 3);
    }

  As another example, the following call to ``strncpy`` results in copying
  to ``d`` just the characters preceding the terminating NUL, without
  appending the NUL to the end.  Assuming the result of ``strncpy`` is
  necessarily a NUL-terminated string is a common mistake, and so the call
  is diagnosed.  To avoid the warning when the result is not expected to be
  NUL-terminated, call ``memcpy`` instead.

  .. code-block:: c++

    void copy (char *d, const char *s)
    {
      strncpy (d, s, strlen (s));
    }

  In the following example, the call to ``strncpy`` specifies the size
  of the destination buffer as the bound.  If the length of the source
  string is equal to or greater than this size the result of the copy will
  not be NUL-terminated.  Therefore, the call is also diagnosed.  To avoid
  the warning, specify ``sizeof buf - 1`` as the bound and set the last
  element of the buffer to ``NUL``.

  .. code-block:: c++

    void copy (const char *s)
    {
      char buf[80];
      strncpy (buf, s, sizeof buf);
      ...
    }

  In situations where a character array is intended to store a sequence
  of bytes with no terminating ``NUL`` such an array may be annotated
  with attribute :var-attr:`nonstring` to avoid this warning.  Such arrays,
  however, are not suitable arguments to functions that expect
  ``NUL`` -terminated strings.  To help detect accidental misuses of
  such arrays GCC issues warnings unless it can prove that the use is
  safe.  See :ref:`common-variable-attributes`.

.. option:: -Wstringop-truncation

  Default setting; overrides :option:`-Wno-stringop-truncation`.

.. option:: -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]

  Warn for cases where adding an attribute may be beneficial. The
  attributes currently supported are listed below.

  .. option:: -Wsuggest-attribute=pure, -Wno-suggest-attribute=pure, -Wno-suggest-attribute=const, -Wno-suggest-attribute=noreturn, -Wno-missing-noreturn, -Wno-suggest-attribute=malloc

    Warn about functions that might be candidates for attributes
    :fn-attr:`pure`, :fn-attr:`const` or :fn-attr:`noreturn` or ``malloc``. The compiler
    only warns for functions visible in other compilation units or (in the case of
    :fn-attr:`pure` and :fn-attr:`const`) if it cannot prove that the function returns
    normally. A function returns normally if it doesn't contain an infinite loop or
    return abnormally by throwing, calling ``abort`` or trapping.  This analysis
    requires option :option:`-fipa-pure-const`, which is enabled by default at
    :option:`-O` and higher.  Higher optimization levels improve the accuracy
    of the analysis.

  .. option:: -Wsuggest-attribute=format, -Wno-suggest-attribute=format, -Wno-missing-format-attribute

    Warn about function pointers that might be candidates for ``format``
    attributes.  Note these are only possible candidates, not absolute ones.
    GCC guesses that function pointers with ``format`` attributes that
    are used in assignment, initialization, parameter passing or return
    statements should have a corresponding ``format`` attribute in the
    resulting type.  I.e. the left-hand side of the assignment or
    initialization, the type of the parameter variable, or the return type
    of the containing function respectively should also have a ``format``
    attribute to avoid the warning.

    GCC also warns about function definitions that might be
    candidates for ``format`` attributes.  Again, these are only
    possible candidates.  GCC guesses that ``format`` attributes
    might be appropriate for any function that calls a function like
    ``vprintf`` or ``vscanf``, but this might not always be the
    case, and some functions for which ``format`` attributes are
    appropriate may not be detected.

  .. option:: -Wsuggest-attribute=cold

    Warn about functions that might be candidates for :fn-attr:`cold` attribute.  This
    is based on static detection and generally only warns about functions which
    always leads to a call to another :fn-attr:`cold` function such as wrappers of
    C++ ``throw`` or fatal error reporting functions leading to ``abort``.

  .. option:: -Wno-suggest-attribute=cold

    Default setting; overrides :option:`-Wsuggest-attribute=cold`.

.. option:: -Walloc-zero

  Warn about calls to allocation functions decorated with attribute
  ``alloc_size`` that specify zero bytes, including those to the built-in
  forms of the functions ``aligned_alloc``, ``alloca``, ``calloc``,
  ``malloc``, and ``realloc``.  Because the behavior of these functions
  when called with a zero size differs among implementations (and in the case
  of ``realloc`` has been deprecated) relying on it may result in subtle
  portability bugs and should be avoided.

.. option:: -Wno-alloc-zero

  Default setting; overrides :option:`-Walloc-zero`.

.. option:: -Walloc-size-larger-than={byte-size}

  Warn about calls to functions decorated with attribute ``alloc_size``
  that attempt to allocate objects larger than the specified number of bytes,
  or where the result of the size computation in an integer type with infinite
  precision would exceed the value of :samp:`PTRDIFF_MAX` on the target.
  :option:`-Walloc-size-larger-than=PTRDIFF_MAX` is enabled by default.
  Warnings controlled by the option can be disabled either by specifying
  :samp:`{byte-size}` of :samp:`SIZE_MAX` or more or by
  :option:`-Wno-alloc-size-larger-than`.
  See :ref:`function-attributes`.

.. option:: -Wno-alloc-size-larger-than

  Disable :option:`-Walloc-size-larger-than=` warnings.  The option is
  equivalent to :option:`-Walloc-size-larger-than=SIZE_MAX` or
  larger.

.. option:: -Walloca

  This option warns on all uses of ``alloca`` in the source.

.. option:: -Wno-alloca

  Default setting; overrides :option:`-Walloca`.

.. option:: -Walloca-larger-than={byte-size}

  This option warns on calls to ``alloca`` with an integer argument whose
  value is either zero, or that is not bounded by a controlling predicate
  that limits its value to at most :samp:`{byte-size}`.  It also warns for calls
  to ``alloca`` where the bound value is unknown.  Arguments of non-integer
  types are considered unbounded even if they appear to be constrained to
  the expected range.

  For example, a bounded case of ``alloca`` could be:

  .. code-block:: c++

    void func (size_t n)
    {
      void *p;
      if (n <= 1000)
        p = alloca (n);
      else
        p = malloc (n);
      f (p);
    }

  In the above example, passing ``-Walloca-larger-than=1000`` would not
  issue a warning because the call to ``alloca`` is known to be at most
  1000 bytes.  However, if ``-Walloca-larger-than=500`` were passed,
  the compiler would emit a warning.

  Unbounded uses, on the other hand, are uses of ``alloca`` with no
  controlling predicate constraining its integer argument.  For example:

  .. code-block:: c++

    void func ()
    {
      void *p = alloca (n);
      f (p);
    }

  If ``-Walloca-larger-than=500`` were passed, the above would trigger
  a warning, but this time because of the lack of bounds checking.

  Note, that even seemingly correct code involving signed integers could
  cause a warning:

  .. code-block:: c++

    void func (signed int n)
    {
      if (n < 500)
        {
          p = alloca (n);
          f (p);
        }
    }

  In the above example, :samp:`{n}` could be negative, causing a larger than
  expected argument to be implicitly cast into the ``alloca`` call.

  This option also warns when ``alloca`` is used in a loop.

  :option:`-Walloca-larger-than=PTRDIFF_MAX` is enabled by default
  but is usually only effective  when :option:`-ftree-vrp` is active (default
  for :option:`-O2` and above).

  See also :option:`-Wvla-larger-than=byte-size`.

.. option:: -Wno-alloca-larger-than

  Disable :option:`-Walloca-larger-than=` warnings.  The option is
  equivalent to :option:`-Walloca-larger-than=SIZE_MAX` or larger.

.. option:: -Warith-conversion

  Do warn about implicit conversions from arithmetic operations even
  when conversion of the operands to the same type cannot change their
  values.  This affects warnings from :option:`-Wconversion`,
  :option:`-Wfloat-conversion`, and :option:`-Wsign-conversion`.

  .. code-block:: c++

    void f (char c, int i)
    {
      c = c + i; // warns with -Wconversion
      c = c + 1; // only warns with -Warith-conversion
    }

.. option:: -Wno-arith-conversion

  Default setting; overrides :option:`-Warith-conversion`.

.. option:: -Warray-bounds, -Warray-bounds={n}

  Warn about out of bounds subscripts or offsets into arrays.  This warning
  is enabled by :option:`-Wall`.  It is more effective when :option:`-ftree-vrp`
  is active (the default for :option:`-O2` and above) but a subset of instances
  are issued even without optimization.

  ``-Warray-bounds=1``
    This is the default warning level of :option:`-Warray-bounds` and is enabled
    by :option:`-Wall` ; higher levels are not, and must be explicitly requested.

  ``-Warray-bounds=2``
    This warning level also warns about out of bounds accesses to trailing
    struct members of one-element array types (see :ref:`zero-length`) and about
    the intermediate results of pointer arithmetic that may yield out of bounds
    values.  This warning level may give a larger number of false positives and
    is deactivated by default.

.. option:: -Wno-array-bounds

  Default setting; overrides :option:`-Warray-bounds`.

.. option:: -Warray-compare

  Warn about equality and relational comparisons between two operands of array
  type.  This comparison was deprecated in C++20.  For example:

  .. code-block:: c++

    int arr1[5];
    int arr2[5];
    bool same = arr1 == arr2;

  :option:`-Warray-compare` is enabled by :option:`-Wall`.

.. option:: -Wno-array-compare

  Default setting; overrides :option:`-Warray-compare`.

.. option:: -Warray-parameter, -Warray-parameter={n}

  Warn about redeclarations of functions involving arguments of array or
  pointer types of inconsistent kinds or forms, and enable the detection
  of out-of-bounds accesses to such parameters by warnings such as
  :option:`-Warray-bounds`.

  If the first function declaration uses the array form the bound specified
  in the array is assumed to be the minimum number of elements expected to
  be provided in calls to the function and the maximum number of elements
  accessed by it.  Failing to provide arguments of sufficient size or accessing
  more than the maximum number of elements may be diagnosed by warnings such
  as :option:`-Warray-bounds`.  At level 1 the warning diagnoses inconsistencies
  involving array parameters declared using the ``T[static N]`` form.

  For example, the warning triggers for the following redeclarations because
  the first one allows an array of any size to be passed to ``f`` while
  the second one with the keyword ``static`` specifies that the array
  argument must have at least four elements.

  .. code-block:: c++

    void f (int[static 4]);
    void f (int[]);           // warning (inconsistent array form)

    void g (void)
    {
      int *p = (int *)malloc (4);
      f (p);                  // warning (array too small)
      ...
    }

  At level 2 the warning also triggers for redeclarations involving any other
  inconsistency in array or pointer argument forms denoting array sizes.
  Pointers and arrays of unspecified bound are considered equivalent and do
  not trigger a warning.

  .. code-block:: c++

    void g (int*);
    void g (int[]);     // no warning
    void g (int[8]);    // warning (inconsistent array bound)

  :option:`-Warray-parameter=2` is included in :option:`-Wall`.  The
  :option:`-Wvla-parameter` option triggers warnings for similar inconsistencies
  involving Variable Length Array arguments.

.. option:: -Wno-array-parameter

  Default setting; overrides :option:`-Warray-parameter`.

.. option:: -Wattribute-alias={n}

  Warn about declarations using the ``alias`` and similar attributes whose
  target is incompatible with the type of the alias.
  See :ref:`function-attributes`.

  ``-Wattribute-alias=1``
    The default warning level of the :option:`-Wattribute-alias` option diagnoses
    incompatibilities between the type of the alias declaration and that of its
    target.  Such incompatibilities are typically indicative of bugs.

  ``-Wattribute-alias=2``
    At this level :option:`-Wattribute-alias` also diagnoses cases where
    the attributes of the alias declaration are more restrictive than the
    attributes applied to its target.  These mismatches can potentially
    result in incorrect code generation.  In other cases they may be
    benign and could be resolved simply by adding the missing attribute to
    the target.  For comparison, see the :option:`-Wmissing-attributes`
    option, which controls diagnostics when the alias declaration is less
    restrictive than the target, rather than more restrictive.

    Attributes considered include ``alloc_align``, ``alloc_size``,
    :fn-attr:`cold`, :fn-attr:`const`, :fn-attr:`hot`, :fn-attr:`leaf`, :fn-attr:`malloc`,
    :fn-attr:`nonnull`, :fn-attr:`noreturn`, :fn-attr:`nothrow`, :fn-attr:`pure`,
    :fn-attr:`returns_nonnull`, and :fn-attr:`returns_twice`.

  :option:`-Wattribute-alias` is equivalent to :option:`-Wattribute-alias=1`.
  This is the default.  You can disable these warnings with either
  :option:`-Wno-attribute-alias` or :option:`-Wattribute-alias=0`.

.. option:: -Wattribute-alias

  Default setting; overrides :option:`-Wno-attribute-alias`.

.. option:: -Wbidi-chars=[none|unpaired|any|ucn]

  Warn about possibly misleading UTF-8 bidirectional control characters in
  comments, string literals, character constants, and identifiers.  Such
  characters can change left-to-right writing direction into right-to-left
  (and vice versa), which can cause confusion between the logical order and
  visual order.  This may be dangerous; for instance, it may seem that a piece
  of code is not commented out, whereas it in fact is.

  There are three levels of warning supported by GCC.  The default is
  :option:`-Wbidi-chars=unpaired`, which warns about improperly terminated
  bidi contexts.  :option:`-Wbidi-chars=none` turns the warning off.
  :option:`-Wbidi-chars=any` warns about any use of bidirectional control
  characters.

  By default, this warning does not warn about UCNs.  It is, however, possible
  to turn on such checking by using :option:`-Wbidi-chars=unpaired,ucn` or
  :option:`-Wbidi-chars=any,ucn`.  Using :option:`-Wbidi-chars=ucn` is valid,
  and is equivalent to :option:`-Wbidi-chars=unpaired,ucn`, if no previous
  :option:`-Wbidi-chars=any` was specified.

.. option:: -Wbool-compare

  Warn about boolean expression compared with an integer value different from
  ``true`` / ``false``.  For instance, the following comparison is
  always false:

  .. code-block:: c++

    int n = 5;
    ...
    if ((n > 1) == 2) { ... }

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-bool-compare

  Default setting; overrides :option:`-Wbool-compare`.

.. option:: -Wbool-operation

  Warn about suspicious operations on expressions of a boolean type.  For
  instance, bitwise negation of a boolean is very likely a bug in the program.
  For C, this warning also warns about incrementing or decrementing a boolean,
  which rarely makes sense.  (In C++, decrementing a boolean is always invalid.
  Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-bool-operation

  Default setting; overrides :option:`-Wbool-operation`.

.. option:: -Wduplicated-branches

  Warn when an if-else has identical branches.  This warning detects cases like

  .. code-block:: c++

    if (p != NULL)
      return 0;
    else
      return 0;

  It doesn't warn when both branches contain just a null statement.  This warning
  also warn for conditional operators:

  .. code-block:: c++

      int i = x ? *p : *p;

.. option:: -Wno-duplicated-branches

  Default setting; overrides :option:`-Wduplicated-branches`.

.. option:: -Wduplicated-cond

  Warn about duplicated conditions in an if-else-if chain.  For instance,
  warn for the following code:

  .. code-block:: c++

    if (p->q != NULL) { ... }
    else if (p->q != NULL) { ... }

.. option:: -Wno-duplicated-cond

  Default setting; overrides :option:`-Wduplicated-cond`.

.. option:: -Wframe-address

  Warn when the :samp:`__builtin_frame_address` or :samp:`__builtin_return_address`
  is called with an argument greater than 0.  Such calls may return indeterminate
  values or crash the program.  The warning is included in :option:`-Wall`.

.. option:: -Wno-frame-address

  Default setting; overrides :option:`-Wframe-address`.

.. option:: -Wno-discarded-qualifiers

  .. note::

    C and Objective-C only

  Do not warn if type qualifiers on pointers are being discarded.
  Typically, the compiler warns if a ``const char *`` variable is
  passed to a function that takes a ``char *`` parameter.  This option
  can be used to suppress such a warning.

.. option:: -Wdiscarded-qualifiers

  Default setting; overrides :option:`-Wno-discarded-qualifiers`.

.. option:: -Wno-discarded-array-qualifiers

  .. note::

    C and Objective-C only

  Do not warn if type qualifiers on arrays which are pointer targets
  are being discarded.  Typically, the compiler warns if a
  ``const int (*)[]`` variable is passed to a function that
  takes a ``int (*)[]`` parameter.  This option can be used to
  suppress such a warning.

.. option:: -Wdiscarded-array-qualifiers

  Default setting; overrides :option:`-Wno-discarded-array-qualifiers`.

.. option:: -Wno-incompatible-pointer-types

  .. note::

    C and Objective-C only

  Do not warn when there is a conversion between pointers that have incompatible
  types.  This warning is for cases not covered by :option:`-Wno-pointer-sign`,
  which warns for pointer argument passing or assignment with different
  signedness.

.. option:: -Wincompatible-pointer-types

  Default setting; overrides :option:`-Wno-incompatible-pointer-types`.

.. option:: -Wno-int-conversion

  .. note::

    C and Objective-C only

  Do not warn about incompatible integer to pointer and pointer to integer
  conversions.  This warning is about implicit conversions; for explicit
  conversions the warnings :option:`-Wno-int-to-pointer-cast` and
  :option:`-Wno-pointer-to-int-cast` may be used.

.. option:: -Wint-conversion

  Default setting; overrides :option:`-Wno-int-conversion`.

.. option:: -Wzero-length-bounds

  Warn about accesses to elements of zero-length array members that might
  overlap other members of the same object.  Declaring interior zero-length
  arrays is discouraged because accesses to them are undefined.  See
  See :ref:`zero-length`.

  For example, the first two stores in function ``bad`` are diagnosed
  because the array elements overlap the subsequent members ``b`` and
  ``c``.  The third store is diagnosed by :option:`-Warray-bounds`
  because it is beyond the bounds of the enclosing object.

  .. code-block:: c++

    struct X { int a[0]; int b, c; };
    struct X x;

    void bad (void)
    {
      x.a[0] = 0;   // -Wzero-length-bounds
      x.a[1] = 1;   // -Wzero-length-bounds
      x.a[2] = 2;   // -Warray-bounds
    }

  Option :option:`-Wzero-length-bounds` is enabled by :option:`-Warray-bounds`.

.. option:: -Wno-div-by-zero

  Do not warn about compile-time integer division by zero.  Floating-point
  division by zero is not warned about, as it can be a legitimate way of
  obtaining infinities and NaNs.

.. option:: -Wdiv-by-zero

  Default setting; overrides :option:`-Wno-div-by-zero`.

.. index:: warnings from system headers, system headers, warnings from

.. option:: -Wsystem-headers

  Print warning messages for constructs found in system header files.
  Warnings from system headers are normally suppressed, on the assumption
  that they usually do not indicate real problems and would only make the
  compiler output harder to read.  Using this command-line option tells
  GCC to emit warnings from system headers as if they occurred in user
  code.  However, note that using :option:`-Wall` in conjunction with this
  option does *not* warn about unknown pragmas in system
  headers --- for that, :option:`-Wunknown-pragmas` must also be used.

.. option:: -Wno-system-headers

  Default setting; overrides :option:`-Wsystem-headers`.

.. option:: -Wtautological-compare

  Warn if a self-comparison always evaluates to true or false.  This
  warning detects various mistakes such as:

  .. code-block:: c++

    int i = 1;
    ...
    if (i > i) { ... }

  This warning also warns about bitwise comparisons that always evaluate
  to true or false, for instance:

  .. code-block:: c++

    if ((a & 16) == 10) { ... }

  will always be false.

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-tautological-compare

  Default setting; overrides :option:`-Wtautological-compare`.

.. option:: -Wtrampolines

  Warn about trampolines generated for pointers to nested functions.
  A trampoline is a small piece of data or code that is created at run
  time on the stack when the address of a nested function is taken, and is
  used to call the nested function indirectly.  For some targets, it is
  made up of data only and thus requires no special treatment.  But, for
  most targets, it is made up of code and thus requires the stack to be
  made executable in order for the program to work properly.

.. option:: -Wno-trampolines

  Default setting; overrides :option:`-Wtrampolines`.

.. option:: -Wfloat-equal

  Warn if floating-point values are used in equality comparisons.

  The idea behind this is that sometimes it is convenient (for the
  programmer) to consider floating-point values as approximations to
  infinitely precise real numbers.  If you are doing this, then you need
  to compute (by analyzing the code, or in some other way) the maximum or
  likely maximum error that the computation introduces, and allow for it
  when performing comparisons (and when producing output, but that's a
  different problem).  In particular, instead of testing for equality, you
  should check to see whether the two values have ranges that overlap; and
  this is done with the relational operators, so equality comparisons are
  probably mistaken.

.. option:: -Wno-float-equal

  Default setting; overrides :option:`-Wfloat-equal`.

.. option:: -Wtraditional

  .. note::

    C and Objective-C only

  Warn about certain constructs that behave differently in traditional and
  ISO C.  Also warn about ISO C constructs that have no traditional C
  equivalent, and/or problematic constructs that should be avoided.

  * Macro parameters that appear within string literals in the macro body.
    In traditional C macro replacement takes place within string literals,
    but in ISO C it does not.

  * In traditional C, some preprocessor directives did not exist.
    Traditional preprocessors only considered a line to be a directive
    if the :samp:`#` appeared in column 1 on the line.  Therefore
    :option:`-Wtraditional` warns about directives that traditional C
    understands but ignores because the :samp:`#` does not appear as the
    first character on the line.  It also suggests you hide directives like
    ``#pragma`` not understood by traditional C by indenting them.  Some
    traditional implementations do not recognize ``#elif``, so this option
    suggests avoiding it altogether.

  * A function-like macro that appears without arguments.

  * The unary plus operator.

  * The :samp:`U` integer constant suffix, or the :samp:`F` or :samp:`L` floating-point
    constant suffixes.  (Traditional C does support the :samp:`L` suffix on integer
    constants.)  Note, these suffixes appear in macros defined in the system
    headers of most modern systems, e.g. the :samp:`_MIN`/:samp:`_MAX` macros in ``<limits.h>``.
    Use of these macros in user code might normally lead to spurious
    warnings, however GCC's integrated preprocessor has enough context to
    avoid warning in these cases.

  * A function declared external in one block and then used after the end of
    the block.

  * A ``switch`` statement has an operand of type ``long``.

  * A non- ``static`` function declaration follows a ``static`` one.
    This construct is not accepted by some traditional C compilers.

  * The ISO type of an integer constant has a different width or
    signedness from its traditional type.  This warning is only issued if
    the base of the constant is ten.  I.e. hexadecimal or octal values, which
    typically represent bit patterns, are not warned about.

  * Usage of ISO string concatenation is detected.

  * Initialization of automatic aggregates.

  * Identifier conflicts with labels.  Traditional C lacks a separate
    namespace for labels.

  * Initialization of unions.  If the initializer is zero, the warning is
    omitted.  This is done under the assumption that the zero initializer in
    user code appears conditioned on e.g. ``__STDC__`` to avoid missing
    initializer warnings and relies on default initialization to zero in the
    traditional C case.

  * Conversions by prototypes between fixed/floating-point values and vice
    versa.  The absence of these prototypes when compiling with traditional
    C causes serious problems.  This is a subset of the possible
    conversion warnings; for the full set use :option:`-Wtraditional-conversion`.

  * Use of ISO C style function definitions.  This warning intentionally is
    *not* issued for prototype declarations or variadic functions
    because these ISO C features appear in your code when using
    libiberty's traditional C compatibility macros, ``PARAMS`` and
    ``VPARAMS``.  This warning is also bypassed for nested functions
    because that feature is already a GCC extension and thus not relevant to
    traditional C compatibility.

.. option:: -Wno-traditional

  Default setting; overrides :option:`-Wtraditional`.

.. option:: -Wtraditional-conversion

  .. note::

    C and Objective-C only

  Warn if a prototype causes a type conversion that is different from what
  would happen to the same argument in the absence of a prototype.  This
  includes conversions of fixed point to floating and vice versa, and
  conversions changing the width or signedness of a fixed-point argument
  except when the same as the default promotion.

.. option:: -Wno-traditional-conversion

  Default setting; overrides :option:`-Wtraditional-conversion`.

.. option:: -Wdeclaration-after-statement

  .. note::

    C and Objective-C only

  Warn when a declaration is found after a statement in a block.  This
  construct, known from C++, was introduced with ISO C99 and is by default
  allowed in GCC.  It is not supported by ISO C90.  See :ref:`mixed-labels-and-declarations`.

.. option:: -Wno-declaration-after-statement

  Default setting; overrides :option:`-Wdeclaration-after-statement`.

.. option:: -Wshadow

  Warn whenever a local variable or type declaration shadows another
  variable, parameter, type, class member (in C++), or instance variable
  (in Objective-C) or whenever a built-in function is shadowed.  Note
  that in C++, the compiler warns if a local variable shadows an
  explicit typedef, but not if it shadows a struct/class/enum.
  If this warning is enabled, it includes also all instances of
  local shadowing.  This means that :option:`-Wno-shadow=local`
  and :option:`-Wno-shadow=compatible-local` are ignored when
  :option:`-Wshadow` is used.
  Same as :option:`-Wshadow=global`.

.. option:: -Wno-shadow

  Default setting; overrides :option:`-Wshadow`.

.. option:: -Wno-shadow-ivar

  .. note::

    Objective-C only

  Do not warn whenever a local variable shadows an instance variable in an
  Objective-C method.

.. option:: -Wshadow-ivar

  Default setting; overrides :option:`-Wno-shadow-ivar`.

.. option:: -Wshadow=global

  Warn for any shadowing.
  Same as :option:`-Wshadow`.

.. option:: -Wshadow=local

  Warn when a local variable shadows another local variable or parameter.

.. option:: -Wshadow=compatible-local

  Warn when a local variable shadows another local variable or parameter
  whose type is compatible with that of the shadowing variable.  In C++,
  type compatibility here means the type of the shadowing variable can be
  converted to that of the shadowed variable.  The creation of this flag
  (in addition to :option:`-Wshadow=local`) is based on the idea that when
  a local variable shadows another one of incompatible type, it is most
  likely intentional, not a bug or typo, as shown in the following example:

  .. code-block:: c++

    for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
    {
      for (int i = 0; i < N; ++i)
      {
        ...
      }
      ...
    }

  Since the two variable ``i`` in the example above have incompatible types,
  enabling only :option:`-Wshadow=compatible-local` does not emit a warning.
  Because their types are incompatible, if a programmer accidentally uses one
  in place of the other, type checking is expected to catch that and emit an
  error or warning.  Use of this flag instead of :option:`-Wshadow=local` can
  possibly reduce the number of warnings triggered by intentional shadowing.
  Note that this also means that shadowing ``const char *i`` by
  ``char *i`` does not emit a warning.

  This warning is also enabled by :option:`-Wshadow=local`.

.. index:: Wlarger-than-byte-size

.. option:: -Wlarger-than={byte-size}

  Warn whenever an object is defined whose size exceeds :samp:`{byte-size}`.
  :option:`-Wlarger-than=PTRDIFF_MAX` is enabled by default.
  Warnings controlled by the option can be disabled either by specifying
  :samp:`{byte-size}` of :samp:`SIZE_MAX` or more or by :option:`-Wno-larger-than`.

  Also warn for calls to bounded functions such as ``memchr`` or
  ``strnlen`` that specify a bound greater than the largest possible
  object, which is :samp:`PTRDIFF_MAX` bytes by default.  These warnings
  can only be disabled by :option:`-Wno-larger-than`.

.. option:: -Wno-larger-than

  Disable :option:`-Wlarger-than=` warnings.  The option is equivalent
  to :option:`-Wlarger-than=SIZE_MAX` or larger.

.. option:: -Wframe-larger-than={byte-size}

  Warn if the size of a function frame exceeds :samp:`{byte-size}`.
  The computation done to determine the stack frame size is approximate
  and not conservative.
  The actual requirements may be somewhat greater than :samp:`{byte-size}`
  even if you do not get a warning.  In addition, any space allocated
  via ``alloca``, variable-length arrays, or related constructs
  is not included by the compiler when determining
  whether or not to issue a warning.
  :option:`-Wframe-larger-than=PTRDIFF_MAX` is enabled by default.
  Warnings controlled by the option can be disabled either by specifying
  :samp:`{byte-size}` of :samp:`SIZE_MAX` or more or by
  :option:`-Wno-frame-larger-than`.

.. option:: -Wno-frame-larger-than

  Disable :option:`-Wframe-larger-than=` warnings.  The option is equivalent
  to :option:`-Wframe-larger-than=SIZE_MAX` or larger.

.. option:: -Wfree-nonheap-object

  Warn when attempting to deallocate an object that was either not allocated
  on the heap, or by using a pointer that was not returned from a prior call
  to the corresponding allocation function.  For example, because the call
  to ``stpcpy`` returns a pointer to the terminating nul character and
  not to the beginning of the object, the call to ``free`` below is
  diagnosed.

  .. code-block:: c++

    void f (char *p)
    {
      p = stpcpy (p, "abc");
      // ...
      free (p);   // warning
    }

  :option:`-Wfree-nonheap-object` is included in :option:`-Wall`.

.. option:: -Wno-free-nonheap-object

  Default setting; overrides :option:`-Wfree-nonheap-object`.

.. option:: -Wstack-usage={byte-size}

  Warn if the stack usage of a function might exceed :samp:`{byte-size}`.
  The computation done to determine the stack usage is conservative.
  Any space allocated via ``alloca``, variable-length arrays, or related
  constructs is included by the compiler when determining whether or not to
  issue a warning.

  The message is in keeping with the output of :option:`-fstack-usage`.

  * If the stack usage is fully static but exceeds the specified amount, it's:

    .. code-block:: c++

        warning: stack usage is 1120 bytes

  * If the stack usage is (partly) dynamic but bounded, it's:

    .. code-block:: c++

        warning: stack usage might be 1648 bytes

  * If the stack usage is (partly) dynamic and not bounded, it's:

    .. code-block:: c++

        warning: stack usage might be unbounded

  :option:`-Wstack-usage=PTRDIFF_MAX` is enabled by default.
  Warnings controlled by the option can be disabled either by specifying
  :samp:`{byte-size}` of :samp:`SIZE_MAX` or more or by
  :option:`-Wno-stack-usage`.

.. option:: -Wno-stack-usage

  Disable :option:`-Wstack-usage=` warnings.  The option is equivalent
  to :option:`-Wstack-usage=SIZE_MAX` or larger.

.. option:: -Wunsafe-loop-optimizations

  Warn if the loop cannot be optimized because the compiler cannot
  assume anything on the bounds of the loop indices.  With
  :option:`-funsafe-loop-optimizations` warn if the compiler makes
  such assumptions.

.. option:: -Wno-unsafe-loop-optimizations

  Default setting; overrides :option:`-Wunsafe-loop-optimizations`.

.. option:: -Wno-pedantic-ms-format

  .. note::

    MinGW targets only

  When used in combination with :option:`-Wformat`
  and :option:`-pedantic` without GNU extensions, this option
  disables the warnings about non-ISO ``printf`` / ``scanf`` format
  width specifiers ``I32``, ``I64``, and ``I`` used on Windows targets,
  which depend on the MS runtime.

.. option:: -Wpedantic-ms-format

  Default setting; overrides :option:`-Wno-pedantic-ms-format`.

.. option:: -Wpointer-arith

  Warn about anything that depends on the 'size of' a function type or
  of ``void``.  GNU C assigns these types a size of 1, for
  convenience in calculations with ``void *`` pointers and pointers
  to functions.  In C++, warn also when an arithmetic operation involves
  ``NULL``.  This warning is also enabled by :option:`-Wpedantic`.

.. option:: -Wno-pointer-arith

  Default setting; overrides :option:`-Wpointer-arith`.

.. option:: -Wno-pointer-compare

  Do not warn if a pointer is compared with a zero character constant.
  This usually
  means that the pointer was meant to be dereferenced.  For example:

  .. code-block:: c++

    const char *p = foo ();
    if (p == '\0')
      return 42;

  Note that the code above is invalid in C++11.

  This warning is enabled by default.

.. option:: -Wpointer-compare

  Default setting; overrides :option:`-Wno-pointer-compare`.

.. option:: -Wtsan

  Warn about unsupported features in ThreadSanitizer.

  ThreadSanitizer does not support ``std::atomic_thread_fence`` and
  can report false positives.

  This warning is enabled by default.

.. option:: -Wno-tsan

  Default setting; overrides :option:`-Wtsan`.

.. option:: -Wtype-limits

  Warn if a comparison is always true or always false due to the limited
  range of the data type, but do not warn for constant expressions.  For
  example, warn if an unsigned variable is compared against zero with
  ``<`` or ``>=``.  This warning is also enabled by
  :option:`-Wextra`.

.. option:: -Wno-type-limits

  Default setting; overrides :option:`-Wtype-limits`.

.. option:: -Wabsolute-value

  .. note::

    C and Objective-C only

  Warn for calls to standard functions that compute the absolute value
  of an argument when a more appropriate standard function is available.
  For example, calling ``abs(3.14)`` triggers the warning because the
  appropriate function to call to compute the absolute value of a double
  argument is ``fabs``.  The option also triggers warnings when the
  argument in a call to such a function has an unsigned type.  This
  warning can be suppressed with an explicit type cast and it is also
  enabled by :option:`-Wextra`.

.. option:: -Wno-absolute-value

  Default setting; overrides :option:`-Wabsolute-value`.

.. include:: ../../../../doc/cppwarnopts.rst


.. option:: -Wendif-labels

  Default setting; overrides :option:`-Wno-endif-labels`.

.. option:: -Wbad-function-cast

  .. note::

    C and Objective-C only

  Warn when a function call is cast to a non-matching type.
  For example, warn if a call to a function returning an integer type
  is cast to a pointer type.

.. option:: -Wno-bad-function-cast

  Default setting; overrides :option:`-Wbad-function-cast`.

.. option:: -Wc90-c99-compat

  .. note::

    C and Objective-C only

  Warn about features not present in ISO C90, but present in ISO C99.
  For instance, warn about use of variable length arrays, ``long long``
  type, ``bool`` type, compound literals, designated initializers, and so
  on.  This option is independent of the standards mode.  Warnings are disabled
  in the expression that follows ``__extension__``.

.. option:: -Wno-c90-c99-compat

  Default setting; overrides :option:`-Wc90-c99-compat`.

.. option:: -Wc99-c11-compat

  .. note::

    C and Objective-C only

  Warn about features not present in ISO C99, but present in ISO C11.
  For instance, warn about use of anonymous structures and unions,
  ``_Atomic`` type qualifier, ``_Thread_local`` storage-class specifier,
  ``_Alignas`` specifier, ``Alignof`` operator, ``_Generic`` keyword,
  and so on.  This option is independent of the standards mode.  Warnings are
  disabled in the expression that follows ``__extension__``.

.. option:: -Wno-c99-c11-compat

  Default setting; overrides :option:`-Wc99-c11-compat`.

.. option:: -Wc11-c2x-compat

  .. note::

    C and Objective-C only

  Warn about features not present in ISO C11, but present in ISO C2X.
  For instance, warn about omitting the string in ``_Static_assert``,
  use of :samp:`[[]]` syntax for attributes, use of decimal
  floating-point types, and so on.  This option is independent of the
  standards mode.  Warnings are disabled in the expression that follows
  ``__extension__``.

.. option:: -Wno-c11-c2x-compat

  Default setting; overrides :option:`-Wc11-c2x-compat`.

.. option:: -Wc++-compat

  .. note::

    C and Objective-C only

  Warn about ISO C constructs that are outside of the common subset of
  ISO C and ISO C++, e.g. request for implicit conversion from
  ``void *`` to a pointer to non- ``void`` type.

.. option:: -Wno-c++-compat

  Default setting; overrides :option:`-Wc++-compat`.

.. option:: -Wc++11-compat

  .. note::

    C++ and Objective-C++ only

  Warn about C++ constructs whose meaning differs between ISO C++ 1998
  and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
  in ISO C++ 2011.  This warning turns on :option:`-Wnarrowing` and is
  enabled by :option:`-Wall`.

.. option:: -Wno-c++11-compat

  Default setting; overrides :option:`-Wc++11-compat`.

.. option:: -Wc++14-compat

  .. note::

    C++ and Objective-C++ only

  Warn about C++ constructs whose meaning differs between ISO C++ 2011
  and ISO C++ 2014.  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-c++14-compat

  Default setting; overrides :option:`-Wc++14-compat`.

.. option:: -Wc++17-compat

  .. note::

    C++ and Objective-C++ only

  Warn about C++ constructs whose meaning differs between ISO C++ 2014
  and ISO C++ 2017.  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-c++17-compat

  Default setting; overrides :option:`-Wc++17-compat`.

.. option:: -Wc++20-compat

  .. note::

    C++ and Objective-C++ only

  Warn about C++ constructs whose meaning differs between ISO C++ 2017
  and ISO C++ 2020.  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-c++20-compat

  Default setting; overrides :option:`-Wc++20-compat`.

.. option:: -Wno-c++11-extensions

  .. note::

    C++ and Objective-C++ only

  Do not warn about C++11 constructs in code being compiled using
  an older C++ standard.  Even without this option, some C++11 constructs
  will only be diagnosed if :option:`-Wpedantic` is used.

.. option:: -Wc++11-extensions

  Default setting; overrides :option:`-Wno-c++11-extensions`.

.. option:: -Wno-c++14-extensions

  .. note::

    C++ and Objective-C++ only

  Do not warn about C++14 constructs in code being compiled using
  an older C++ standard.  Even without this option, some C++14 constructs
  will only be diagnosed if :option:`-Wpedantic` is used.

.. option:: -Wc++14-extensions

  Default setting; overrides :option:`-Wno-c++14-extensions`.

.. option:: -Wno-c++17-extensions

  .. note::

    C++ and Objective-C++ only

  Do not warn about C++17 constructs in code being compiled using
  an older C++ standard.  Even without this option, some C++17 constructs
  will only be diagnosed if :option:`-Wpedantic` is used.

.. option:: -Wc++17-extensions

  Default setting; overrides :option:`-Wno-c++17-extensions`.

.. option:: -Wno-c++20-extensions

  .. note::

    C++ and Objective-C++ only

  Do not warn about C++20 constructs in code being compiled using
  an older C++ standard.  Even without this option, some C++20 constructs
  will only be diagnosed if :option:`-Wpedantic` is used.

.. option:: -Wc++20-extensions

  Default setting; overrides :option:`-Wno-c++20-extensions`.

.. option:: -Wno-c++23-extensions

  .. note::

    C++ and Objective-C++ only

  Do not warn about C++23 constructs in code being compiled using
  an older C++ standard.  Even without this option, some C++23 constructs
  will only be diagnosed if :option:`-Wpedantic` is used.

.. option:: -Wc++23-extensions

  Default setting; overrides :option:`-Wno-c++23-extensions`.

.. option:: -Wcast-qual

  Warn whenever a pointer is cast so as to remove a type qualifier from
  the target type.  For example, warn if a ``const char *`` is cast
  to an ordinary ``char *``.

  Also warn when making a cast that introduces a type qualifier in an
  unsafe way.  For example, casting ``char **`` to ``const char **``
  is unsafe, as in this example:

  .. code-block:: c++

      /* p is char ** value.  */
      const char **q = (const char **) p;
      /* Assignment of readonly string to const char * is OK.  */
      *q = "string";
      /* Now char** pointer points to read-only memory.  */
      **p = 'b';

.. option:: -Wno-cast-qual

  Default setting; overrides :option:`-Wcast-qual`.

.. option:: -Wcast-align

  Warn whenever a pointer is cast such that the required alignment of the
  target is increased.  For example, warn if a ``char *`` is cast to
  an ``int *`` on machines where integers can only be accessed at
  two- or four-byte boundaries.

.. option:: -Wno-cast-align

  Default setting; overrides :option:`-Wcast-align`.

.. option:: -Wcast-align=strict

  Warn whenever a pointer is cast such that the required alignment of the
  target is increased.  For example, warn if a ``char *`` is cast to
  an ``int *`` regardless of the target machine.

.. option:: -Wcast-function-type

  Warn when a function pointer is cast to an incompatible function pointer.
  In a cast involving function types with a variable argument list only
  the types of initial arguments that are provided are considered.
  Any parameter of pointer-type matches any other pointer-type.  Any benign
  differences in integral types are ignored, like ``int`` vs. ``long``
  on ILP32 targets.  Likewise type qualifiers are ignored.  The function
  type ``void (*) (void)`` is special and matches everything, which can
  be used to suppress this warning.
  In a cast involving pointer to member types this warning warns whenever
  the type cast is changing the pointer to member type.
  This warning is enabled by :option:`-Wextra`.

.. option:: -Wno-cast-function-type

  Default setting; overrides :option:`-Wcast-function-type`.

.. option:: -Wwrite-strings

  When compiling C, give string constants the type ``const
  char[length]`` so that copying the address of one into a
  non- ``const`` ``char *`` pointer produces a warning.  These
  warnings help you find at compile time code that can try to write
  into a string constant, but only if you have been very careful about
  using ``const`` in declarations and prototypes.  Otherwise, it is
  just a nuisance. This is why we did not make :option:`-Wall` request
  these warnings.

  When compiling C++, warn about the deprecated conversion from string
  literals to ``char *``.  This warning is enabled by default for C++
  programs.

.. option:: -Wno-write-strings

  Default setting; overrides :option:`-Wwrite-strings`.

.. option:: -Wclobbered

  Warn for variables that might be changed by ``longjmp`` or
  ``vfork``.  This warning is also enabled by :option:`-Wextra`.

.. option:: -Wno-clobbered

  Default setting; overrides :option:`-Wclobbered`.

.. option:: -Wconversion

  Warn for implicit conversions that may alter a value. This includes
  conversions between real and integer, like ``abs (x)`` when
  ``x`` is ``double`` ; conversions between signed and unsigned,
  like ``unsigned ui = -1`` ; and conversions to smaller types, like
  ``sqrtf (M_PI)``. Do not warn for explicit casts like ``abs
  ((int) x)`` and ``ui = (unsigned) -1``, or if the value is not
  changed by the conversion like in ``abs (2.0)``.  Warnings about
  conversions between signed and unsigned integers can be disabled by
  using :option:`-Wno-sign-conversion`.

  For C++, also warn for confusing overload resolution for user-defined
  conversions; and conversions that never use a type conversion
  operator: conversions to ``void``, the same type, a base class or a
  reference to them. Warnings about conversions between signed and
  unsigned integers are disabled by default in C++ unless
  :option:`-Wsign-conversion` is explicitly enabled.

  Warnings about conversion from arithmetic on a small type back to that
  type are only given with :option:`-Warith-conversion`.

.. option:: -Wno-conversion

  Default setting; overrides :option:`-Wconversion`.

.. option:: -Wdangling-else

  Warn about constructions where there may be confusion to which
  ``if`` statement an ``else`` branch belongs.  Here is an example of
  such a case:

  .. code-block:: c++

    {
      if (a)
        if (b)
          foo ();
      else
        bar ();
    }

  In C/C++, every ``else`` branch belongs to the innermost possible
  ``if`` statement, which in this example is ``if (b)``.  This is
  often not what the programmer expected, as illustrated in the above
  example by indentation the programmer chose.  When there is the
  potential for this confusion, GCC issues a warning when this flag
  is specified.  To eliminate the warning, add explicit braces around
  the innermost ``if`` statement so there is no way the ``else``
  can belong to the enclosing ``if``.  The resulting code
  looks like this:

  .. code-block:: c++

    {
      if (a)
        {
          if (b)
            foo ();
          else
            bar ();
        }
    }

  This warning is enabled by :option:`-Wparentheses`.

.. option:: -Wno-dangling-else

  Default setting; overrides :option:`-Wdangling-else`.

.. option:: -Wdangling-pointer, -Wdangling-pointer={n}

  Warn about uses of pointers (or C++ references) to objects with automatic
  storage duration after their lifetime has ended.  This includes local
  variables declared in nested blocks, compound literals and other unnamed
  temporary objects.  In addition, warn about storing the address of such
  objects in escaped pointers.  The warning is enabled at all optimization
  levels but may yield different results with optimization than without.

  ``-Wdangling-pointer=1``
    At level 1 the warning diagnoses only unconditional uses of dangling pointers.
    For example

    .. code-block:: c++

      int f (int c1, int c2, x)
      {
        char *p = strchr ((char[]){ c1, c2 }, c3);
        return p ? *p : 'x';   // warning: dangling pointer to a compound literal
      }

    In the following function the store of the address of the local variable
    ``x`` in the escaped pointer ``*p`` also triggers the warning.

    .. code-block:: c++

      void g (int **p)
      {
        int x = 7;
        *p = &x;   // warning: storing the address of a local variable in *p
      }

  ``-Wdangling-pointer=2``
    At level 2, in addition to unconditional uses the warning also diagnoses
    conditional uses of dangling pointers.

    For example, because the array :samp:`{a}` in the following function is out of
    scope when the pointer :samp:`{s}` that was set to point is used, the warning
    triggers at this level.

    .. code-block:: c++

      void f (char *s)
      {
        if (!s)
          {
            char a[12] = "tmpname";
            s = a;
          }
        strcat (s, ".tmp");   // warning: dangling pointer to a may be used
        ...
      }

    :option:`-Wdangling-pointer=2` is included in :option:`-Wall`.

.. option:: -Wno-dangling-pointer

  Default setting; overrides :option:`-Wdangling-pointer`.

.. option:: -Wdate-time

  Warn when macros ``__TIME__``, ``__DATE__`` or ``__TIMESTAMP__``
  are encountered as they might prevent bit-wise-identical reproducible
  compilations.

.. option:: -Wno-date-time

  Default setting; overrides :option:`-Wdate-time`.

.. option:: -Wempty-body

  Warn if an empty body occurs in an ``if``, ``else`` or ``do
  while`` statement.  This warning is also enabled by :option:`-Wextra`.

.. option:: -Wno-empty-body

  Default setting; overrides :option:`-Wempty-body`.

.. option:: -Wno-endif-labels

  Do not warn about stray tokens after ``#else`` and ``#endif``.

.. option:: -Wendif-labels

  Default setting; overrides :option:`-Wno-endif-labels`.

.. option:: -Wenum-compare

  Warn about a comparison between values of different enumerated types.
  In C++ enumerated type mismatches in conditional expressions are also
  diagnosed and the warning is enabled by default.  In C this warning is
  enabled by :option:`-Wall`.

.. option:: -Wno-enum-compare

  Default setting; overrides :option:`-Wenum-compare`.

.. option:: -Wenum-conversion

  Warn when a value of enumerated type is implicitly converted to a
  different enumerated type.  This warning is enabled by :option:`-Wextra`
  in C.

.. option:: -Wno-enum-conversion

  Default setting; overrides :option:`-Wenum-conversion`.

.. option:: -Wenum-int-mismatch

  .. note::

    C and Objective-C only

  Warn about mismatches between an enumerated type and an integer type in
  declarations.  For example:

  .. code-block:: c++

    enum E { l = -1, z = 0, g = 1 };
    int foo(void);
    enum E foo(void);

  In C, an enumerated type is compatible with ``char``, a signed
  integer type, or an unsigned integer type.  However, since the choice
  of the underlying type of an enumerated type is implementation-defined,
  such mismatches may cause portability issues.  In C++, such mismatches
  are an error.  In C, this warning is enabled by :option:`-Wall` and
  :option:`-Wc++-compat`.

.. option:: -Wno-enum-int-mismatch

  Default setting; overrides :option:`-Wenum-int-mismatch`.

.. option:: -Wjump-misses-init

  .. note::

    C, Objective-C only

  Warn if a ``goto`` statement or a ``switch`` statement jumps
  forward across the initialization of a variable, or jumps backward to a
  label after the variable has been initialized.  This only warns about
  variables that are initialized when they are declared.  This warning is
  only supported for C and Objective-C; in C++ this sort of branch is an
  error in any case.

  :option:`-Wjump-misses-init` is included in :option:`-Wc++-compat`.  It
  can be disabled with the :option:`-Wno-jump-misses-init` option.

.. option:: -Wno-jump-misses-init

  Default setting; overrides :option:`-Wjump-misses-init`.

.. index:: warning for comparison of signed and unsigned values, comparison of signed and unsigned values, warning, signed and unsigned values, comparison warning

.. option:: -Wsign-compare

  Warn when a comparison between signed and unsigned values could produce
  an incorrect result when the signed value is converted to unsigned.
  In C++, this warning is also enabled by :option:`-Wall`.  In C, it is
  also enabled by :option:`-Wextra`.

.. option:: -Wno-sign-compare

  Default setting; overrides :option:`-Wsign-compare`.

.. option:: -Wsign-conversion

  Warn for implicit conversions that may change the sign of an integer
  value, like assigning a signed integer expression to an unsigned
  integer variable. An explicit cast silences the warning. In C, this
  option is enabled also by :option:`-Wconversion`.

.. option:: -Wno-sign-conversion

  Default setting; overrides :option:`-Wsign-conversion`.

.. option:: -Wfloat-conversion

  Warn for implicit conversions that reduce the precision of a real value.
  This includes conversions from real to integer, and from higher precision
  real to lower precision real values.  This option is also enabled by
  :option:`-Wconversion`.

.. option:: -Wno-float-conversion

  Default setting; overrides :option:`-Wfloat-conversion`.

.. option:: -Wno-scalar-storage-order

  Do not warn on suspicious constructs involving reverse scalar storage order.

.. option:: -Wscalar-storage-order

  Default setting; overrides :option:`-Wno-scalar-storage-order`.

.. option:: -Wsizeof-array-div

  Warn about divisions of two sizeof operators when the first one is applied
  to an array and the divisor does not equal the size of the array element.
  In such a case, the computation will not yield the number of elements in the
  array, which is likely what the user intended.  This warning warns e.g. about

  .. code-block:: c++

    int fn ()
    {
      int arr[10];
      return sizeof (arr) / sizeof (short);
    }

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-sizeof-array-div

  Default setting; overrides :option:`-Wsizeof-array-div`.

.. option:: -Wsizeof-pointer-div

  Warn for suspicious divisions of two sizeof expressions that divide
  the pointer size by the element size, which is the usual way to compute
  the array size but won't work out correctly with pointers.  This warning
  warns e.g. about ``sizeof (ptr) / sizeof (ptr[0])`` if ``ptr`` is
  not an array, but a pointer.  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-sizeof-pointer-div

  Default setting; overrides :option:`-Wsizeof-pointer-div`.

.. option:: -Wsizeof-pointer-memaccess

  Warn for suspicious length parameters to certain string and memory built-in
  functions if the argument uses ``sizeof``.  This warning triggers for
  example for ``memset (ptr, 0, sizeof (ptr));`` if ``ptr`` is not
  an array, but a pointer, and suggests a possible fix, or about
  ``memcpy (&foo, ptr, sizeof (&foo));``.  :option:`-Wsizeof-pointer-memaccess`
  also warns about calls to bounded string copy functions like ``strncat``
  or ``strncpy`` that specify as the bound a ``sizeof`` expression of
  the source array.  For example, in the following function the call to
  ``strncat`` specifies the size of the source string as the bound.  That
  is almost certainly a mistake and so the call is diagnosed.

  .. code-block:: c++

    void make_file (const char *name)
    {
      char path[PATH_MAX];
      strncpy (path, name, sizeof path - 1);
      strncat (path, ".text", sizeof ".text");
      ...
    }

  The :option:`-Wsizeof-pointer-memaccess` option is enabled by :option:`-Wall`.

.. option:: -Wno-sizeof-pointer-memaccess

  Default setting; overrides :option:`-Wsizeof-pointer-memaccess`.

.. option:: -Wno-sizeof-array-argument

  Do not warn when the ``sizeof`` operator is applied to a parameter that is
  declared as an array in a function definition.  This warning is enabled by
  default for C and C++ programs.

.. option:: -Wsizeof-array-argument

  Default setting; overrides :option:`-Wno-sizeof-array-argument`.

.. option:: -Wmemset-elt-size

  Warn for suspicious calls to the ``memset`` built-in function, if the
  first argument references an array, and the third argument is a number
  equal to the number of elements, but not equal to the size of the array
  in memory.  This indicates that the user has omitted a multiplication by
  the element size.  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-memset-elt-size

  Default setting; overrides :option:`-Wmemset-elt-size`.

.. option:: -Wmemset-transposed-args

  Warn for suspicious calls to the ``memset`` built-in function where
  the second argument is not zero and the third argument is zero.  For
  example, the call ``memset (buf, sizeof buf, 0)`` is diagnosed because
  ``memset (buf, 0, sizeof buf)`` was meant instead.  The diagnostic
  is only emitted if the third argument is a literal zero.  Otherwise, if
  it is an expression that is folded to zero, or a cast of zero to some
  type, it is far less likely that the arguments have been mistakenly
  transposed and no warning is emitted.  This warning is enabled
  by :option:`-Wall`.

.. option:: -Wno-memset-transposed-args

  Default setting; overrides :option:`-Wmemset-transposed-args`.

.. option:: -Waddress

  Warn about suspicious uses of address expressions. These include comparing
  the address of a function or a declared object to the null pointer constant
  such as in

  .. code-block:: c++

    void f (void);
    void g (void)
    {
      if (!f)   // warning: expression evaluates to false
        abort ();
    }

  comparisons of a pointer to a string literal, such as in

  .. code-block:: c++

    void f (const char *x)
    {
      if (x == "abc")   // warning: expression evaluates to false
        puts ("equal");
    }

  and tests of the results of pointer addition or subtraction for equality
  to null, such as in

  .. code-block:: c++

    void f (const int *p, int i)
    {
      return p + i == NULL;
    }

  Such uses typically indicate a programmer error: the address of most
  functions and objects necessarily evaluates to true (the exception are
  weak symbols), so their use in a conditional might indicate missing
  parentheses in a function call or a missing dereference in an array
  expression.  The subset of the warning for object pointers can be
  suppressed by casting the pointer operand to an integer type such
  as ``intptr_t`` or ``uintptr_t``.
  Comparisons against string literals result in unspecified behavior
  and are not portable, and suggest the intent was to call ``strcmp``.
  The warning is suppressed if the suspicious expression is the result
  of macro expansion.
  :option:`-Waddress` warning is enabled by :option:`-Wall`.

.. option:: -Wno-address

  Default setting; overrides :option:`-Waddress`.

.. option:: -Wno-address-of-packed-member

  Do not warn when the address of packed member of struct or union is taken,
  which usually results in an unaligned pointer value.  This is
  enabled by default.

.. option:: -Waddress-of-packed-member

  Default setting; overrides :option:`-Wno-address-of-packed-member`.

.. option:: -Wlogical-op

  Warn about suspicious uses of logical operators in expressions.
  This includes using logical operators in contexts where a
  bit-wise operator is likely to be expected.  Also warns when
  the operands of a logical operator are the same:

  .. code-block:: c++

    extern int a;
    if (a < 0 && a < 0) { ... }

.. option:: -Wno-logical-op

  Default setting; overrides :option:`-Wlogical-op`.

.. option:: -Wlogical-not-parentheses

  Warn about logical not used on the left hand side operand of a comparison.
  This option does not warn if the right operand is considered to be a boolean
  expression.  Its purpose is to detect suspicious code like the following:

  .. code-block:: c++

    int a;
    ...
    if (!a > 1) { ... }

  It is possible to suppress the warning by wrapping the LHS into
  parentheses:

  .. code-block:: c++

    if ((!a) > 1) { ... }

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-logical-not-parentheses

  Default setting; overrides :option:`-Wlogical-not-parentheses`.

.. option:: -Waggregate-return

  Warn if any functions that return structures or unions are defined or
  called.  (In languages where you can return an array, this also elicits
  a warning.)

.. option:: -Wno-aggregate-return

  Default setting; overrides :option:`-Waggregate-return`.

.. option:: -Wno-aggressive-loop-optimizations

  Warn if in a loop with constant number of iterations the compiler detects
  undefined behavior in some statement during one or more of the iterations.

.. option:: -Waggressive-loop-optimizations

  Default setting; overrides :option:`-Wno-aggressive-loop-optimizations`.

.. option:: -Wno-attributes

  Do not warn if an unexpected ``__attribute__`` is used, such as
  unrecognized attributes, function attributes applied to variables,
  etc.  This does not stop errors for incorrect use of supported
  attributes.

  Additionally, using :option:`-Wno-attributes=`, it is possible to suppress
  warnings about unknown scoped attributes (in C++11 and C2X).  For example,
  :option:`-Wno-attributes=vendor::attr` disables warning about the following
  declaration:

  .. code-block:: c++

    [[vendor::attr]] void f();

  It is also possible to disable warning about all attributes in a namespace
  using :option:`-Wno-attributes=vendor::` which prevents warning about both
  of these declarations:

  .. code-block:: c++

    [[vendor::safe]] void f();
    [[vendor::unsafe]] void f2();

  Note that :option:`-Wno-attributes=` does not imply :option:`-Wno-attributes`.

.. option:: -Wattributes

  Default setting; overrides :option:`-Wno-attributes`.

.. option:: -Wno-builtin-declaration-mismatch

  Warn if a built-in function is declared with an incompatible signature
  or as a non-function, or when a built-in function declared with a type
  that does not include a prototype is called with arguments whose promoted
  types do not match those expected by the function.  When :option:`-Wextra`
  is specified, also warn when a built-in function that takes arguments is
  declared without a prototype.  The :option:`-Wbuiltin-declaration-mismatch`
  warning is enabled by default.  To avoid the warning include the appropriate
  header to bring the prototypes of built-in functions into scope.

  For example, the call to ``memset`` below is diagnosed by the warning
  because the function expects a value of type ``size_t`` as its argument
  but the type of ``32`` is ``int``.  With :option:`-Wextra`,
  the declaration of the function is diagnosed as well.

  .. code-block:: c++

    extern void* memset ();
    void f (void *d)
    {
      memset (d, '\0', 32);
    }

.. option:: -Wbuiltin-declaration-mismatch

  Default setting; overrides :option:`-Wno-builtin-declaration-mismatch`.

.. option:: -Wno-builtin-macro-redefined

  Do not warn if certain built-in macros are redefined.  This suppresses
  warnings for redefinition of ``__TIMESTAMP__``, ``__TIME__``,
  ``__DATE__``, ``__FILE__``, and ``__BASE_FILE__``.

.. option:: -Wbuiltin-macro-redefined

  Default setting; overrides :option:`-Wno-builtin-macro-redefined`.

.. option:: -Wstrict-prototypes

  .. note::

    C and Objective-C only

  Warn if a function is declared or defined without specifying the
  argument types.  (An old-style function definition is permitted without
  a warning if preceded by a declaration that specifies the argument
  types.)

.. option:: -Wno-strict-prototypes

  Default setting; overrides :option:`-Wstrict-prototypes`.

.. option:: -Wold-style-declaration

  .. note::

    C and Objective-C only

  Warn for obsolescent usages, according to the C Standard, in a
  declaration. For example, warn if storage-class specifiers like
  ``static`` are not the first things in a declaration.  This warning
  is also enabled by :option:`-Wextra`.

.. option:: -Wno-old-style-declaration

  Default setting; overrides :option:`-Wold-style-declaration`.

.. option:: -Wold-style-definition

  .. note::

    C and Objective-C only

  Warn if an old-style function definition is used.  A warning is given
  even if there is a previous prototype.  A definition using :samp:`()`
  is not considered an old-style definition in C2X mode, because it is
  equivalent to :samp:`(void)` in that case, but is considered an
  old-style definition for older standards.

.. option:: -Wno-old-style-definition

  Default setting; overrides :option:`-Wold-style-definition`.

.. option:: -Wmissing-parameter-type

  .. note::

    C and Objective-C only

  A function parameter is declared without a type specifier in K&R-style
  functions:

  .. code-block:: c++

    void foo(bar) { }

  This warning is also enabled by :option:`-Wextra`.

.. option:: -Wno-missing-parameter-type

  Default setting; overrides :option:`-Wmissing-parameter-type`.

.. option:: -Wmissing-prototypes

  .. note::

    C and Objective-C only

  Warn if a global function is defined without a previous prototype
  declaration.  This warning is issued even if the definition itself
  provides a prototype.  Use this option to detect global functions
  that do not have a matching prototype declaration in a header file.
  This option is not valid for C++ because all function declarations
  provide prototypes and a non-matching declaration declares an
  overload rather than conflict with an earlier declaration.
  Use :option:`-Wmissing-declarations` to detect missing declarations in C++.

.. option:: -Wno-missing-prototypes

  Default setting; overrides :option:`-Wmissing-prototypes`.

.. option:: -Wmissing-declarations

  Warn if a global function is defined without a previous declaration.
  Do so even if the definition itself provides a prototype.
  Use this option to detect global functions that are not declared in
  header files.  In C, no warnings are issued for functions with previous
  non-prototype declarations; use :option:`-Wmissing-prototypes` to detect
  missing prototypes.  In C++, no warnings are issued for function templates,
  or for inline functions, or for functions in anonymous namespaces.

.. option:: -Wno-missing-declarations

  Default setting; overrides :option:`-Wmissing-declarations`.

.. option:: -Wmissing-field-initializers

  Warn if a structure's initializer has some fields missing.  For
  example, the following code causes such a warning, because
  ``x.h`` is implicitly zero:

  .. code-block:: c++

    struct s { int f, g, h; };
    struct s x = { 3, 4 };

  This option does not warn about designated initializers, so the following
  modification does not trigger a warning:

  .. code-block:: c++

    struct s { int f, g, h; };
    struct s x = { .f = 3, .g = 4 };

  In C this option does not warn about the universal zero initializer
  :samp:`{ 0 }`:

  .. code-block:: c++

    struct s { int f, g, h; };
    struct s x = { 0 };

  Likewise, in C++ this option does not warn about the empty { }
  initializer, for example:

  .. code-block:: c++

    struct s { int f, g, h; };
    s x = { };

  This warning is included in :option:`-Wextra`.  To get other :option:`-Wextra`
  warnings without this one, use :option:`-Wextra -Wno-missing-field-initializers`.

.. option:: -Wno-missing-field-initializers

  Default setting; overrides :option:`-Wmissing-field-initializers`.

.. option:: -Wno-missing-requires

  By default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:

  .. code-block:: c++

    bool satisfied = requires { C<T> };

  Here :samp:`satisfied` will be true if :samp:`C<T>` is a valid
  expression, which it is for all T.  Presumably the user meant to write

  .. code-block:: c++

    bool satisfied = requires { requires C<T> };

  so :samp:`satisfied` is only true if concept :samp:`C` is satisfied for
  type :samp:`T`.

  This warning can be disabled with :option:`-Wno-missing-requires`.

.. option:: -Wmissing-requires

  Default setting; overrides :option:`-Wno-missing-requires`.

.. option:: -Wno-missing-template-keyword

  The member access tokens ., -> and :: must be followed by the ``template``
  keyword if the parent object is dependent and the member being named is a
  template.

  .. code-block:: c++

    template <class X>
    void DoStuff (X x)
    {
      x.template DoSomeOtherStuff<X>(); // Good.
      x.DoMoreStuff<X>(); // Warning, x is dependent.
    }

  In rare cases it is possible to get false positives. To silence this, wrap
  the expression in parentheses. For example, the following is treated as a
  template, even where m and N are integers:

  .. code-block:: c++

    void NotATemplate (my_class t)
    {
      int N = 5;

      bool test = t.m < N > (0); // Treated as a template.
      test = (t.m < N) > (0); // Same meaning, but not treated as a template.
    }

  This warning can be disabled with :option:`-Wno-missing-template-keyword`.

.. option:: -Wmissing-template-keyword

  Default setting; overrides :option:`-Wno-missing-template-keyword`.

.. option:: -Wno-multichar

  Do not warn if a multicharacter constant (:samp:`'FOOF'`) is used.
  Usually they indicate a typo in the user's code, as they have
  implementation-defined values, and should not be used in portable code.

.. option:: -Wmultichar

  Default setting; overrides :option:`-Wno-multichar`.

.. index:: NFC, NFKC, character set, input normalization

.. option:: -Wnormalized=[none|id|nfc|nfkc]

  In ISO C and ISO C++, two identifiers are different if they are
  different sequences of characters.  However, sometimes when characters
  outside the basic ASCII character set are used, you can have two
  different character sequences that look the same.  To avoid confusion,
  the ISO 10646 standard sets out some :dfn:`normalization rules` which
  when applied ensure that two sequences that look the same are turned into
  the same sequence.  GCC can warn you if you are using identifiers that
  have not been normalized; this option controls that warning.

  There are four levels of warning supported by GCC.  The default is
  :option:`-Wnormalized=nfc`, which warns about any identifier that is
  not in the ISO 10646 'C' normalized form, :dfn:`NFC`.  NFC is the
  recommended form for most uses.  It is equivalent to
  :option:`-Wnormalized`.

  Unfortunately, there are some characters allowed in identifiers by
  ISO C and ISO C++ that, when turned into NFC, are not allowed in
  identifiers.  That is, there's no way to use these symbols in portable
  ISO C or C++ and have all your identifiers in NFC.
  :option:`-Wnormalized=id` suppresses the warning for these characters.
  It is hoped that future versions of the standards involved will correct
  this, which is why this option is not the default.

  You can switch the warning off for all characters by writing
  :option:`-Wnormalized=none` or :option:`-Wno-normalized`.  You should
  only do this if you are using some other normalization scheme (like
  'D'), because otherwise you can easily create bugs that are
  literally impossible to see.

  Some characters in ISO 10646 have distinct meanings but look identical
  in some fonts or display methodologies, especially once formatting has
  been applied.  For instance ``\u207F``, 'SUPERSCRIPT LATIN SMALL
  LETTER N', displays just like a regular ``n`` that has been
  placed in a superscript.  ISO 10646 defines the :dfn:`NFKC`
  normalization scheme to convert all these into a standard form as
  well, and GCC warns if your code is not in NFKC if you use
  :option:`-Wnormalized=nfkc`.  This warning is comparable to warning
  about every identifier that contains the letter O because it might be
  confused with the digit 0, and so is not the default, but may be
  useful as a local coding convention if the programming environment
  cannot be fixed to display these characters distinctly.

.. option:: -Wno-attribute-warning

  Do not warn about usage of functions (see :ref:`function-attributes`)
  declared with ``warning`` attribute.  By default, this warning is
  enabled.  :option:`-Wno-attribute-warning` can be used to disable the
  warning or :option:`-Wno-error=attribute-warning` can be used to
  disable the error when compiled with :option:`-Werror` flag.

.. option:: -Wattribute-warning

  Default setting; overrides :option:`-Wno-attribute-warning`.

.. option:: -Wno-deprecated

  Do not warn about usage of deprecated features.  See :ref:`deprecated-features`.

.. option:: -Wdeprecated

  Default setting; overrides :option:`-Wno-deprecated`.

.. option:: -Wno-deprecated-declarations

  Do not warn about uses of functions (see :ref:`function-attributes`),
  variables (see :ref:`variable-attributes`), and types (see :ref:`type-attributes`) marked as deprecated by using the ``deprecated``
  attribute.

.. option:: -Wdeprecated-declarations

  Default setting; overrides :option:`-Wno-deprecated-declarations`.

.. option:: -Wno-overflow

  Do not warn about compile-time overflow in constant expressions.

.. option:: -Woverflow

  Default setting; overrides :option:`-Wno-overflow`.

.. option:: -Wno-odr

  Warn about One Definition Rule violations during link-time optimization.
  Enabled by default.

.. option:: -Wodr

  Default setting; overrides :option:`-Wno-odr`.

.. index:: OpenACC accelerator programming

.. option:: -Wopenacc-parallelism

  Warn about potentially suboptimal choices related to OpenACC parallelism.

.. option:: -Wno-openacc-parallelism

  Default setting; overrides :option:`-Wopenacc-parallelism`.

.. option:: -Wopenmp-simd

  Warn if the vectorizer cost model overrides the OpenMP
  simd directive set by user.  The :option:`-fsimd-cost-model=unlimited`
  option can be used to relax the cost model.

.. option:: -Wno-openmp-simd

  Default setting; overrides :option:`-Wopenmp-simd`.

.. option:: -Woverride-init

  .. note::

    C and Objective-C only

  Warn if an initialized field without side effects is overridden when
  using designated initializers (see :ref:`designated-inits`).

  This warning is included in :option:`-Wextra`.  To get other
  :option:`-Wextra` warnings without this one, use :option:`-Wextra
  -Wno-override-init`.

.. option:: -Wno-override-init

  Default setting; overrides :option:`-Woverride-init`.

.. option:: -Wno-override-init-side-effects

  .. note::

    C and Objective-C only

  Do not warn if an initialized field with side effects is overridden when
  using designated initializers (see :ref:`designated-inits`).  This warning is enabled by default.

.. option:: -Woverride-init-side-effects

  Default setting; overrides :option:`-Wno-override-init-side-effects`.

.. option:: -Wpacked

  Warn if a structure is given the packed attribute, but the packed
  attribute has no effect on the layout or size of the structure.
  Such structures may be mis-aligned for little benefit.  For
  instance, in this code, the variable ``f.x`` in ``struct bar``
  is misaligned even though ``struct bar`` does not itself
  have the packed attribute:

  .. code-block:: c++

    struct foo {
      int x;
      char a, b, c, d;
    } __attribute__((packed));
    struct bar {
      char z;
      struct foo f;
    };

.. option:: -Wno-packed

  Default setting; overrides :option:`-Wpacked`.

.. option:: -Wnopacked-bitfield-compat

  The 4.1, 4.2 and 4.3 series of GCC ignore the :var-attr:`packed` attribute
  on bit-fields of type ``char``.  This was fixed in GCC 4.4 but
  the change can lead to differences in the structure layout.  GCC
  informs you when the offset of such a field has changed in GCC 4.4.
  For example there is no longer a 4-bit padding between field ``a``
  and ``b`` in this structure:

  .. code-block:: c++

    struct foo
    {
      char a:4;
      char b:8;
    } __attribute__ ((packed));

  This warning is enabled by default.  Use
  :option:`-Wno-packed-bitfield-compat` to disable this warning.

.. option:: -Wpacked-not-aligned

  .. note::

    C, C++, Objective-C and Objective-C++ only

  Warn if a structure field with explicitly specified alignment in a
  packed struct or union is misaligned.  For example, a warning will
  be issued on ``struct S``, like, ``warning: alignment 1 of
  'struct S' is less than 8``, in this code:

  .. code-block:: c++

    struct __attribute__ ((aligned (8))) S8 { char a[8]; };
    struct __attribute__ ((packed)) S {
      struct S8 s8;
    };

  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-packed-not-aligned

  Default setting; overrides :option:`-Wpacked-not-aligned`.

.. option:: -Wpadded

  Warn if padding is included in a structure, either to align an element
  of the structure or to align the whole structure.  Sometimes when this
  happens it is possible to rearrange the fields of the structure to
  reduce the padding and so make the structure smaller.

.. option:: -Wno-padded

  Default setting; overrides :option:`-Wpadded`.

.. option:: -Wredundant-decls

  Warn if anything is declared more than once in the same scope, even in
  cases where multiple declaration is valid and changes nothing.

.. option:: -Wno-redundant-decls

  Default setting; overrides :option:`-Wredundant-decls`.

.. option:: -Wrestrict

  Warn when an object referenced by a ``restrict`` -qualified parameter
  (or, in C++, a ``__restrict`` -qualified parameter) is aliased by another
  argument, or when copies between such objects overlap.  For example,
  the call to the ``strcpy`` function below attempts to truncate the string
  by replacing its initial characters with the last four.  However, because
  the call writes the terminating NUL into ``a[4]``, the copies overlap and
  the call is diagnosed.

  .. code-block:: c++

    void foo (void)
    {
      char a[] = "abcd1234";
      strcpy (a, a + 4);
      ...
    }

  The :option:`-Wrestrict` option detects some instances of simple overlap
  even without optimization but works best at :option:`-O2` and above.  It
  is included in :option:`-Wall`.

.. option:: -Wno-restrict

  Default setting; overrides :option:`-Wrestrict`.

.. option:: -Wnested-externs

  .. note::

    C and Objective-C only

  Warn if an ``extern`` declaration is encountered within a function.

.. option:: -Wno-nested-externs

  Default setting; overrides :option:`-Wnested-externs`.

.. option:: -Winline

  Warn if a function that is declared as inline cannot be inlined.
  Even with this option, the compiler does not warn about failures to
  inline functions declared in system headers.

  The compiler uses a variety of heuristics to determine whether or not
  to inline a function.  For example, the compiler takes into account
  the size of the function being inlined and the amount of inlining
  that has already been done in the current function.  Therefore,
  seemingly insignificant changes in the source program can cause the
  warnings produced by :option:`-Winline` to appear or disappear.

.. option:: -Wno-inline

  Default setting; overrides :option:`-Winline`.

.. option:: -Winterference-size

  Warn about use of C++17 ``std::hardware_destructive_interference_size``
  without specifying its value with :option:`--param destructive-interference-size`.
  Also warn about questionable values for that option.

  This variable is intended to be used for controlling class layout, to
  avoid false sharing in concurrent code:

  .. code-block:: c++

    struct independent_fields {
      alignas(std::hardware_destructive_interference_size) std::atomic<int> one;
      alignas(std::hardware_destructive_interference_size) std::atomic<int> two;
    };

  Here :samp:`one` and :samp:`two` are intended to be far enough apart
  that stores to one won't require accesses to the other to reload the
  cache line.

  By default, :option:`--param destructive-interference-size` and
  :option:`--param constructive-interference-size` are set based on the
  current :option:`-mtune` option, typically to the L1 cache line size
  for the particular target CPU, sometimes to a range if tuning for a
  generic target.  So all translation units that depend on ABI
  compatibility for the use of these variables must be compiled with
  the same :option:`-mtune` (or :option:`-mcpu`).

  If ABI stability is important, such as if the use is in a header for a
  library, you should probably not use the hardware interference size
  variables at all.  Alternatively, you can force a particular value
  with :option:`--param`.

  If you are confident that your use of the variable does not affect ABI
  outside a single build of your project, you can turn off the warning
  with :option:`-Wno-interference-size`.

.. option:: -Wint-in-bool-context

  Warn for suspicious use of integer values where boolean values are expected,
  such as conditional expressions (?:) using non-boolean integer constants in
  boolean context, like ``if (a <= b ? 2 : 3)``.  Or left shifting of signed
  integers in boolean context, like ``for (a = 0; 1 << a; a++);``.  Likewise
  for all kinds of multiplications regardless of the data type.
  This warning is enabled by :option:`-Wall`.

.. option:: -Wno-int-in-bool-context

  Default setting; overrides :option:`-Wint-in-bool-context`.

.. option:: -Wno-int-to-pointer-cast

  Suppress warnings from casts to pointer type of an integer of a
  different size. In C++, casting to a pointer type of smaller size is
  an error. Wint-to-pointer-cast is enabled by default.

.. option:: -Wint-to-pointer-cast

  Default setting; overrides :option:`-Wno-int-to-pointer-cast`.

.. option:: -Wno-pointer-to-int-cast

  .. note::

    C and Objective-C only

  Suppress warnings from casts from a pointer to an integer type of a
  different size.

.. option:: -Wpointer-to-int-cast

  Default setting; overrides :option:`-Wno-pointer-to-int-cast`.

.. option:: -Winvalid-pch

  Warn if a precompiled header (see :ref:`precompiled-headers`) is found in
  the search path but cannot be used.

.. option:: -Wno-invalid-pch

  Default setting; overrides :option:`-Winvalid-pch`.

.. option:: -Winvalid-utf8

  Warn if an invalid UTF-8 character is found.
  This warning is on by default for C++23 if :option:`-finput-charset=UTF-8`
  is used and turned into error with :option:`-pedantic-errors`.

.. option:: -Wno-invalid-utf8

  Default setting; overrides :option:`-Winvalid-utf8`.

.. option:: -Wno-unicode

  Don't diagnose invalid forms of delimited or named escape sequences which are
  treated as separate tokens.  Wunicode is enabled by default.

.. option:: -Wunicode

  Default setting; overrides :option:`-Wno-unicode`.

.. option:: -Wlong-long

  Warn if ``long long`` type is used.  This is enabled by either
  :option:`-Wpedantic` or :option:`-Wtraditional` in ISO C90 and C++98
  modes.  To inhibit the warning messages, use :option:`-Wno-long-long`.

.. option:: -Wno-long-long

  Default setting; overrides :option:`-Wlong-long`.

.. option:: -Wvariadic-macros

  Warn if variadic macros are used in ISO C90 mode, or if the GNU
  alternate syntax is used in ISO C99 mode.  This is enabled by either
  :option:`-Wpedantic` or :option:`-Wtraditional`.  To inhibit the warning
  messages, use :option:`-Wno-variadic-macros`.

.. option:: -Wno-variadic-macros

  Default setting; overrides :option:`-Wvariadic-macros`.

.. option:: -Wno-varargs

  Do not warn upon questionable usage of the macros used to handle variable
  arguments like ``va_start``.  These warnings are enabled by default.

.. option:: -Wvarargs

  Default setting; overrides :option:`-Wno-varargs`.

.. option:: -Wvector-operation-performance

  Warn if vector operation is not implemented via SIMD capabilities of the
  architecture.  Mainly useful for the performance tuning.
  Vector operation can be implemented ``piecewise``, which means that the
  scalar operation is performed on every vector element;
  ``in parallel``, which means that the vector operation is implemented
  using scalars of wider type, which normally is more performance efficient;
  and ``as a single scalar``, which means that vector fits into a
  scalar type.

.. option:: -Wno-vector-operation-performance

  Default setting; overrides :option:`-Wvector-operation-performance`.

.. option:: -Wvla

  Warn if a variable-length array is used in the code.
  :option:`-Wno-vla` prevents the :option:`-Wpedantic` warning of
  the variable-length array.

.. option:: -Wno-vla

  Default setting; overrides :option:`-Wvla`.

.. option:: -Wvla-larger-than={byte-size}

  If this option is used, the compiler warns for declarations of
  variable-length arrays whose size is either unbounded, or bounded
  by an argument that allows the array size to exceed :samp:`{byte-size}`
  bytes.  This is similar to how :option:`-Walloca-larger-than=byte-size`
  works, but with variable-length arrays.

  Note that GCC may optimize small variable-length arrays of a known
  value into plain arrays, so this warning may not get triggered for
  such arrays.

  :option:`-Wvla-larger-than=PTRDIFF_MAX` is enabled by default but
  is typically only effective when :option:`-ftree-vrp` is active (default
  for :option:`-O2` and above).

  See also :option:`-Walloca-larger-than=byte-size`.

.. option:: -Wno-vla-larger-than

  Disable :option:`-Wvla-larger-than=` warnings.  The option is equivalent
  to :option:`-Wvla-larger-than=SIZE_MAX` or larger.

.. option:: -Wvla-parameter

  Warn about redeclarations of functions involving arguments of Variable
  Length Array types of inconsistent kinds or forms, and enable the detection
  of out-of-bounds accesses to such parameters by warnings such as
  :option:`-Warray-bounds`.

  If the first function declaration uses the VLA form the bound specified
  in the array is assumed to be the minimum number of elements expected to
  be provided in calls to the function and the maximum number of elements
  accessed by it.  Failing to provide arguments of sufficient size or
  accessing more than the maximum number of elements may be diagnosed.

  For example, the warning triggers for the following redeclarations because
  the first one allows an array of any size to be passed to ``f`` while
  the second one specifies that the array argument must have at least ``n``
  elements.  In addition, calling ``f`` with the associated VLA bound
  parameter in excess of the actual VLA bound triggers a warning as well.

  .. code-block:: c++

    void f (int n, int[n]);
    void f (int, int[]);     // warning: argument 2 previously declared as a VLA

    void g (int n)
    {
        if (n > 4)
          return;
        int a[n];
        f (sizeof a, a);     // warning: access to a by f may be out of bounds
      ...
    }

  :option:`-Wvla-parameter` is included in :option:`-Wall`.  The
  :option:`-Warray-parameter` option triggers warnings for similar problems
  involving ordinary array arguments.

.. option:: -Wno-vla-parameter

  Default setting; overrides :option:`-Wvla-parameter`.

.. option:: -Wvolatile-register-var

  Warn if a register variable is declared volatile.  The volatile
  modifier does not inhibit all optimizations that may eliminate reads
  and/or writes to register variables.  This warning is enabled by
  :option:`-Wall`.

.. option:: -Wno-volatile-register-var

  Default setting; overrides :option:`-Wvolatile-register-var`.

.. option:: -Wxor-used-as-pow

  .. note::

    C, C++, Objective-C and Objective-C++ only

  Warn about uses of ``^``, the exclusive or operator, where it appears
  the user meant exponentiation.  Specifically, the warning occurs when the
  left-hand side is the decimal constant 2 or 10 and the right-hand side
  is also a decimal constant.

  In C and C++, ``^`` means exclusive or, whereas in some other languages
  (e.g. TeX and some versions of BASIC) it means exponentiation.

  This warning is enabled by default.  It can be silenced by converting one
  of the operands to hexadecimal.

.. option:: -Wno-xor-used-as-pow

  Default setting; overrides :option:`-Wxor-used-as-pow`.

.. option:: -Wdisabled-optimization

  Warn if a requested optimization pass is disabled.  This warning does
  not generally indicate that there is anything wrong with your code; it
  merely indicates that GCC's optimizers are unable to handle the code
  effectively.  Often, the problem is that your code is too big or too
  complex; GCC refuses to optimize programs when the optimization
  itself is likely to take inordinate amounts of time.

.. option:: -Wno-disabled-optimization

  Default setting; overrides :option:`-Wdisabled-optimization`.

.. option:: -Wpointer-sign

  .. note::

    C and Objective-C only

  Warn for pointer argument passing or assignment with different signedness.
  This option is only supported for C and Objective-C.  It is implied by
  :option:`-Wall` and by :option:`-Wpedantic`, which can be disabled with
  :option:`-Wno-pointer-sign`.

.. option:: -Wno-pointer-sign

  Default setting; overrides :option:`-Wpointer-sign`.

.. option:: -Wstack-protector

  This option is only active when :option:`-fstack-protector` is active.  It
  warns about functions that are not protected against stack smashing.

.. option:: -Wno-stack-protector

  Default setting; overrides :option:`-Wstack-protector`.

.. option:: -Woverlength-strings

  Warn about string constants that are longer than the 'minimum
  maximum' length specified in the C standard.  Modern compilers
  generally allow string constants that are much longer than the
  standard's minimum limit, but very portable programs should avoid
  using longer strings.

  The limit applies *after* string constant concatenation, and does
  not count the trailing NUL.  In C90, the limit was 509 characters; in
  C99, it was raised to 4095.  C++98 does not specify a normative
  minimum maximum, so we do not diagnose overlength strings in C++.

  This option is implied by :option:`-Wpedantic`, and can be disabled with
  :option:`-Wno-overlength-strings`.

.. option:: -Wno-overlength-strings

  Default setting; overrides :option:`-Woverlength-strings`.

.. option:: -Wunsuffixed-float-constants

  .. note::

    C and Objective-C only

  Issue a warning for any floating constant that does not have
  a suffix.  When used together with :option:`-Wsystem-headers` it
  warns about such constants in system header files.  This can be useful
  when preparing code to use with the ``FLOAT_CONST_DECIMAL64`` pragma
  from the decimal floating-point extension to C99.

.. option:: -Wno-unsuffixed-float-constants

  Default setting; overrides :option:`-Wunsuffixed-float-constants`.

.. option:: -Wno-lto-type-mismatch

  During the link-time optimization, do not warn about type mismatches in
  global declarations from different compilation units.
  Requires :option:`-flto` to be enabled.  Enabled by default.

.. option:: -Wlto-type-mismatch

  Default setting; overrides :option:`-Wno-lto-type-mismatch`.

.. option:: -Wno-designated-init

  .. note::

    C and Objective-C only

  Suppress warnings when a positional initializer is used to initialize
  a structure that has been marked with the :type-attr:`designated_init`
  attribute.

.. option:: -Wdesignated-init

  Default setting; overrides :option:`-Wno-designated-init`.