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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gcc: (gcc).                  The GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the use of the GNU compilers.

   Published by the Free Software Foundation
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File: gcc.info,  Node: Warning Options,  Next: Debugging Options,  Prev: Language Independent Options,  Up: Invoking GCC

Options to Request or Suppress Warnings
=======================================

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

   You can request many specific warnings with options beginning `-W',
for example `-Wimplicit' to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'.
This manual lists only one of the two forms, whichever is not the
default.

   The following options control the amount and kinds of warnings
produced by GCC; for further, language-specific options also refer to
*Note C++ Dialect Options:: and *Note Objective-C Dialect Options::.

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

`-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 `-std' option
     used.

     Valid ISO C and ISO C++ programs should compile properly with or
     without this option (though a rare few will require `-ansi' or a
     `-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.

     `-pedantic' does not cause warning messages for use of the
     alternate keywords whose names begin and end with `__'.  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.
     *Note Alternate Keywords::.

     Some users try to use `-pedantic' 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 `-pedantic'.  We
     don't have plans to support such a feature in the near future.

     Where the standard specified with `-std' represents a GNU extended
     dialect of C, such as `gnu89' or `gnu99', there is a corresponding
     "base standard", the version of ISO C on which the GNU extended
     dialect is based.  Warnings from `-pedantic' are given where they
     are required by the base standard.  (It would 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.)

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

`-w'
     Inhibit all warning messages.

`-Wno-import'
     Inhibit warning messages about the use of `#import'.

`-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.

`-Wcomment'
     Warn whenever a comment-start sequence `/*' appears in a `/*'
     comment, or whenever a Backslash-Newline appears in a `//' comment.

`-Wformat'
     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 (*note Function Attributes::), in
     the `printf', `scanf', `strftime' and `strfmon' (an X/Open
     extension, not in the C standard) families.

     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 `-pedantic' is used with `-Wformat', warnings will be
     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).  *Note Options Controlling C Dialect: C
     Dialect Options.

     Since `-Wformat' also checks for null format arguments for several
     functions, `-Wformat' also implies `-Wnonnull'.

     `-Wformat' is included in `-Wall'.  For more control over some
     aspects of format checking, the options `-Wno-format-y2k',
     `-Wno-format-extra-args', `-Wno-format-zero-length',
     `-Wformat-nonliteral', `-Wformat-security', and `-Wformat=2' are
     available, but are not included in `-Wall'.

`-Wno-format-y2k'
     If `-Wformat' is specified, do not warn about `strftime' formats
     which may yield only a two-digit year.

`-Wno-format-extra-args'
     If `-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 `$' 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 will suppress
     the warning if the unused arguments are all pointers, since the
     Single Unix Specification says that such unused arguments are
     allowed.

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

`-Wformat-nonliteral'
     If `-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'.

`-Wformat-security'
     If `-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 `%n'.
     (This is currently a subset of what `-Wformat-nonliteral' warns
     about, but in future warnings may be added to `-Wformat-security'
     that are not included in `-Wformat-nonliteral'.)

`-Wformat=2'
     Enable `-Wformat' plus format checks not included in `-Wformat'.
     Currently equivalent to `-Wformat -Wformat-nonliteral
     -Wformat-security'.

`-Wnonnull'
     Enable warning about passing a null pointer for arguments marked as
     requiring a non-null value by the `nonnull' function attribute.

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

`-Winit-self (C, C++, and Objective-C only)'
     Enable warning about uninitialized variables which are initalized
     with themselves.  Note this option can only be used with the
     `-Wuninitialized' option and that only works with `-O'.

     For an example, the following code will not warn about i being
     uninitialized without this option:
          int f()
          {
            int i = i;
            return i;
          }

`-Wimplicit-int'
     Warn when a declaration does not specify a type.

`-Wimplicit-function-declaration'
`-Werror-implicit-function-declaration'
     Give a warning (or error) whenever a function is used before being
     declared.

`-Wimplicit'
     Same as `-Wimplicit-int' and `-Wimplicit-function-declaration'.

`-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.

`-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.

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

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

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

     In 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 will issue 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' could
     belong to the enclosing `if'.  The resulting code would look like
     this:

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

`-Wsequence-point'
     Warn about code that may have undefined semantics because of
     violations of sequence point rules in the C standard.

     The C standard defines the order in which expressions in a C
     program are evaluated in terms of "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 standard specifies
     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 present implementation of this option only works for C
     programs.  A future implementation may also work for C++ programs.

     The C 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 our readings page, at
     `http://gcc.gnu.org/readings.html'.

`-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'.

     For C++, a function without return type always produces a
     diagnostic message, even when `-Wno-return-type' is specified.
     The only exceptions are `main' and functions defined in system
     headers.

`-Wswitch'
     Warn whenever a `switch' statement has an index of enumeral 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.

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

`-Wswitch-enum'
     Warn whenever a `switch' statement has an index of enumeral 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.

`-Wtrigraphs'
     Warn if any trigraphs are encountered that might change the
     meaning of the program (trigraphs within comments are not warned
     about).

`-Wunused-function'
     Warn whenever a static function is declared but not defined or a
     non\-inline static function is unused.

`-Wunused-label'
     Warn whenever a label is declared but not used.

     To suppress this warning use the `unused' attribute (*note
     Variable Attributes::).

`-Wunused-parameter'
     Warn whenever a function parameter is unused aside from its
     declaration.

     To suppress this warning use the `unused' attribute (*note
     Variable Attributes::).

`-Wunused-variable'
     Warn whenever a local variable or non-constant static variable is
     unused aside from its declaration

     To suppress this warning use the `unused' attribute (*note
     Variable Attributes::).

`-Wunused-value'
     Warn whenever a statement computes a result that is explicitly not
     used.

     To suppress this warning cast the expression to `void'.

`-Wunused'
     All the above `-Wunused' options combined.

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

`-Wuninitialized'
     Warn if an automatic variable is used without first being
     initialized or if a variable may be clobbered by a `setjmp' call.

     These warnings are possible only in optimizing compilation,
     because they require data flow information that is computed only
     when optimizing.  If you don't specify `-O', you simply won't get
     these warnings.

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

     These warnings occur only for variables that are candidates for
     register allocation.  Therefore, they do not occur for a variable
     that is declared `volatile', or whose address is taken, or whose
     size is other than 1, 2, 4 or 8 bytes.  Also, they do not occur for
     structures, unions or arrays, even when they are in registers.

     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.

     These warnings are made optional because GCC is not smart enough
     to see all the reasons why the code might be correct despite
     appearing to have an error.  Here is one example of how this can
     happen:

          {
            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.  Here is another common
     case:

          {
            int save_y;
            if (change_y) save_y = y, y = new_y;
            ...
            if (change_y) y = save_y;
          }

     This has no bug because `save_y' is used only if it is set.

     This option also warns when a non-volatile automatic variable
     might be changed by a call to `longjmp'.  These warnings as well
     are possible only in optimizing compilation.

     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 which would cause a problem.

     Some spurious warnings can be avoided if you declare all the
     functions you use that never return as `noreturn'.  *Note Function
     Attributes::.

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

`-Wstrict-aliasing'
     This option is only active when `-fstrict-aliasing' is active.  It
     warns about code which 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 `-Wall'.

`-Wall'
     All of the above `-W' options combined.  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 *Note C++ Dialect
     Options:: and *Note Objective-C Dialect Options::.

   The following `-W...' options are not implied by `-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.

`-Wextra'
     (This option used to be called `-W'.  The older name is still
     supported, but the newer name is more descriptive.)  Print extra
     warning messages for these events:

        * A function can return either with or without a value.
          (Falling off the end of the function body is considered
          returning without a value.)  For example, this function would
          evoke such a warning:

               foo (a)
               {
                 if (a > 0)
                   return a;
               }

        * An expression-statement or the left-hand side of a comma
          expression contains no side effects.  To suppress the
          warning, cast the unused expression to void.  For example, an
          expression such as `x[i,j]' will cause a warning, but
          `x[(void)i,j]' will not.

        * An unsigned value is compared against zero with `<' or `>='.

        * 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.

        * Storage-class specifiers like `static' are not the first
          things in a declaration.  According to the C Standard, this
          usage is obsolescent.

        * The return type of a function has a type qualifier such as
          `const'.  Such a type qualifier has no effect, since the
          value returned by a function is not an lvalue.  (But don't
          warn about the GNU extension of `volatile void' return types.
          That extension will be warned about if `-pedantic' is
          specified.)

        * If `-Wall' or `-Wunused' is also specified, warn about unused
          arguments.

        * A comparison between signed and unsigned values could produce
          an incorrect result when the signed value is converted to
          unsigned.  (But don't warn if `-Wno-sign-compare' is also
          specified.)

        * An aggregate has an initializer which does not initialize all
          members.  For example, the following code would cause such a
          warning, because `x.h' would be implicitly initialized to
          zero:

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

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

               void foo(bar) { }

        * An empty body occurs in an `if' or `else' statement.

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

        * A variable might be changed by `longjmp' or `vfork'.

        * Any of several floating-point events that often indicate
          errors, such as overflow, underflow, loss of precision, etc.

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

        * (C++ only) A non-static reference or non-static `const'
          member appears in a class without constructors.

        * (C++ only) Ambiguous virtual bases.

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

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

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

`-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.

`-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
     `-Wall' in conjunction with this option will _not_ warn about
     unknown pragmas in system headers--for that, `-Wunknown-pragmas'
     must also be used.

`-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 would 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.

`-Wtraditional (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 which
     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 does not in ISO C.

        * In traditional C, some preprocessor directives did not exist.
          Traditional preprocessors would only consider a line to be a
          directive if the `#' appeared in column 1 on the line.
          Therefore `-Wtraditional' warns about directives that
          traditional C understands but would ignore because the `#'
          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 would not recognize `#elif', so it suggests
          avoiding it altogether.

        * A function-like macro that appears without arguments.

        * The unary plus operator.

        * The `U' integer constant suffix, or the `F' or `L' floating
          point constant suffixes.  (Traditional C does support the `L'
          suffix on integer constants.)  Note, these suffixes appear in
          macros defined in the system headers of most modern systems,
          e.g. the `_MIN'/`_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 would cause serious problems.
          This is a subset of the possible conversion warnings, for the
          full set use `-Wconversion'.

        * Use of ISO C style function definitions.  This warning
          intentionally is _not_ issued for prototype declarations or
          variadic functions because these ISO C features will 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.

`-Wdeclaration-after-statement (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 and was
     not supported by GCC versions before GCC 3.0.  *Note Mixed
     Declarations::.

`-Wundef'
     Warn if an undefined identifier is evaluated in an `#if' directive.

`-Wendif-labels'
     Warn whenever an `#else' or an `#endif' are followed by text.

`-Wshadow'
     Warn whenever a local variable shadows another local variable,
     parameter or global variable or whenever a built-in function is
     shadowed.

`-Wlarger-than-LEN'
     Warn whenever an object of larger than LEN bytes is defined.

`-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.

`-Wbad-function-cast (C only)'
     Warn whenever a function call is cast to a non-matching type.  For
     example, warn if `int malloc()' is cast to `anything *'.

`-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 *'.

`-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.

`-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 will get a warning; when compiling
     C++, warn about the deprecated conversion from string constants to
     `char *'.  These warnings will 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 will just be a nuisance; this is why we
     did not make `-Wall' request these warnings.

`-Wconversion'
     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.

     Also, warn if a negative integer constant expression is implicitly
     converted to an unsigned type.  For example, warn about the
     assignment `x = -1' if `x' is unsigned.  But do not warn about
     explicit casts like `(unsigned) -1'.

`-Wsign-compare'
     Warn when a comparison between signed and unsigned values could
     produce an incorrect result when the signed value is converted to
     unsigned.  This warning is also enabled by `-Wextra'; to get the
     other warnings of `-Wextra' without this warning, use `-Wextra
     -Wno-sign-compare'.

`-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.)

`-Wstrict-prototypes (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 which specifies the
     argument types.)

`-Wmissing-prototypes (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.  The aim is to detect global functions that
     fail to be declared in header files.

`-Wmissing-declarations (C only)'
     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.

`-Wmissing-noreturn'
     Warn about functions which might be candidates for attribute
     `noreturn'.  Note these are only possible candidates, not absolute
     ones.  Care should be taken to manually verify functions actually
     do not ever return before adding the `noreturn' attribute,
     otherwise subtle code generation bugs could be introduced.  You
     will not get a warning for `main' in hosted C environments.

`-Wmissing-format-attribute'
     If `-Wformat' is enabled, also warn about functions which might be
     candidates for `format' attributes.  Note these are only possible
     candidates, not absolute ones.  GCC will guess 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.  This option has no effect unless
     `-Wformat' is enabled (possibly by `-Wall').

`-Wno-multichar'
     Do not warn if a multicharacter constant (`'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.

`-Wno-deprecated-declarations'
     Do not warn about uses of functions, variables, and types marked as
     deprecated by using the `deprecated' attribute.  (*note Function
     Attributes::, *note Variable Attributes::, *note Type
     Attributes::.)

`-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' will be
     misaligned even though `struct bar' does not itself have the
     packed attribute:

          struct foo {
            int x;
            char a, b, c, d;
          } __attribute__((packed));
          struct bar {
            char z;
            struct foo f;
          };

`-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.

`-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.

`-Wnested-externs (C only)'
     Warn if an `extern' declaration is encountered within a function.

`-Wunreachable-code'
     Warn if the compiler detects that code will never be executed.

     This option is intended to warn when the compiler detects that at
     least a whole line of source code will never be executed, because
     some condition is never satisfied or because it is after a
     procedure that never returns.

     It is possible for this option to produce a warning even though
     there are circumstances under which part of the affected line can
     be executed, so care should be taken when removing
     apparently-unreachable code.

     For instance, when a function is inlined, a warning may mean that
     the line is unreachable in only one inlined copy of the function.

     This option is not made part of `-Wall' because in a debugging
     version of a program there is often substantial code which checks
     correct functioning of the program and is, hopefully, unreachable
     because the program does work.  Another common use of unreachable
     code is to provide behavior which is selectable at compile-time.

`-Winline'
     Warn if a function can not be inlined and it was declared as
     inline.  Even with this option, the compiler will 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 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 `-Winline' to appear or
     disappear.

`-Wno-invalid-offsetof (C++ only)'
     Suppress warnings from applying the `offsetof' macro to a non-POD
     type.  According to the 1998 ISO C++ standard, applying `offsetof'
     to a non-POD type is undefined.  In existing C++ implementations,
     however, `offsetof' typically gives meaningful results even when
     applied to certain kinds of non-POD types. (Such as a simple
     `struct' that fails to be a POD type only by virtue of having a
     constructor.)  This flag is for users who are aware that they are
     writing nonportable code and who have deliberately chosen to
     ignore the warning about it.

     The restrictions on `offsetof' may be relaxed in a future version
     of the C++ standard.

`-Winvalid-pch'
     Warn if a precompiled header (*note Precompiled Headers::) is
     found in the search path but can't be used.

`-Wlong-long'
     Warn if `long long' type is used.  This is default.  To inhibit
     the warning messages, use `-Wno-long-long'.  Flags `-Wlong-long'
     and `-Wno-long-long' are taken into account only when `-pedantic'
     flag is used.

`-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 were unable to
     handle the code effectively.  Often, the problem is that your code
     is too big or too complex; GCC will refuse to optimize programs
     when the optimization itself is likely to take inordinate amounts
     of time.

`-Werror'
     Make all warnings into errors.

